all: REVERSE MERGE dev.typeparams (7cdfa49) into master

This commit is a REVERSE MERGE.
It merges dev.typeparams back into its parent branch, master.
This marks the end of development on dev.typeparams.

Merge List:

+ 2021-02-19 7cdfa4969a [dev.typeparams] all: merge master (06b86e9) into dev.typeparams
+ 2021-02-19 6521c7b378 [dev.typeparams] cmd/compile/internal/types2: resolve decl cycle the same way as in go/types
+ 2021-02-19 dfe0ef961b [dev.typeparams] go/types, types2: revert fancy struct printing (fixes x/tools tests)
+ 2021-02-19 2f37939a21 go/parser: improve error recovery from invalid selector exprs
+ 2021-02-19 8654db4555 [dev.typeparams] go/types: adjust printing of embedded struct fields (fixes x/tools/cmd/guru tests)
+ 2021-02-19 47e4b0739e Merge "[dev.typeparams] all: merge master (eb98272) into dev.typeparams" into dev.typeparams
+ 2021-02-18 a789be7814 [dev.typeparams] cmd/compile: use new converter functions rather than methods (fix build)
+ 2021-02-18 20050a15fe [dev.typeparams] cmd/compile:  support generic types (with stenciling of method calls)
+ 2021-02-18 e7493a9c74 [dev.typeparams] all: merge master (eb98272) into dev.typeparams
+ 2021-02-18 2ff1e05a4c [dev.typeparams] all: update parent repository
+ 2021-02-18 8960ce7735 [dev.typeparams] cmd/compile/internal/types2: minor adjustments to match go/types more closely
+ 2021-02-18 6f3878b942 [dev.typeparams] cmd/compile/internal/types: review of typestring_test.go
+ 2021-02-18 d6bdd1aeef [dev.typeparams] cmd/compile/internal/types: review of typestring.go
+ 2021-02-18 c2314babb8 [dev.typeparams] cmd/compile/internal/types: review of type.go
+ 2021-02-18 099374b55e [dev.typeparams] cmd/compile/internal/types2: remove Type.Under method in favor of function
+ 2021-02-18 653386a89a [dev.typeparams] cmd/compile/internal/types2: replace Named, TypeParam methods with functions
+ 2021-02-18 5e4da8670b [dev.typeparams] cmd/compile/internal/types2: use converter functions rather than methods
+ 2021-02-18 5ecb9a7887 [dev.typeparams] go/types: use a new ast.ListExpr for multi-type instances
+ 2021-02-18 7b679617f3 [dev.typeparams] go/types: conversions to type parameters are not constant
+ 2021-02-17 f5d0c653e6 [dev.typeparams] merge master (2f0da6d) into dev.typeparams
+ 2021-02-17 e196cb8258 [dev.typeparams] cmd/dist:  disable -G=3 on the std go tests for now
+ 2021-02-12 a06bd9fecb [dev.typeparams] cmd/compile/internal/types: review of resolver_test.go
+ 2021-02-12 042f88fe30 [dev.typeparams] cmd/compile/internal/types: review of errors_test.go
+ 2021-02-12 0abd7b768b [dev.typeparams] cmd/compile/internal/types: review of universe.go
+ 2021-02-12 1b6f0bf1b2 [dev.typeparams] cmd/compile/internal/types: review of sizes_test.go
+ 2021-02-12 1758780181 [dev.typeparams] cmd/compile/internal/types: review of sizes.go
+ 2021-02-12 3aee461d5c [dev.typeparams] cmd/compile/internal/types: review of return.go
+ 2021-02-12 7428318af6 [dev.typeparams] cmd/compile/internal/types: review of object_test.go
+ 2021-02-12 b20f9e2da1 [dev.typeparams] cmd/compile/internal/types: review of object.go
+ 2021-02-12 20746b2f37 [dev.typeparams] cmd/compile/internal/types: review of labels.go
+ 2021-02-12 bab3461123 [dev.typeparams] cmd/compile/internal/types: review of infer.go
+ 2021-02-12 9168590977 [dev.typeparams] cmd/compile/internal/types: review of builtin_test.go
+ 2021-02-12 f1777cf84c [dev.typeparams] cmd/compile/internal/types: review of builtin.go
+ 2021-02-12 0f43973b4b [dev.typeparams] go/types: make predeclared "any" alias for interface{}
+ 2021-02-12 58758e0a21 [dev.typeparams] go/types: better error message for invalid ... use
+ 2021-02-11 c0aa7bd760 [dev.typeparams] cmd/compile: small fixes for stenciling
+ 2021-02-10 df23540dde [dev.typeparams] cmd/gofmt: add the -G flag to allow generic code
+ 2021-02-10 ddec18cf82 [dev.typeparams] cmd/compile/internal/types2: overlapping embedded interfaces requires go1.14
+ 2021-02-10 fdf3496fcc [dev.typeparams] cmd/compile:  make type conversions by type parameters work
+ 2021-02-10 12e15d430d [dev.typeparams] cmd/compile: handle calling a method on a type param in stenciling
+ 2021-02-08 ca18c42054 [dev.typeparams] merge dev.regabi (618e3c1) into dev.typeparams
+ 2021-02-08 a360eeb528 [dev.typeparams] cmd/compile/internal/types2: conversions to type parameters are not constant
+ 2021-02-08 0fbde54ea6 [dev.typeparams] cmd/compile: allow generic funcs to call other generic funcs for stenciling
+ 2021-02-05 dcb5e0392e [dev.typeparams] cmd/compile: add stenciling of simple generic functions
+ 2021-02-04 f37b0c6c12 [dev.typeparams] cmd/compile/internal/types2: type alias decl requires go1.9
+ 2021-02-04 721488498a [dev.typeparams] cmd/compile: pass -lang flag value to new type checker
+ 2021-02-04 1ff2fdaaf1 [dev.typeparams] cmd/compile/internal/types2: add support for language version checking
+ 2021-02-04 370e9f5843 [dev.typeparams] cmd/compile/internal/types2: use 512 bits as max. integer precision
+ 2021-02-04 ca2f152893 [dev.typeparams] go/types: add missing test from dev.go2go
+ 2021-02-03 dc122c7a9c [dev.typeparams] test: exclude a failing test again (fix 32bit builds)
+ 2021-02-03 c910fd7b77 [dev.typeparams] cmd/compile: refuse excessively long constants
+ 2021-02-03 3db6e18468 [dev.typeparams] test: enable more errorcheck tests
+ 2021-02-03 bb53a5ad43 [dev.typeparams] cmd/compile/internal/importer: adjust importer to match compiler importer
+ 2021-02-03 3f845b3b45 [dev.typeparams] cmd/compile: deal with inferred type arguments
+ 2021-02-03 e633f343ba [dev.typeparams] cmd/compile:  add OFUNCINST/OTYPEINST nodes for generic func/type instantiation
+ 2021-02-02 0d2d6c7464 [dev.typeparams] all: merge dev.regabi (23b0c1f) into dev.typeparams
+ 2021-02-02 3d5c715bf2 [dev.typeparams] Handling multiple type arguments for call via new node OLIST
+ 2021-02-01 13a7412983 [dev.typeparams] Parse a generic type arg for generic function call
+ 2021-01-30 0aafd69124 [dev.typeparams] cmd/compile: start translating type params in noder2
+ 2021-01-30 a59cb5109d [dev.typeparams] cmd/compile/internal/types2: handle untyped constant arithmetic overflow
+ 2021-01-29 507e641963 [dev.typeparams] cmd/compile/internal/typecheck: declutter generated builtin.go (cleanup)
+ 2021-01-28 2440dd457a [dev.typeparams] cmd/compile:  start adding info needed for typeparams in types & ir
+ 2021-01-28 c0bf904ddf [dev.typeparams] cmd/compile/internal/types2: translate syntax to token constants via tables
+ 2021-01-28 f7d1c5990b [dev.typeparams] cmd/compile/internal/types2: must not import a package called "init"
+ 2021-01-27 217a461f56 [dev.typeparams] cmd/compile/internal/types2: report unused packages in source order
+ 2021-01-26 08a598f8c1 [dev.typeparams] cmd/compile: fix MethodExpr handling with embedded fields
+ 2021-01-26 cecc1dfcba [dev.typeparams] test: enable excluded test fixedbugs/issue7742.go
+ 2021-01-26 e48d7d3b21 [dev.typeparams] go/constant: faster match implementation
+ 2021-01-26 d39685e5e9 [dev.typeparams] go/constant: choose internal float representations more consistently
+ 2021-01-25 34704e374f [dev.typeparams] all: merge dev.regabi (5e4a0cd) into dev.typeparams
+ 2021-01-26 c97af0036b [dev.typeparams] cmd/compile: force untyped constants from types2 to expected kind
+ 2021-01-25 3663a437a7 [dev.typeparams] go/constant: in ToFloat, convert to rational numbers, not floats
+ 2021-01-25 3432d24bab Merge "[dev.typeparams] all: merge dev.regabi (063c72f) into dev.typeparams" into dev.typeparams
+ 2021-01-25 493eb6e6ec [dev.typeparams] cmd/compile: fix -G=3 handling of blank methods
+ 2021-01-25 13f02018af [dev.typeparams] test: enable more errorcheck tests w/ -G=3
+ 2021-01-24 6d8d118762 [dev.typeparams] all: merge dev.regabi (063c72f) into dev.typeparams
+ 2021-01-24 9456804e86 [dev.typeparams] test: fix excluded files lookup so it works on Windows
+ 2021-01-23 7947df436d [dev.typeparams] test: set -G=3 and enable more errorcheck tests in run.go
+ 2021-01-23 a49e941027 [dev.typeparams] cmd/compile/internal/types2: remove MethodSet code - not used by types2
+ 2021-01-23 5347241b5e [dev.typeparams] cmd/compile/internal/types2: use same sort criteria for methods as compiler
+ 2021-01-23 2b95c28b18 [dev.typeparams] cmd/compile: refactor SelectorExpr code into helpers
+ 2021-01-23 1946a77e69 Merge "[dev.typeparams] all: merge dev.regabi (7e0a81d) into dev.typeparams" into dev.typeparams
+ 2021-01-23 6923019a71 [dev.typeparams] cmd/compile/internal/types2: factor out sorting of methods
+ 2021-01-22 6e46c8fbb5 [dev.typeparams] all: merge dev.regabi (7e0a81d) into dev.typeparams
+ 2021-01-22 e4ef30a667 [dev.typeparams] cmd/compile: refactor irgen's handling of ":="
+ 2021-01-22 626406b703 [dev.typeparams] go/types: import api_test.go changes from dev.go2go
+ 2021-01-22 12cd9cf7e0 [dev.typeparams] cmd/compile: disambiguate OXDOT in noder using types2 Selection info
+ 2021-01-22 f8654579cd [dev.typeparams] cmd/compile/internal/types2: adjust errors in branch checking code, fix a bug
+ 2021-01-21 18bd7aa625 [dev.typeparams] cmd/compile: use nil instead of syntax.ImplicitOne
+ 2021-01-21 2427f6e6c0 [dev.typeparams] cmd/compile: directly set some simple expression types
+ 2021-01-21 455c29af83 [dev.typeparams] cmd/compile/internal/types2: convert untyped arguments to delete
+ 2021-01-21 f03f934ede [dev.typeparams] cmd/compile/internal/types2: make predeclared "any" alias for interface{}
+ 2021-01-20 0f054c5be0 [dev.typeparams] cmd/dist: add -G=3 test coverage
+ 2021-01-20 89ec17be9a [dev.typeparams] cmd/compile: simplify how irgen handles qualified idents
+ 2021-01-20 fa01ade41e [dev.typeparams] go/types: add tests from dev.go2go
+ 2021-01-20 734cb8be0a [dev.typeparams] go/types: refactor untyped conversion for typeparams
+ 2021-01-20 d8796b5670 [dev.typeparams] cmd/compile/internal/types2: report type of nil based on context
+ 2021-01-19 48a3cb399d [dev.typeparams] go/types: fix some merge errors in call.go
+ 2021-01-19 3e15bf7716 [dev.typeparams] go/types: don't modify Named.underlying in validType
+ 2021-01-19 f38f862417 [dev.typeparams] go/types: strip annotations from errors
+ 2021-01-19 2e64511ac9 [dev.typeparams] go/types: unify methods in missingMethod
+ 2021-01-19 958927c824 [dev.typeparams] go/parser: error for type instances without ParseTypeParams
+ 2021-01-19 90bfc73071 [dev.typeparams] cmd/compile: cache mapped types during irgen
+ 2021-01-19 3c0a39c964 [dev.typeparams] cmd/compile/internal/types: minor fixes/cleanups around testing
+ 2021-01-15 502198c8dc [dev.typeparams] cmd/compile/internal/types2: consistently report nil type as "untyped nil"
+ 2021-01-14 82c3f0a358 [dev.typeparams] cmd/compile/internal/types2: untyped shift counts must fit into uint
+ 2021-01-14 67bf62d939 [dev.typeparams] cmd/compile/internal/types2: better error message for invalid ... use
+ 2021-01-14 ef5285fbd0 [dev.typeparams] cmd/compile: add types2-based noder
+ 2021-01-12 f065ff221b [dev.typeparams] all: merge dev.regabi (d9acf6f) into dev.typeparams
+ 2021-01-12 099599662d [dev.typeparams] cmd/compile: refactor import logic
+ 2021-01-12 106aa941df [dev.typeparams] cmd/compile: refactor DWARF scope marking
+ 2021-01-12 6a56c6c870 [dev.typeparams] go/types: import dev.go2go changes to check tests
+ 2021-01-11 9e746e4255 [dev.typeparams] cmd/compile: refactor varEmbed logic
+ 2021-01-11 3e1a87ac2a [dev.typeparams] cmd/compile: extract posMap from noder
+ 2021-01-11 2e8f29b79d [dev.typeparams] cmd/compile: add types2.Sizes implementation
+ 2021-01-11 44d1a8523a [dev.typeparams] cmd/compile/internal/types2: fixes for all.bash
+ 2021-01-11 8123bc90b8 [dev.typeparams] cmd/go: relax test expectation
+ 2021-01-11 8c5aa42c79 [dev.typeparams] cmd/compile: calculate variable sizes in walk
+ 2021-01-11 1ce0854157 [dev.typeparams] import stmt changes from dev.go2go
+ 2021-01-11 eb53a6c7cf [dev.typeparams] import operand.go changes from dev.go2go
+ 2021-01-11 81cd99858d [dev.typeparams] go/types: import expr changes from dev.go2go
+ 2021-01-08 822aeacd9e [dev.typeparams] cmd/compile/internal/syntax: remove ShortString, use String instead
+ 2021-01-08 d017a1b649 [dev.typeparams] cmd/compile/internal/syntax: add Walk node vistor from types2
+ 2021-01-08 7903214fcc [dev.typeparams] cmd/compile/internal/syntax: add ShortString tests
+ 2021-01-08 0aede1205b [dev.typeparams] cmd/compile/internal/types2: use syntax printer to print expressions
+ 2021-01-08 934f9dc0ef [dev.typeparams] cmd/compile/internal/syntax: clean up node printing API
+ 2021-01-07 5b9152de57 [dev.typeparams] all: merge dev.regabi (cb05a0a) into dev.typeparams
+ 2021-01-07 7e689f86e3 [dev.typeparams] go/types: move use and useLHS to match dev.go2go
+ 2021-01-06 d76cefed1f [dev.typeparams] go/types: remove disabled code related to type lists
+ 2021-01-06 0e286579c5 [dev.typeparams] go/types: import typexpr.go from dev.go2go
+ 2021-01-06 9546596d77 [dev.typeparams] cmd/compile/internal/types2: remove disabled code related to type lists
+ 2021-01-06 196102d046 [dev.typeparams] cmd/compile/internal/types2: review of typexpr.go
+ 2021-01-05 a8fe098a12 [dev.typeparams] all: merge dev.regabi (77365c5) into dev.typeparams
+ 2020-12-28 a800acaae1 [dev.typeparams] all: merge dev.regabi (07569da) into dev.typeparams
+ 2020-12-27 dd40bbc57b [dev.typeparams] cmd/compile: re-enable internal/types2 test
+ 2020-12-23 d99dd17827 [dev.typeparams] all: merge dev.regabi (37f138d) into dev.typeparams
+ 2020-12-22 8b1fbd8215 [dev.typeparams] all: merge dev.regabi (6d03cde) into dev.typeparams
+ 2020-12-22 23922cf83b [dev.typeparams] all: merge dev.regabi (d1d1099) into dev.typeparams
+ 2020-12-23 788dad53c5 [dev.typeparams] cmd/compile/internal/types2: disable external test temporarily
+ 2020-12-22 91cc51e005 [dev.typeparams] all: merge dev.regabi (ec741b0) into dev.typeparams
+ 2020-12-22 e02a007ffd [dev.typeparams] codereview.cfg: add config for dev.typeparams
+ 2020-12-22 41e7901ca4 [dev.typeparams] cmd/compile/internal/types2: report error for invalid main function signature
+ 2020-12-22 53c4c17b09 [dev.typeparams] all: merge dev.regabi into dev.typeparams
+ 2020-12-17 060cdbc7b5 [dev.typeparams] go/types: import object resolution from dev.go2go
+ 2020-12-17 c4f0da5750 [dev.typeparams] cmd/compile/internal/types2: remove code for implicit type arguments
+ 2020-12-16 068dd0470b [dev.typeparams] cmd/compile/internal/syntax: don't panic when providing -verify
+ 2020-12-16 7909d6ec28 [dev.typeparams] cmd/compile/internal/syntax: type parameters must always be named
+ 2020-12-16 3b5918c757 [dev.typeparams] cmd/compile/internal/types2: review of predicates.go
+ 2020-12-16 09abd23d9e [dev.typeparams] go/types: import predicates.go from dev.go2go
+ 2020-12-16 f38da2cbb6 [dev.typeparams] cmd/compile/internal/types2: review of unify.go
+ 2020-12-16 ceb77db24f [dev.typeparams] go/types: import some support functions from dev.go2go
+ 2020-12-15 a4d4c10340 [dev.typeparams] go/types: import lookup logic from dev.go2go
+ 2020-12-15 1306435103 [dev.typeparams] go/types: import changes to types.Info from dev.go2go
+ 2020-12-15 6b18081d01 [dev.typeparams] cmd/compile/internal/types2: don't crash if import path is missing
+ 2020-12-15 14e4267c34 [dev.typeparams] cmd/compile/internal/types2: report error for invalid (but empty) expr switch
+ 2020-12-15 96999296e6 [dev.typeparams] go/types: import unify.go and infer.go from dev.go2go
+ 2020-12-15 5aff757efc [dev.typeparams] cmd/compile/internal/gc: provide types2 selection info to noder
+ 2020-12-14 c8e73489c3 go/types: import instance.expand and subst.go from dev.go2go
+ 2020-12-14 3a912f279f [dev.typeparams] cmd/compile/internal/syntax: export NewName and use it
+ 2020-12-14 8ec9e89000 [dev.typeparams] cmd/compile/internal/types2: fix stray ')' in error
+ 2020-12-14 8fe8e29c9f [dev.typeparams] cmd/compile/internal/types2: report error for invalid type expression
+ 2020-12-14 f8930a2413 [dev.typeparams] cmd/compile/internal/types2: report invalid ... in conversions
+ 2020-12-14 5aca6e7857 [dev.typeparams] test: finish triaging all outstanding failing tests
+ 2020-12-14 df58f3368e [dev.typeparams] cmd/compile/internal/types2: don't report two errors for bad strings
+ 2020-12-14 91803a2df3 [dev.typeparams] merge: merge branch 'dev.regabi' into 'dev.typeparams'
+ 2020-12-11 a20021227e [dev.typeparams] cmd/compile/internal/types2: bring over subst.go changes from go/types
+ 2020-12-10 ddf44904f1 [dev.typeparams] test: exclude 32bit-specific test that fails on 32bit platforms (fix build)
+ 2020-12-10 dbce27d29c [dev.typeparams] cmd/compile/internal/types2: report correct line number for missing key
+ 2020-12-09 edf80c4209 [dev.typeparams] cmd/compile/internal/types2: adjusted more error messages for compiler
+ 2020-12-09 43c7b214db [dev.typeparams] cmd/compile/internal/types2: adjusted qualified identifier error message for compiler
+ 2020-12-09 6812eae2e2 [dev.typeparams] cmd/compile/internal/types2: adjust init cycle error message for compiler
+ 2020-12-09 810957b155 [dev.typeparams] cmd/compile/internal/types2: adjusted array error message for compiler
+ 2020-12-09 c32566c336 [dev.typeparams] cmd/compile/internal/types2: avoid endless recursion in Comparable predicate
+ 2020-12-08 6015c4e543 [dev.typeparams] go/*: add TODOs from CLs importing dev.go2go changes
+ 2020-12-08 b6e678573d [dev.typeparams] go/types: import universe changes from dev.go2go
+ 2020-12-08 14dc2d2345 [dev.typeparams] go/types: import the Type API from dev.go2go
+ 2020-12-07 cd15a48036 [dev.typeparams] cmd/compile/internal/types2: correct error position for inherited const init expression
+ 2020-12-04 02820d61a9 [dev.typeparams] test: enable some more errorcheck tests
+ 2020-12-03 9ff27e9fad [dev.typeparams] test: run all errorcheck tests that pass compiler with -G flag
+ 2020-12-03 7a1aa7dfaf [dev.typeparams] test: adjust more test cases to match compiler -G output
+ 2020-12-03 07d32c8183 [dev.typeparams] cmd/compile/internal/types: adjust some error messages to match the compiler
+ 2020-12-03 6b4da14dd3 [dev.typeparams] cmd/compile: provide scaffolding to get types2 types during noding
+ 2020-12-02 72cc2353f0 [dev.typeparams] go/printer: adapt changes from dev.go2go
+ 2020-12-02 ab18125567 [dev.typeparams] cmd/compile/internal/types2: no "declared but not used" errors for invalid var decls
+ 2020-12-02 036245862a [dev.typeparams] cmd/compile/internal/types2: set compiler error message for undeclared variable
+ 2020-12-01 bdc4ffe9a8 [dev.typeparams] cmd/compile/internal/types2: add Config.IgnoreBranches flag
+ 2020-12-01 72ad2f44ea [dev.typeparams] test: add scaffolding to run.go to check compiler with -G flag
+ 2020-12-01 87bc85a846 [dev.typeparams] cmd/compile/internal/types2: adjustments toward matching compiler error messages
+ 2020-12-01 fbc4c6a3ae [dev.typeparams] cmd/compile/internal/types2: remove support for type parameter pointer designation
+ 2020-11-25 ba9c35aa12 [dev.typeparams] merge dev.regabi 40f5bc4d55 into dev.typeparams
+ 2020-11-25 5c2e14872c [dev.typeparams] merge dev.regabi 41f3af9d04 into dev.typeparams
+ 2020-11-25 2c25cd5ba7 [dev.typeparams] cmd/compile/internal/types2: a type parameter is a valid type case in a type switch
+ 2020-11-24 0fb733b7f7 [dev.typeparams] go/parser: support the ParseTypeParams mode
+ 2020-11-24 b56762129e [dev.typeparams] import go2go changes to parse type parameters
+ 2020-11-24 a324aebb7d [dev.typeparams] go/types, cmd/compile/internal/types2: fix incorrect string(int) conversion (regression)
+ 2020-11-20 8fbdacf64c [dev.typeparams] cmd/compile/internal/types2: report constant overflow in binary ops
+ 2020-11-19 b1ae0a0646 [dev.typeparams] Merge branch 'master' into dev.typeparams
+ 2020-11-19 0123c9b321 [dev.typeparams] cmd/compile/internal/types2: report an error for invalid constant values
+ 2020-11-18 2140049172 [dev.typeparams] cmd/compile/internal/types2: port of https://golang.org/cl/270957
+ 2020-11-11 6877ee1e07 [dev.typeparams] cmd/compile: use existing findpkg algorithm when importing through types2
+ 2020-11-03 165ceb09f9 [dev.typeparams] merge master into dev.typeparams
+ 2020-10-28 41ff51ae00 [dev.typeparams] cmd/compile/internal/types2: review of scopes.go
+ 2020-10-27 9392b82919 [dev.typeparams] cmd/compile/internal/types: review of objset.go
+ 2020-10-27 c32ac6c15f [dev.typeparams] cmd/compile/internal/types: review of selection.go
+ 2020-10-27 ff6ab114c9 [dev.typeparams] cmd/compile/internal/types: review of gccgosizes.go
+ 2020-10-27 abb31c2558 [dev.typeparams] cmd/compile/internal/types2: review of hilbert_test.go
+ 2020-10-27 6e98406ac3 [dev.typeparams] cmd/compile/internal/types2: review of initorder.go
+ 2020-10-27 38af45b4cb [dev.typeparams] cmd/compile/internal/types2: review of package.go
+ 2020-10-27 87eab74628 [dev.typeparams] cmd/compile: enable type-checking of generic code
+ 2020-10-23 5bfd2964a6 [dev.typeparams] merge master into dev.typeparams
+ 2020-10-22 a10fe9f6e7 go/ast: import AST changes supporting typeparams from dev.go2go
+ 2020-10-21 7a8a720c80 [dev.typeparams] cmd/compile/internal/types2: add *.go2 (generic) tests
+ 2020-10-21 755d6de1d8 [dev.typeparams] cmd/compile/internal/types2: add some more tests
+ 2020-10-21 1d07306a44 [dev.typeparams] cmd/compile/internal/types2: adjust tests, enable Testdata tests
+ 2020-10-21 befc62a2c4 [dev.typeparams] cmd/compile/internal/types2: add testdata directory
+ 2020-10-21 ca36ba83ab [dev.typeparams] cmd/compile/internal/importer, types2: initial check-in of types2 and importer
+ 2020-10-20 6ff16fe3ee [dev.typeparams] cmd/compile/internal/syntax: add utility functions for testing
+ 2020-10-19 5e46c6a10f [dev.typeparams] cmd/compile/internal/syntax: add Pos method
+ 2020-10-15 e9e58a4d49 [dev.typeparams] cmd/compile/internal/syntax: fix printing of channel types
+ 2020-10-14 73f529845c [dev.typeparams] cmd/compile/internal/syntax: always use IndexExpr node for type instantiation
+ 2020-10-13 48755e06aa [dev.typeparams] cmd/compile: enable parsing of generic code with new -G flag
+ 2020-10-13 7668f02dec [dev.typeparams] cmd/compile/internal/syntax: add type parameter tests
+ 2020-10-13 b627988b0c [dev.typeparams] cmd/compile/internal/syntax: implement parsing of type parameters
+ 2020-10-13 617b633917 [dev.typeparams] cmd/compile/internal/syntax: prepare syntax nodes for type parameters
+ 2020-10-12 986cad14e2 [dev.typeparams] merge master into dev.typeparams

Change-Id: Ie0796a53af1fb5db49f4c748144461d3af598575

src/cmd/compile/internal/base/flag.go

@@ -54,6 +54,7 @@ type CmdFlags struct {
 	C CountFlag    "help:\"disable printing of columns in error messages\""
 	D string       "help:\"set relative `path` for local imports\""
 	E CountFlag    "help:\"debug symbol export\""
+	G CountFlag    "help:\"accept generic code\""
 	I func(string) "help:\"add `directory` to import search path\""
 	K CountFlag    "help:\"debug missing line numbers\""
 	L CountFlag    "help:\"show full file names in error messages\""

src/cmd/compile/internal/gc/export.go

@@ -25,6 +25,11 @@ func exportf(bout *bio.Writer, format string, args ...interface{}) {
 func dumpexport(bout *bio.Writer) {
 	p := &exporter{marked: make(map[*types.Type]bool)}
 	for _, n := range typecheck.Target.Exports {
+		// Must catch it here rather than Export(), because the type can be
+		// not fully set (still TFORW) when Export() is called.
+		if n.Type() != nil && n.Type().HasTParam() {
+			base.Fatalf("Cannot (yet) export a generic type: %v", n)
+		}
 		p.markObject(n)
 	}
 

src/cmd/compile/internal/importer/exportdata.go

@@ -0,0 +1,92 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements FindExportData.
+
+package importer
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+)
+
+func readGopackHeader(r *bufio.Reader) (name string, size int, err error) {
+	// See $GOROOT/include/ar.h.
+	hdr := make([]byte, 16+12+6+6+8+10+2)
+	_, err = io.ReadFull(r, hdr)
+	if err != nil {
+		return
+	}
+	// leave for debugging
+	if false {
+		fmt.Printf("header: %s", hdr)
+	}
+	s := strings.TrimSpace(string(hdr[16+12+6+6+8:][:10]))
+	size, err = strconv.Atoi(s)
+	if err != nil || hdr[len(hdr)-2] != '`' || hdr[len(hdr)-1] != '\n' {
+		err = fmt.Errorf("invalid archive header")
+		return
+	}
+	name = strings.TrimSpace(string(hdr[:16]))
+	return
+}
+
+// FindExportData positions the reader r at the beginning of the
+// export data section of an underlying GC-created object/archive
+// file by reading from it. The reader must be positioned at the
+// start of the file before calling this function. The hdr result
+// is the string before the export data, either "$$" or "$$B".
+//
+func FindExportData(r *bufio.Reader) (hdr string, err error) {
+	// Read first line to make sure this is an object file.
+	line, err := r.ReadSlice('\n')
+	if err != nil {
+		err = fmt.Errorf("can't find export data (%v)", err)
+		return
+	}
+
+	if string(line) == "!<arch>\n" {
+		// Archive file. Scan to __.PKGDEF.
+		var name string
+		if name, _, err = readGopackHeader(r); err != nil {
+			return
+		}
+
+		// First entry should be __.PKGDEF.
+		if name != "__.PKGDEF" {
+			err = fmt.Errorf("go archive is missing __.PKGDEF")
+			return
+		}
+
+		// Read first line of __.PKGDEF data, so that line
+		// is once again the first line of the input.
+		if line, err = r.ReadSlice('\n'); err != nil {
+			err = fmt.Errorf("can't find export data (%v)", err)
+			return
+		}
+	}
+
+	// Now at __.PKGDEF in archive or still at beginning of file.
+	// Either way, line should begin with "go object ".
+	if !strings.HasPrefix(string(line), "go object ") {
+		err = fmt.Errorf("not a Go object file")
+		return
+	}
+
+	// Skip over object header to export data.
+	// Begins after first line starting with $$.
+	for line[0] != '$' {
+		if line, err = r.ReadSlice('\n'); err != nil {
+			err = fmt.Errorf("can't find export data (%v)", err)
+			return
+		}
+	}
+	hdr = string(line)
+
+	return
+}

src/cmd/compile/internal/importer/gcimporter.go

@@ -0,0 +1,175 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// package importer implements Import for gc-generated object files.
+package importer
+
+import (
+	"bufio"
+	"cmd/compile/internal/types2"
+	"fmt"
+	"go/build"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// debugging/development support
+const debug = false
+
+var pkgExts = [...]string{".a", ".o"}
+
+// FindPkg returns the filename and unique package id for an import
+// path based on package information provided by build.Import (using
+// the build.Default build.Context). A relative srcDir is interpreted
+// relative to the current working directory.
+// If no file was found, an empty filename is returned.
+//
+func FindPkg(path, srcDir string) (filename, id string) {
+	if path == "" {
+		return
+	}
+
+	var noext string
+	switch {
+	default:
+		// "x" -> "$GOPATH/pkg/$GOOS_$GOARCH/x.ext", "x"
+		// Don't require the source files to be present.
+		if abs, err := filepath.Abs(srcDir); err == nil { // see issue 14282
+			srcDir = abs
+		}
+		bp, _ := build.Import(path, srcDir, build.FindOnly|build.AllowBinary)
+		if bp.PkgObj == "" {
+			id = path // make sure we have an id to print in error message
+			return
+		}
+		noext = strings.TrimSuffix(bp.PkgObj, ".a")
+		id = bp.ImportPath
+
+	case build.IsLocalImport(path):
+		// "./x" -> "/this/directory/x.ext", "/this/directory/x"
+		noext = filepath.Join(srcDir, path)
+		id = noext
+
+	case filepath.IsAbs(path):
+		// for completeness only - go/build.Import
+		// does not support absolute imports
+		// "/x" -> "/x.ext", "/x"
+		noext = path
+		id = path
+	}
+
+	if false { // for debugging
+		if path != id {
+			fmt.Printf("%s -> %s\n", path, id)
+		}
+	}
+
+	// try extensions
+	for _, ext := range pkgExts {
+		filename = noext + ext
+		if f, err := os.Stat(filename); err == nil && !f.IsDir() {
+			return
+		}
+	}
+
+	filename = "" // not found
+	return
+}
+
+// Import imports a gc-generated package given its import path and srcDir, adds
+// the corresponding package object to the packages map, and returns the object.
+// The packages map must contain all packages already imported.
+//
+func Import(packages map[string]*types2.Package, path, srcDir string, lookup func(path string) (io.ReadCloser, error)) (pkg *types2.Package, err error) {
+	var rc io.ReadCloser
+	var id string
+	if lookup != nil {
+		// With custom lookup specified, assume that caller has
+		// converted path to a canonical import path for use in the map.
+		if path == "unsafe" {
+			return types2.Unsafe, nil
+		}
+		id = path
+
+		// No need to re-import if the package was imported completely before.
+		if pkg = packages[id]; pkg != nil && pkg.Complete() {
+			return
+		}
+		f, err := lookup(path)
+		if err != nil {
+			return nil, err
+		}
+		rc = f
+	} else {
+		var filename string
+		filename, id = FindPkg(path, srcDir)
+		if filename == "" {
+			if path == "unsafe" {
+				return types2.Unsafe, nil
+			}
+			return nil, fmt.Errorf("can't find import: %q", id)
+		}
+
+		// no need to re-import if the package was imported completely before
+		if pkg = packages[id]; pkg != nil && pkg.Complete() {
+			return
+		}
+
+		// open file
+		f, err := os.Open(filename)
+		if err != nil {
+			return nil, err
+		}
+		defer func() {
+			if err != nil {
+				// add file name to error
+				err = fmt.Errorf("%s: %v", filename, err)
+			}
+		}()
+		rc = f
+	}
+	defer rc.Close()
+
+	var hdr string
+	buf := bufio.NewReader(rc)
+	if hdr, err = FindExportData(buf); err != nil {
+		return
+	}
+
+	switch hdr {
+	case "$$\n":
+		err = fmt.Errorf("import %q: old textual export format no longer supported (recompile library)", path)
+
+	case "$$B\n":
+		var data []byte
+		data, err = ioutil.ReadAll(buf)
+		if err != nil {
+			break
+		}
+
+		// The indexed export format starts with an 'i'; the older
+		// binary export format starts with a 'c', 'd', or 'v'
+		// (from "version"). Select appropriate importer.
+		if len(data) > 0 && data[0] == 'i' {
+			_, pkg, err = iImportData(packages, data[1:], id)
+		} else {
+			err = fmt.Errorf("import %q: old binary export format no longer supported (recompile library)", path)
+		}
+
+	default:
+		err = fmt.Errorf("import %q: unknown export data header: %q", path, hdr)
+	}
+
+	return
+}
+
+type byPath []*types2.Package
+
+func (a byPath) Len() int           { return len(a) }
+func (a byPath) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a byPath) Less(i, j int) bool { return a[i].Path() < a[j].Path() }

src/cmd/compile/internal/importer/gcimporter_test.go

@@ -0,0 +1,611 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importer
+
+import (
+	"bytes"
+	"cmd/compile/internal/types2"
+	"fmt"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+// skipSpecialPlatforms causes the test to be skipped for platforms where
+// builders (build.golang.org) don't have access to compiled packages for
+// import.
+func skipSpecialPlatforms(t *testing.T) {
+	switch platform := runtime.GOOS + "-" + runtime.GOARCH; platform {
+	case "darwin-arm64":
+		t.Skipf("no compiled packages available for import on %s", platform)
+	}
+}
+
+// compile runs the compiler on filename, with dirname as the working directory,
+// and writes the output file to outdirname.
+func compile(t *testing.T, dirname, filename, outdirname string) string {
+	// filename must end with ".go"
+	if !strings.HasSuffix(filename, ".go") {
+		t.Fatalf("filename doesn't end in .go: %s", filename)
+	}
+	basename := filepath.Base(filename)
+	outname := filepath.Join(outdirname, basename[:len(basename)-2]+"o")
+	cmd := exec.Command(testenv.GoToolPath(t), "tool", "compile", "-o", outname, filename)
+	cmd.Dir = dirname
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Logf("%s", out)
+		t.Fatalf("go tool compile %s failed: %s", filename, err)
+	}
+	return outname
+}
+
+func testPath(t *testing.T, path, srcDir string) *types2.Package {
+	t0 := time.Now()
+	pkg, err := Import(make(map[string]*types2.Package), path, srcDir, nil)
+	if err != nil {
+		t.Errorf("testPath(%s): %s", path, err)
+		return nil
+	}
+	t.Logf("testPath(%s): %v", path, time.Since(t0))
+	return pkg
+}
+
+const maxTime = 30 * time.Second
+
+func testDir(t *testing.T, dir string, endTime time.Time) (nimports int) {
+	dirname := filepath.Join(runtime.GOROOT(), "pkg", runtime.GOOS+"_"+runtime.GOARCH, dir)
+	list, err := ioutil.ReadDir(dirname)
+	if err != nil {
+		t.Fatalf("testDir(%s): %s", dirname, err)
+	}
+	for _, f := range list {
+		if time.Now().After(endTime) {
+			t.Log("testing time used up")
+			return
+		}
+		switch {
+		case !f.IsDir():
+			// try extensions
+			for _, ext := range pkgExts {
+				if strings.HasSuffix(f.Name(), ext) {
+					name := f.Name()[0 : len(f.Name())-len(ext)] // remove extension
+					if testPath(t, filepath.Join(dir, name), dir) != nil {
+						nimports++
+					}
+				}
+			}
+		case f.IsDir():
+			nimports += testDir(t, filepath.Join(dir, f.Name()), endTime)
+		}
+	}
+	return
+}
+
+func mktmpdir(t *testing.T) string {
+	tmpdir, err := ioutil.TempDir("", "gcimporter_test")
+	if err != nil {
+		t.Fatal("mktmpdir:", err)
+	}
+	if err := os.Mkdir(filepath.Join(tmpdir, "testdata"), 0700); err != nil {
+		os.RemoveAll(tmpdir)
+		t.Fatal("mktmpdir:", err)
+	}
+	return tmpdir
+}
+
+func TestImportTestdata(t *testing.T) {
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+
+	compile(t, "testdata", "exports.go", filepath.Join(tmpdir, "testdata"))
+
+	if pkg := testPath(t, "./testdata/exports", tmpdir); pkg != nil {
+		// The package's Imports list must include all packages
+		// explicitly imported by exports.go, plus all packages
+		// referenced indirectly via exported objects in exports.go.
+		// With the textual export format, the list may also include
+		// additional packages that are not strictly required for
+		// import processing alone (they are exported to err "on
+		// the safe side").
+		// TODO(gri) update the want list to be precise, now that
+		// the textual export data is gone.
+		got := fmt.Sprint(pkg.Imports())
+		for _, want := range []string{"go/ast", "go/token"} {
+			if !strings.Contains(got, want) {
+				t.Errorf(`Package("exports").Imports() = %s, does not contain %s`, got, want)
+			}
+		}
+	}
+}
+
+func TestVersionHandling(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	const dir = "./testdata/versions"
+	list, err := ioutil.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	corruptdir := filepath.Join(tmpdir, "testdata", "versions")
+	if err := os.Mkdir(corruptdir, 0700); err != nil {
+		t.Fatal(err)
+	}
+
+	for _, f := range list {
+		name := f.Name()
+		if !strings.HasSuffix(name, ".a") {
+			continue // not a package file
+		}
+		if strings.Contains(name, "corrupted") {
+			continue // don't process a leftover corrupted file
+		}
+		pkgpath := "./" + name[:len(name)-2]
+
+		if testing.Verbose() {
+			t.Logf("importing %s", name)
+		}
+
+		// test that export data can be imported
+		_, err := Import(make(map[string]*types2.Package), pkgpath, dir, nil)
+		if err != nil {
+			// ok to fail if it fails with a no longer supported error for select files
+			if strings.Contains(err.Error(), "no longer supported") {
+				switch name {
+				case "test_go1.7_0.a", "test_go1.7_1.a",
+					"test_go1.8_4.a", "test_go1.8_5.a",
+					"test_go1.11_6b.a", "test_go1.11_999b.a":
+					continue
+				}
+				// fall through
+			}
+			// ok to fail if it fails with a newer version error for select files
+			if strings.Contains(err.Error(), "newer version") {
+				switch name {
+				case "test_go1.11_999i.a":
+					continue
+				}
+				// fall through
+			}
+			t.Errorf("import %q failed: %v", pkgpath, err)
+			continue
+		}
+
+		// create file with corrupted export data
+		// 1) read file
+		data, err := ioutil.ReadFile(filepath.Join(dir, name))
+		if err != nil {
+			t.Fatal(err)
+		}
+		// 2) find export data
+		i := bytes.Index(data, []byte("\n$$B\n")) + 5
+		j := bytes.Index(data[i:], []byte("\n$$\n")) + i
+		if i < 0 || j < 0 || i > j {
+			t.Fatalf("export data section not found (i = %d, j = %d)", i, j)
+		}
+		// 3) corrupt the data (increment every 7th byte)
+		for k := j - 13; k >= i; k -= 7 {
+			data[k]++
+		}
+		// 4) write the file
+		pkgpath += "_corrupted"
+		filename := filepath.Join(corruptdir, pkgpath) + ".a"
+		ioutil.WriteFile(filename, data, 0666)
+
+		// test that importing the corrupted file results in an error
+		_, err = Import(make(map[string]*types2.Package), pkgpath, corruptdir, nil)
+		if err == nil {
+			t.Errorf("import corrupted %q succeeded", pkgpath)
+		} else if msg := err.Error(); !strings.Contains(msg, "version skew") {
+			t.Errorf("import %q error incorrect (%s)", pkgpath, msg)
+		}
+	}
+}
+
+func TestImportStdLib(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	dt := maxTime
+	if testing.Short() && testenv.Builder() == "" {
+		dt = 10 * time.Millisecond
+	}
+	nimports := testDir(t, "", time.Now().Add(dt)) // installed packages
+	t.Logf("tested %d imports", nimports)
+}
+
+var importedObjectTests = []struct {
+	name string
+	want string
+}{
+	// non-interfaces
+	{"crypto.Hash", "type Hash uint"},
+	{"go/ast.ObjKind", "type ObjKind int"},
+	{"go/types.Qualifier", "type Qualifier func(*Package) string"},
+	{"go/types.Comparable", "func Comparable(T Type) bool"},
+	{"math.Pi", "const Pi untyped float"},
+	{"math.Sin", "func Sin(x float64) float64"},
+	{"go/ast.NotNilFilter", "func NotNilFilter(_ string, v reflect.Value) bool"},
+	{"go/internal/gcimporter.FindPkg", "func FindPkg(path string, srcDir string) (filename string, id string)"},
+
+	// interfaces
+	{"context.Context", "type Context interface{Deadline() (deadline time.Time, ok bool); Done() <-chan struct{}; Err() error; Value(key interface{}) interface{}}"},
+	{"crypto.Decrypter", "type Decrypter interface{Decrypt(rand io.Reader, msg []byte, opts DecrypterOpts) (plaintext []byte, err error); Public() PublicKey}"},
+	{"encoding.BinaryMarshaler", "type BinaryMarshaler interface{MarshalBinary() (data []byte, err error)}"},
+	{"io.Reader", "type Reader interface{Read(p []byte) (n int, err error)}"},
+	{"io.ReadWriter", "type ReadWriter interface{Reader; Writer}"},
+	{"go/ast.Node", "type Node interface{End() go/token.Pos; Pos() go/token.Pos}"},
+	// go/types.Type has grown much larger - excluded for now
+	// {"go/types.Type", "type Type interface{String() string; Underlying() Type}"},
+}
+
+func TestImportedTypes(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	for _, test := range importedObjectTests {
+		s := strings.Split(test.name, ".")
+		if len(s) != 2 {
+			t.Fatal("inconsistent test data")
+		}
+		importPath := s[0]
+		objName := s[1]
+
+		pkg, err := Import(make(map[string]*types2.Package), importPath, ".", nil)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		obj := pkg.Scope().Lookup(objName)
+		if obj == nil {
+			t.Errorf("%s: object not found", test.name)
+			continue
+		}
+
+		got := types2.ObjectString(obj, types2.RelativeTo(pkg))
+		if got != test.want {
+			t.Errorf("%s: got %q; want %q", test.name, got, test.want)
+		}
+
+		if named, _ := obj.Type().(*types2.Named); named != nil {
+			verifyInterfaceMethodRecvs(t, named, 0)
+		}
+	}
+}
+
+// verifyInterfaceMethodRecvs verifies that method receiver types
+// are named if the methods belong to a named interface type.
+func verifyInterfaceMethodRecvs(t *testing.T, named *types2.Named, level int) {
+	// avoid endless recursion in case of an embedding bug that lead to a cycle
+	if level > 10 {
+		t.Errorf("%s: embeds itself", named)
+		return
+	}
+
+	iface, _ := named.Underlying().(*types2.Interface)
+	if iface == nil {
+		return // not an interface
+	}
+
+	// check explicitly declared methods
+	for i := 0; i < iface.NumExplicitMethods(); i++ {
+		m := iface.ExplicitMethod(i)
+		recv := m.Type().(*types2.Signature).Recv()
+		if recv == nil {
+			t.Errorf("%s: missing receiver type", m)
+			continue
+		}
+		if recv.Type() != named {
+			t.Errorf("%s: got recv type %s; want %s", m, recv.Type(), named)
+		}
+	}
+
+	// check embedded interfaces (if they are named, too)
+	for i := 0; i < iface.NumEmbeddeds(); i++ {
+		// embedding of interfaces cannot have cycles; recursion will terminate
+		if etype, _ := iface.EmbeddedType(i).(*types2.Named); etype != nil {
+			verifyInterfaceMethodRecvs(t, etype, level+1)
+		}
+	}
+}
+
+func TestIssue5815(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	pkg := importPkg(t, "strings", ".")
+
+	scope := pkg.Scope()
+	for _, name := range scope.Names() {
+		obj := scope.Lookup(name)
+		if obj.Pkg() == nil {
+			t.Errorf("no pkg for %s", obj)
+		}
+		if tname, _ := obj.(*types2.TypeName); tname != nil {
+			named := tname.Type().(*types2.Named)
+			for i := 0; i < named.NumMethods(); i++ {
+				m := named.Method(i)
+				if m.Pkg() == nil {
+					t.Errorf("no pkg for %s", m)
+				}
+			}
+		}
+	}
+}
+
+// Smoke test to ensure that imported methods get the correct package.
+func TestCorrectMethodPackage(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	imports := make(map[string]*types2.Package)
+	_, err := Import(imports, "net/http", ".", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mutex := imports["sync"].Scope().Lookup("Mutex").(*types2.TypeName).Type()
+	obj, _, _ := types2.LookupFieldOrMethod(types2.NewPointer(mutex), false, nil, "Lock")
+	lock := obj.(*types2.Func)
+	if got, want := lock.Pkg().Path(), "sync"; got != want {
+		t.Errorf("got package path %q; want %q", got, want)
+	}
+}
+
+func TestIssue13566(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	testoutdir := filepath.Join(tmpdir, "testdata")
+
+	// b.go needs to be compiled from the output directory so that the compiler can
+	// find the compiled package a. We pass the full path to compile() so that we
+	// don't have to copy the file to that directory.
+	bpath, err := filepath.Abs(filepath.Join("testdata", "b.go"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	compile(t, "testdata", "a.go", testoutdir)
+	compile(t, testoutdir, bpath, testoutdir)
+
+	// import must succeed (test for issue at hand)
+	pkg := importPkg(t, "./testdata/b", tmpdir)
+
+	// make sure all indirectly imported packages have names
+	for _, imp := range pkg.Imports() {
+		if imp.Name() == "" {
+			t.Errorf("no name for %s package", imp.Path())
+		}
+	}
+}
+
+func TestIssue13898(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// import go/internal/gcimporter which imports go/types partially
+	imports := make(map[string]*types2.Package)
+	_, err := Import(imports, "go/internal/gcimporter", ".", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// look for go/types package
+	var goTypesPkg *types2.Package
+	for path, pkg := range imports {
+		if path == "go/types" {
+			goTypesPkg = pkg
+			break
+		}
+	}
+	if goTypesPkg == nil {
+		t.Fatal("go/types not found")
+	}
+
+	// look for go/types2.Object type
+	obj := lookupObj(t, goTypesPkg.Scope(), "Object")
+	typ, ok := obj.Type().(*types2.Named)
+	if !ok {
+		t.Fatalf("go/types2.Object type is %v; wanted named type", typ)
+	}
+
+	// lookup go/types2.Object.Pkg method
+	m, index, indirect := types2.LookupFieldOrMethod(typ, false, nil, "Pkg")
+	if m == nil {
+		t.Fatalf("go/types2.Object.Pkg not found (index = %v, indirect = %v)", index, indirect)
+	}
+
+	// the method must belong to go/types
+	if m.Pkg().Path() != "go/types" {
+		t.Fatalf("found %v; want go/types", m.Pkg())
+	}
+}
+
+func TestIssue15517(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+
+	compile(t, "testdata", "p.go", filepath.Join(tmpdir, "testdata"))
+
+	// Multiple imports of p must succeed without redeclaration errors.
+	// We use an import path that's not cleaned up so that the eventual
+	// file path for the package is different from the package path; this
+	// will expose the error if it is present.
+	//
+	// (Issue: Both the textual and the binary importer used the file path
+	// of the package to be imported as key into the shared packages map.
+	// However, the binary importer then used the package path to identify
+	// the imported package to mark it as complete; effectively marking the
+	// wrong package as complete. By using an "unclean" package path, the
+	// file and package path are different, exposing the problem if present.
+	// The same issue occurs with vendoring.)
+	imports := make(map[string]*types2.Package)
+	for i := 0; i < 3; i++ {
+		if _, err := Import(imports, "./././testdata/p", tmpdir, nil); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestIssue15920(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue15920")
+}
+
+func TestIssue20046(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	// "./issue20046".V.M must exist
+	pkg := compileAndImportPkg(t, "issue20046")
+	obj := lookupObj(t, pkg.Scope(), "V")
+	if m, index, indirect := types2.LookupFieldOrMethod(obj.Type(), false, nil, "M"); m == nil {
+		t.Fatalf("V.M not found (index = %v, indirect = %v)", index, indirect)
+	}
+}
+func TestIssue25301(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue25301")
+}
+
+func TestIssue25596(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue25596")
+}
+
+func importPkg(t *testing.T, path, srcDir string) *types2.Package {
+	pkg, err := Import(make(map[string]*types2.Package), path, srcDir, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return pkg
+}
+
+func compileAndImportPkg(t *testing.T, name string) *types2.Package {
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	compile(t, "testdata", name+".go", filepath.Join(tmpdir, "testdata"))
+	return importPkg(t, "./testdata/"+name, tmpdir)
+}
+
+func lookupObj(t *testing.T, scope *types2.Scope, name string) types2.Object {
+	if obj := scope.Lookup(name); obj != nil {
+		return obj
+	}
+	t.Fatalf("%s not found", name)
+	return nil
+}

src/cmd/compile/internal/importer/iimport.go

@@ -0,0 +1,612 @@
+// UNREVIEWED
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Indexed package import.
+// See cmd/compile/internal/gc/iexport.go for the export data format.
+
+package importer
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/types2"
+	"encoding/binary"
+	"fmt"
+	"go/constant"
+	"go/token"
+	"io"
+	"math/big"
+	"sort"
+)
+
+type intReader struct {
+	*bytes.Reader
+	path string
+}
+
+func (r *intReader) int64() int64 {
+	i, err := binary.ReadVarint(r.Reader)
+	if err != nil {
+		errorf("import %q: read varint error: %v", r.path, err)
+	}
+	return i
+}
+
+func (r *intReader) uint64() uint64 {
+	i, err := binary.ReadUvarint(r.Reader)
+	if err != nil {
+		errorf("import %q: read varint error: %v", r.path, err)
+	}
+	return i
+}
+
+const predeclReserved = 32
+
+type itag uint64
+
+const (
+	// Types
+	definedType itag = iota
+	pointerType
+	sliceType
+	arrayType
+	chanType
+	mapType
+	signatureType
+	structType
+	interfaceType
+)
+
+const io_SeekCurrent = 1 // io.SeekCurrent (not defined in Go 1.4)
+
+// iImportData imports a package from the serialized package data
+// and returns the number of bytes consumed and a reference to the package.
+// If the export data version is not recognized or the format is otherwise
+// compromised, an error is returned.
+func iImportData(imports map[string]*types2.Package, data []byte, path string) (_ int, pkg *types2.Package, err error) {
+	const currentVersion = 1
+	version := int64(-1)
+	defer func() {
+		if e := recover(); e != nil {
+			if version > currentVersion {
+				err = fmt.Errorf("cannot import %q (%v), export data is newer version - update tool", path, e)
+			} else {
+				err = fmt.Errorf("cannot import %q (%v), possibly version skew - reinstall package", path, e)
+			}
+		}
+	}()
+
+	r := &intReader{bytes.NewReader(data), path}
+
+	version = int64(r.uint64())
+	switch version {
+	case currentVersion, 0:
+	default:
+		errorf("unknown iexport format version %d", version)
+	}
+
+	sLen := int64(r.uint64())
+	dLen := int64(r.uint64())
+
+	whence, _ := r.Seek(0, io_SeekCurrent)
+	stringData := data[whence : whence+sLen]
+	declData := data[whence+sLen : whence+sLen+dLen]
+	r.Seek(sLen+dLen, io_SeekCurrent)
+
+	p := iimporter{
+		ipath:   path,
+		version: int(version),
+
+		stringData:  stringData,
+		stringCache: make(map[uint64]string),
+		pkgCache:    make(map[uint64]*types2.Package),
+
+		declData: declData,
+		pkgIndex: make(map[*types2.Package]map[string]uint64),
+		typCache: make(map[uint64]types2.Type),
+	}
+
+	for i, pt := range predeclared {
+		p.typCache[uint64(i)] = pt
+	}
+
+	pkgList := make([]*types2.Package, r.uint64())
+	for i := range pkgList {
+		pkgPathOff := r.uint64()
+		pkgPath := p.stringAt(pkgPathOff)
+		pkgName := p.stringAt(r.uint64())
+		_ = r.uint64() // package height; unused by go/types
+
+		if pkgPath == "" {
+			pkgPath = path
+		}
+		pkg := imports[pkgPath]
+		if pkg == nil {
+			pkg = types2.NewPackage(pkgPath, pkgName)
+			imports[pkgPath] = pkg
+		} else if pkg.Name() != pkgName {
+			errorf("conflicting names %s and %s for package %q", pkg.Name(), pkgName, path)
+		}
+
+		p.pkgCache[pkgPathOff] = pkg
+
+		nameIndex := make(map[string]uint64)
+		for nSyms := r.uint64(); nSyms > 0; nSyms-- {
+			name := p.stringAt(r.uint64())
+			nameIndex[name] = r.uint64()
+		}
+
+		p.pkgIndex[pkg] = nameIndex
+		pkgList[i] = pkg
+	}
+
+	localpkg := pkgList[0]
+
+	names := make([]string, 0, len(p.pkgIndex[localpkg]))
+	for name := range p.pkgIndex[localpkg] {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		p.doDecl(localpkg, name)
+	}
+
+	for _, typ := range p.interfaceList {
+		typ.Complete()
+	}
+
+	// record all referenced packages as imports
+	list := append(([]*types2.Package)(nil), pkgList[1:]...)
+	sort.Sort(byPath(list))
+	localpkg.SetImports(list)
+
+	// package was imported completely and without errors
+	localpkg.MarkComplete()
+
+	consumed, _ := r.Seek(0, io_SeekCurrent)
+	return int(consumed), localpkg, nil
+}
+
+type iimporter struct {
+	ipath   string
+	version int
+
+	stringData  []byte
+	stringCache map[uint64]string
+	pkgCache    map[uint64]*types2.Package
+
+	declData []byte
+	pkgIndex map[*types2.Package]map[string]uint64
+	typCache map[uint64]types2.Type
+
+	interfaceList []*types2.Interface
+}
+
+func (p *iimporter) doDecl(pkg *types2.Package, name string) {
+	// See if we've already imported this declaration.
+	if obj := pkg.Scope().Lookup(name); obj != nil {
+		return
+	}
+
+	off, ok := p.pkgIndex[pkg][name]
+	if !ok {
+		errorf("%v.%v not in index", pkg, name)
+	}
+
+	r := &importReader{p: p, currPkg: pkg}
+	// Reader.Reset is not available in Go 1.4.
+	// Use bytes.NewReader for now.
+	// r.declReader.Reset(p.declData[off:])
+	r.declReader = *bytes.NewReader(p.declData[off:])
+
+	r.obj(name)
+}
+
+func (p *iimporter) stringAt(off uint64) string {
+	if s, ok := p.stringCache[off]; ok {
+		return s
+	}
+
+	slen, n := binary.Uvarint(p.stringData[off:])
+	if n <= 0 {
+		errorf("varint failed")
+	}
+	spos := off + uint64(n)
+	s := string(p.stringData[spos : spos+slen])
+	p.stringCache[off] = s
+	return s
+}
+
+func (p *iimporter) pkgAt(off uint64) *types2.Package {
+	if pkg, ok := p.pkgCache[off]; ok {
+		return pkg
+	}
+	path := p.stringAt(off)
+	errorf("missing package %q in %q", path, p.ipath)
+	return nil
+}
+
+func (p *iimporter) typAt(off uint64, base *types2.Named) types2.Type {
+	if t, ok := p.typCache[off]; ok && (base == nil || !isInterface(t)) {
+		return t
+	}
+
+	if off < predeclReserved {
+		errorf("predeclared type missing from cache: %v", off)
+	}
+
+	r := &importReader{p: p}
+	// Reader.Reset is not available in Go 1.4.
+	// Use bytes.NewReader for now.
+	// r.declReader.Reset(p.declData[off-predeclReserved:])
+	r.declReader = *bytes.NewReader(p.declData[off-predeclReserved:])
+	t := r.doType(base)
+
+	if base == nil || !isInterface(t) {
+		p.typCache[off] = t
+	}
+	return t
+}
+
+type importReader struct {
+	p          *iimporter
+	declReader bytes.Reader
+	currPkg    *types2.Package
+	prevFile   string
+	prevLine   int64
+	prevColumn int64
+}
+
+func (r *importReader) obj(name string) {
+	tag := r.byte()
+	pos := r.pos()
+
+	switch tag {
+	case 'A':
+		typ := r.typ()
+
+		r.declare(types2.NewTypeName(pos, r.currPkg, name, typ))
+
+	case 'C':
+		typ, val := r.value()
+
+		r.declare(types2.NewConst(pos, r.currPkg, name, typ, val))
+
+	case 'F':
+		sig := r.signature(nil)
+
+		r.declare(types2.NewFunc(pos, r.currPkg, name, sig))
+
+	case 'T':
+		// Types can be recursive. We need to setup a stub
+		// declaration before recursing.
+		obj := types2.NewTypeName(pos, r.currPkg, name, nil)
+		named := types2.NewNamed(obj, nil, nil)
+		r.declare(obj)
+
+		underlying := r.p.typAt(r.uint64(), named).Underlying()
+		named.SetUnderlying(underlying)
+
+		if !isInterface(underlying) {
+			for n := r.uint64(); n > 0; n-- {
+				mpos := r.pos()
+				mname := r.ident()
+				recv := r.param()
+				msig := r.signature(recv)
+
+				named.AddMethod(types2.NewFunc(mpos, r.currPkg, mname, msig))
+			}
+		}
+
+	case 'V':
+		typ := r.typ()
+
+		r.declare(types2.NewVar(pos, r.currPkg, name, typ))
+
+	default:
+		errorf("unexpected tag: %v", tag)
+	}
+}
+
+func (r *importReader) declare(obj types2.Object) {
+	obj.Pkg().Scope().Insert(obj)
+}
+
+func (r *importReader) value() (typ types2.Type, val constant.Value) {
+	typ = r.typ()
+
+	switch b := typ.Underlying().(*types2.Basic); b.Info() & types2.IsConstType {
+	case types2.IsBoolean:
+		val = constant.MakeBool(r.bool())
+
+	case types2.IsString:
+		val = constant.MakeString(r.string())
+
+	case types2.IsInteger:
+		var x big.Int
+		r.mpint(&x, b)
+		val = constant.Make(&x)
+
+	case types2.IsFloat:
+		val = r.mpfloat(b)
+
+	case types2.IsComplex:
+		re := r.mpfloat(b)
+		im := r.mpfloat(b)
+		val = constant.BinaryOp(re, token.ADD, constant.MakeImag(im))
+
+	default:
+		errorf("unexpected type %v", typ) // panics
+		panic("unreachable")
+	}
+
+	return
+}
+
+func intSize(b *types2.Basic) (signed bool, maxBytes uint) {
+	if (b.Info() & types2.IsUntyped) != 0 {
+		return true, 64
+	}
+
+	switch b.Kind() {
+	case types2.Float32, types2.Complex64:
+		return true, 3
+	case types2.Float64, types2.Complex128:
+		return true, 7
+	}
+
+	signed = (b.Info() & types2.IsUnsigned) == 0
+	switch b.Kind() {
+	case types2.Int8, types2.Uint8:
+		maxBytes = 1
+	case types2.Int16, types2.Uint16:
+		maxBytes = 2
+	case types2.Int32, types2.Uint32:
+		maxBytes = 4
+	default:
+		maxBytes = 8
+	}
+
+	return
+}
+
+func (r *importReader) mpint(x *big.Int, typ *types2.Basic) {
+	signed, maxBytes := intSize(typ)
+
+	maxSmall := 256 - maxBytes
+	if signed {
+		maxSmall = 256 - 2*maxBytes
+	}
+	if maxBytes == 1 {
+		maxSmall = 256
+	}
+
+	n, _ := r.declReader.ReadByte()
+	if uint(n) < maxSmall {
+		v := int64(n)
+		if signed {
+			v >>= 1
+			if n&1 != 0 {
+				v = ^v
+			}
+		}
+		x.SetInt64(v)
+		return
+	}
+
+	v := -n
+	if signed {
+		v = -(n &^ 1) >> 1
+	}
+	if v < 1 || uint(v) > maxBytes {
+		errorf("weird decoding: %v, %v => %v", n, signed, v)
+	}
+	b := make([]byte, v)
+	io.ReadFull(&r.declReader, b)
+	x.SetBytes(b)
+	if signed && n&1 != 0 {
+		x.Neg(x)
+	}
+}
+
+func (r *importReader) mpfloat(typ *types2.Basic) constant.Value {
+	var mant big.Int
+	r.mpint(&mant, typ)
+	var f big.Float
+	f.SetInt(&mant)
+	if f.Sign() != 0 {
+		f.SetMantExp(&f, int(r.int64()))
+	}
+	return constant.Make(&f)
+}
+
+func (r *importReader) ident() string {
+	return r.string()
+}
+
+func (r *importReader) qualifiedIdent() (*types2.Package, string) {
+	name := r.string()
+	pkg := r.pkg()
+	return pkg, name
+}
+
+func (r *importReader) pos() syntax.Pos {
+	if r.p.version >= 1 {
+		r.posv1()
+	} else {
+		r.posv0()
+	}
+
+	if r.prevFile == "" && r.prevLine == 0 && r.prevColumn == 0 {
+		return syntax.Pos{}
+	}
+	// TODO(gri) fix this
+	// return r.p.fake.pos(r.prevFile, int(r.prevLine), int(r.prevColumn))
+	return syntax.Pos{}
+}
+
+func (r *importReader) posv0() {
+	delta := r.int64()
+	if delta != deltaNewFile {
+		r.prevLine += delta
+	} else if l := r.int64(); l == -1 {
+		r.prevLine += deltaNewFile
+	} else {
+		r.prevFile = r.string()
+		r.prevLine = l
+	}
+}
+
+func (r *importReader) posv1() {
+	delta := r.int64()
+	r.prevColumn += delta >> 1
+	if delta&1 != 0 {
+		delta = r.int64()
+		r.prevLine += delta >> 1
+		if delta&1 != 0 {
+			r.prevFile = r.string()
+		}
+	}
+}
+
+func (r *importReader) typ() types2.Type {
+	return r.p.typAt(r.uint64(), nil)
+}
+
+func isInterface(t types2.Type) bool {
+	_, ok := t.(*types2.Interface)
+	return ok
+}
+
+func (r *importReader) pkg() *types2.Package { return r.p.pkgAt(r.uint64()) }
+func (r *importReader) string() string       { return r.p.stringAt(r.uint64()) }
+
+func (r *importReader) doType(base *types2.Named) types2.Type {
+	switch k := r.kind(); k {
+	default:
+		errorf("unexpected kind tag in %q: %v", r.p.ipath, k)
+		return nil
+
+	case definedType:
+		pkg, name := r.qualifiedIdent()
+		r.p.doDecl(pkg, name)
+		return pkg.Scope().Lookup(name).(*types2.TypeName).Type()
+	case pointerType:
+		return types2.NewPointer(r.typ())
+	case sliceType:
+		return types2.NewSlice(r.typ())
+	case arrayType:
+		n := r.uint64()
+		return types2.NewArray(r.typ(), int64(n))
+	case chanType:
+		dir := chanDir(int(r.uint64()))
+		return types2.NewChan(dir, r.typ())
+	case mapType:
+		return types2.NewMap(r.typ(), r.typ())
+	case signatureType:
+		r.currPkg = r.pkg()
+		return r.signature(nil)
+
+	case structType:
+		r.currPkg = r.pkg()
+
+		fields := make([]*types2.Var, r.uint64())
+		tags := make([]string, len(fields))
+		for i := range fields {
+			fpos := r.pos()
+			fname := r.ident()
+			ftyp := r.typ()
+			emb := r.bool()
+			tag := r.string()
+
+			fields[i] = types2.NewField(fpos, r.currPkg, fname, ftyp, emb)
+			tags[i] = tag
+		}
+		return types2.NewStruct(fields, tags)
+
+	case interfaceType:
+		r.currPkg = r.pkg()
+
+		embeddeds := make([]types2.Type, r.uint64())
+		for i := range embeddeds {
+			_ = r.pos()
+			embeddeds[i] = r.typ()
+		}
+
+		methods := make([]*types2.Func, r.uint64())
+		for i := range methods {
+			mpos := r.pos()
+			mname := r.ident()
+
+			// TODO(mdempsky): Matches bimport.go, but I
+			// don't agree with this.
+			var recv *types2.Var
+			if base != nil {
+				recv = types2.NewVar(syntax.Pos{}, r.currPkg, "", base)
+			}
+
+			msig := r.signature(recv)
+			methods[i] = types2.NewFunc(mpos, r.currPkg, mname, msig)
+		}
+
+		typ := types2.NewInterfaceType(methods, embeddeds)
+		r.p.interfaceList = append(r.p.interfaceList, typ)
+		return typ
+	}
+}
+
+func (r *importReader) kind() itag {
+	return itag(r.uint64())
+}
+
+func (r *importReader) signature(recv *types2.Var) *types2.Signature {
+	params := r.paramList()
+	results := r.paramList()
+	variadic := params.Len() > 0 && r.bool()
+	return types2.NewSignature(recv, params, results, variadic)
+}
+
+func (r *importReader) paramList() *types2.Tuple {
+	xs := make([]*types2.Var, r.uint64())
+	for i := range xs {
+		xs[i] = r.param()
+	}
+	return types2.NewTuple(xs...)
+}
+
+func (r *importReader) param() *types2.Var {
+	pos := r.pos()
+	name := r.ident()
+	typ := r.typ()
+	return types2.NewParam(pos, r.currPkg, name, typ)
+}
+
+func (r *importReader) bool() bool {
+	return r.uint64() != 0
+}
+
+func (r *importReader) int64() int64 {
+	n, err := binary.ReadVarint(&r.declReader)
+	if err != nil {
+		errorf("readVarint: %v", err)
+	}
+	return n
+}
+
+func (r *importReader) uint64() uint64 {
+	n, err := binary.ReadUvarint(&r.declReader)
+	if err != nil {
+		errorf("readUvarint: %v", err)
+	}
+	return n
+}
+
+func (r *importReader) byte() byte {
+	x, err := r.declReader.ReadByte()
+	if err != nil {
+		errorf("declReader.ReadByte: %v", err)
+	}
+	return x
+}

src/cmd/compile/internal/importer/support.go

@@ -0,0 +1,128 @@
+// UNREVIEWED
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements support functionality for iimport.go.
+
+package importer
+
+import (
+	"cmd/compile/internal/types2"
+	"fmt"
+	"go/token"
+	"sync"
+)
+
+func errorf(format string, args ...interface{}) {
+	panic(fmt.Sprintf(format, args...))
+}
+
+const deltaNewFile = -64 // see cmd/compile/internal/gc/bexport.go
+
+// Synthesize a token.Pos
+type fakeFileSet struct {
+	fset  *token.FileSet
+	files map[string]*token.File
+}
+
+func (s *fakeFileSet) pos(file string, line, column int) token.Pos {
+	// TODO(mdempsky): Make use of column.
+
+	// Since we don't know the set of needed file positions, we
+	// reserve maxlines positions per file.
+	const maxlines = 64 * 1024
+	f := s.files[file]
+	if f == nil {
+		f = s.fset.AddFile(file, -1, maxlines)
+		s.files[file] = f
+		// Allocate the fake linebreak indices on first use.
+		// TODO(adonovan): opt: save ~512KB using a more complex scheme?
+		fakeLinesOnce.Do(func() {
+			fakeLines = make([]int, maxlines)
+			for i := range fakeLines {
+				fakeLines[i] = i
+			}
+		})
+		f.SetLines(fakeLines)
+	}
+
+	if line > maxlines {
+		line = 1
+	}
+
+	// Treat the file as if it contained only newlines
+	// and column=1: use the line number as the offset.
+	return f.Pos(line - 1)
+}
+
+var (
+	fakeLines     []int
+	fakeLinesOnce sync.Once
+)
+
+func chanDir(d int) types2.ChanDir {
+	// tag values must match the constants in cmd/compile/internal/gc/go.go
+	switch d {
+	case 1 /* Crecv */ :
+		return types2.RecvOnly
+	case 2 /* Csend */ :
+		return types2.SendOnly
+	case 3 /* Cboth */ :
+		return types2.SendRecv
+	default:
+		errorf("unexpected channel dir %d", d)
+		return 0
+	}
+}
+
+var predeclared = []types2.Type{
+	// basic types
+	types2.Typ[types2.Bool],
+	types2.Typ[types2.Int],
+	types2.Typ[types2.Int8],
+	types2.Typ[types2.Int16],
+	types2.Typ[types2.Int32],
+	types2.Typ[types2.Int64],
+	types2.Typ[types2.Uint],
+	types2.Typ[types2.Uint8],
+	types2.Typ[types2.Uint16],
+	types2.Typ[types2.Uint32],
+	types2.Typ[types2.Uint64],
+	types2.Typ[types2.Uintptr],
+	types2.Typ[types2.Float32],
+	types2.Typ[types2.Float64],
+	types2.Typ[types2.Complex64],
+	types2.Typ[types2.Complex128],
+	types2.Typ[types2.String],
+
+	// basic type aliases
+	types2.Universe.Lookup("byte").Type(),
+	types2.Universe.Lookup("rune").Type(),
+
+	// error
+	types2.Universe.Lookup("error").Type(),
+
+	// untyped types
+	types2.Typ[types2.UntypedBool],
+	types2.Typ[types2.UntypedInt],
+	types2.Typ[types2.UntypedRune],
+	types2.Typ[types2.UntypedFloat],
+	types2.Typ[types2.UntypedComplex],
+	types2.Typ[types2.UntypedString],
+	types2.Typ[types2.UntypedNil],
+
+	// package unsafe
+	types2.Typ[types2.UnsafePointer],
+
+	// invalid type
+	types2.Typ[types2.Invalid], // only appears in packages with errors
+
+	// used internally by gc; never used by this package or in .a files
+	anyType{},
+}
+
+type anyType struct{}
+
+func (t anyType) Underlying() types2.Type { return t }
+func (t anyType) String() string          { return "any" }

src/cmd/compile/internal/importer/testdata/a.go

@@ -0,0 +1,15 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue13566
+
+package a
+
+import "encoding/json"
+
+type A struct {
+	a    *A
+	json json.RawMessage
+}

src/cmd/compile/internal/importer/testdata/b.go

@@ -0,0 +1,12 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue13566
+
+package b
+
+import "./a"
+
+type A a.A

src/cmd/compile/internal/importer/testdata/exports.go

@@ -0,0 +1,89 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is used to generate an object file which
+// serves as test file for gcimporter_test.go.
+
+package exports
+
+import "go/ast"
+
+// Issue 3682: Correctly read dotted identifiers from export data.
+const init1 = 0
+
+func init() {}
+
+const (
+	C0 int = 0
+	C1     = 3.14159265
+	C2     = 2.718281828i
+	C3     = -123.456e-789
+	C4     = +123.456e+789
+	C5     = 1234i
+	C6     = "foo\n"
+	C7     = `bar\n`
+)
+
+type (
+	T1  int
+	T2  [10]int
+	T3  []int
+	T4  *int
+	T5  chan int
+	T6a chan<- int
+	T6b chan (<-chan int)
+	T6c chan<- (chan int)
+	T7  <-chan *ast.File
+	T8  struct{}
+	T9  struct {
+		a    int
+		b, c float32
+		d    []string `go:"tag"`
+	}
+	T10 struct {
+		T8
+		T9
+		_ *T10
+	}
+	T11 map[int]string
+	T12 interface{}
+	T13 interface {
+		m1()
+		m2(int) float32
+	}
+	T14 interface {
+		T12
+		T13
+		m3(x ...struct{}) []T9
+	}
+	T15 func()
+	T16 func(int)
+	T17 func(x int)
+	T18 func() float32
+	T19 func() (x float32)
+	T20 func(...interface{})
+	T21 struct{ next *T21 }
+	T22 struct{ link *T23 }
+	T23 struct{ link *T22 }
+	T24 *T24
+	T25 *T26
+	T26 *T27
+	T27 *T25
+	T28 func(T28) T28
+)
+
+var (
+	V0 int
+	V1         = -991.0
+	V2 float32 = 1.2
+)
+
+func F1()         {}
+func F2(x int)    {}
+func F3() int     { return 0 }
+func F4() float32 { return 0 }
+func F5(a, b, c int, u, v, w struct{ x, y T1 }, more ...interface{}) (p, q, r chan<- T10)
+
+func (p *T1) M1()

src/cmd/compile/internal/importer/testdata/issue15920.go

@@ -0,0 +1,12 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// The underlying type of Error is the underlying type of error.
+// Make sure we can import this again without problems.
+type Error error
+
+func F() Error { return nil }

src/cmd/compile/internal/importer/testdata/issue20046.go

@@ -0,0 +1,10 @@
+// UNREVIEWED
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var V interface {
+	M()
+}

src/cmd/compile/internal/importer/testdata/issue25301.go

@@ -0,0 +1,18 @@
+// UNREVIEWED
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue25301
+
+type (
+	A = interface {
+		M()
+	}
+	T interface {
+		A
+	}
+	S struct{}
+)
+
+func (S) M() { println("m") }

src/cmd/compile/internal/importer/testdata/issue25596.go

@@ -0,0 +1,14 @@
+// UNREVIEWED
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue25596
+
+type E interface {
+	M() T
+}
+
+type T interface {
+	E
+}

src/cmd/compile/internal/importer/testdata/p.go

@@ -0,0 +1,14 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue15517
+
+package p
+
+const C = 0
+
+var V int
+
+func F() {}

src/cmd/compile/internal/importer/testdata/versions/test.go

@@ -0,0 +1,29 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To create a test case for a new export format version,
+// build this package with the latest compiler and store
+// the resulting .a file appropriately named in the versions
+// directory. The VersionHandling test will pick it up.
+//
+// In the testdata/versions:
+//
+// go build -o test_go1.$X_$Y.a test.go
+//
+// with $X = Go version and $Y = export format version
+// (add 'b' or 'i' to distinguish between binary and
+// indexed format starting with 1.11 as long as both
+// formats are supported).
+//
+// Make sure this source is extended such that it exercises
+// whatever export format change has taken place.
+
+package test
+
+// Any release before and including Go 1.7 didn't encode
+// the package for a blank struct field.
+type BlankField struct {
+	_ int
+}

src/cmd/compile/internal/inline/inl.go

@@ -354,6 +354,13 @@ func (v *hairyVisitor) doNode(n ir.Node) bool {
 		return true
 
 	case ir.OCLOSURE:
+		// TODO(danscales,mdempsky): Get working with -G.
+		// Probably after #43818 is fixed.
+		if base.Flag.G > 0 {
+			v.reason = "inlining closures not yet working with -G"
+			return true
+		}
+
 		// TODO(danscales) - fix some bugs when budget is lowered below 15
 		// Maybe make budget proportional to number of closure variables, e.g.:
 		//v.budget -= int32(len(n.(*ir.ClosureExpr).Func.ClosureVars) * 3)
@@ -1221,7 +1228,7 @@ func (subst *inlsubst) closure(n *ir.ClosureExpr) ir.Node {
 		newrecv = newrecvs[0]
 	}
 	newt := types.NewSignature(oldt.Pkg(), newrecv,
-		subst.fields(oldt.Params()), subst.fields(oldt.Results()))
+		nil, subst.fields(oldt.Params()), subst.fields(oldt.Results()))
 
 	newfn.Nname.SetType(newt)
 	newfn.Body = subst.list(oldfn.Body)

src/cmd/compile/internal/ir/class_string.go

@@ -14,12 +14,13 @@ func _() {
 	_ = x[PAUTOHEAP-3]
 	_ = x[PPARAM-4]
 	_ = x[PPARAMOUT-5]
-	_ = x[PFUNC-6]
+	_ = x[PTYPEPARAM-6]
+	_ = x[PFUNC-7]
 }
 
-const _Class_name = "PxxxPEXTERNPAUTOPAUTOHEAPPPARAMPPARAMOUTPFUNC"
+const _Class_name = "PxxxPEXTERNPAUTOPAUTOHEAPPPARAMPPARAMOUTPTYPEPARAMPFUNC"
 
-var _Class_index = [...]uint8{0, 4, 11, 16, 25, 31, 40, 45}
+var _Class_index = [...]uint8{0, 4, 11, 16, 25, 31, 40, 50, 55}
 
 func (i Class) String() string {
 	if i >= Class(len(_Class_index)-1) {

src/cmd/compile/internal/ir/expr.go

@@ -670,6 +670,20 @@ func (n *UnaryExpr) SetOp(op Op) {
 	}
 }
 
+// An InstExpr is a generic function or type instantiation.
+type InstExpr struct {
+	miniExpr
+	X     Node
+	Targs []Node
+}
+
+func NewInstExpr(pos src.XPos, op Op, x Node, targs []Node) *InstExpr {
+	n := &InstExpr{X: x, Targs: targs}
+	n.pos = pos
+	n.op = op
+	return n
+}
+
 func IsZero(n Node) bool {
 	switch n.Op() {
 	case ONIL:

src/cmd/compile/internal/ir/name.go

@@ -479,6 +479,7 @@ const (
 	PAUTOHEAP               // local variables or parameters moved to heap
 	PPARAM                  // input arguments
 	PPARAMOUT               // output results
+	PTYPEPARAM              // type params
 	PFUNC                   // global functions
 
 	// Careful: Class is stored in three bits in Node.flags.

src/cmd/compile/internal/ir/node.go

@@ -278,6 +278,8 @@ const (
 	// OTYPESW:  Left := Right.(type) (appears as .Left of OSWITCH)
 	//   Left is nil if there is no type-switch variable
 	OTYPESW
+	OFUNCINST // instantiation of a generic function
+	OTYPEINST // instantiation of a generic type
 
 	// types
 	OTCHAN   // chan int

src/cmd/compile/internal/ir/node_gen.go

@@ -669,6 +669,33 @@ func (n *InlinedCallExpr) editChildren(edit func(Node) Node) {
 	editNodes(n.ReturnVars, edit)
 }
 
+func (n *InstExpr) Format(s fmt.State, verb rune) { fmtNode(n, s, verb) }
+func (n *InstExpr) copy() Node {
+	c := *n
+	c.init = copyNodes(c.init)
+	c.Targs = copyNodes(c.Targs)
+	return &c
+}
+func (n *InstExpr) doChildren(do func(Node) bool) bool {
+	if doNodes(n.init, do) {
+		return true
+	}
+	if n.X != nil && do(n.X) {
+		return true
+	}
+	if doNodes(n.Targs, do) {
+		return true
+	}
+	return false
+}
+func (n *InstExpr) editChildren(edit func(Node) Node) {
+	editNodes(n.init, edit)
+	if n.X != nil {
+		n.X = edit(n.X).(Node)
+	}
+	editNodes(n.Targs, edit)
+}
+
 func (n *InterfaceType) Format(s fmt.State, verb rune) { fmtNode(n, s, verb) }
 func (n *InterfaceType) copy() Node {
 	c := *n

src/cmd/compile/internal/ir/op_string.go

@@ -137,34 +137,36 @@ func _() {
 	_ = x[OSELECT-126]
 	_ = x[OSWITCH-127]
 	_ = x[OTYPESW-128]
-	_ = x[OTCHAN-129]
+	_ = x[OFUNCINST-129]
-	_ = x[OTMAP-130]
+	_ = x[OTYPEINST-130]
-	_ = x[OTSTRUCT-131]
+	_ = x[OTCHAN-131]
-	_ = x[OTINTER-132]
+	_ = x[OTMAP-132]
-	_ = x[OTFUNC-133]
+	_ = x[OTSTRUCT-133]
-	_ = x[OTARRAY-134]
+	_ = x[OTINTER-134]
-	_ = x[OTSLICE-135]
+	_ = x[OTFUNC-135]
-	_ = x[OINLCALL-136]
+	_ = x[OTARRAY-136]
-	_ = x[OEFACE-137]
+	_ = x[OTSLICE-137]
-	_ = x[OITAB-138]
+	_ = x[OINLCALL-138]
-	_ = x[OIDATA-139]
+	_ = x[OEFACE-139]
-	_ = x[OSPTR-140]
+	_ = x[OITAB-140]
-	_ = x[OCFUNC-141]
+	_ = x[OIDATA-141]
-	_ = x[OCHECKNIL-142]
+	_ = x[OSPTR-142]
-	_ = x[OVARDEF-143]
+	_ = x[OCFUNC-143]
-	_ = x[OVARKILL-144]
+	_ = x[OCHECKNIL-144]
-	_ = x[OVARLIVE-145]
+	_ = x[OVARDEF-145]
-	_ = x[ORESULT-146]
+	_ = x[OVARKILL-146]
-	_ = x[OINLMARK-147]
+	_ = x[OVARLIVE-147]
-	_ = x[OLINKSYMOFFSET-148]
+	_ = x[ORESULT-148]
-	_ = x[OTAILCALL-149]
+	_ = x[OINLMARK-149]
-	_ = x[OGETG-150]
+	_ = x[OLINKSYMOFFSET-150]
-	_ = x[OEND-151]
+	_ = x[OTAILCALL-151]
+	_ = x[OGETG-152]
+	_ = x[OEND-153]
 }
 
-const _Op_name = "XXXNAMENONAMETYPEPACKLITERALNILADDSUBORXORADDSTRADDRANDANDAPPENDBYTES2STRBYTES2STRTMPRUNES2STRSTR2BYTESSTR2BYTESTMPSTR2RUNESASAS2AS2DOTTYPEAS2FUNCAS2MAPRAS2RECVASOPCALLCALLFUNCCALLMETHCALLINTERCALLPARTCAPCLOSECLOSURECOMPLITMAPLITSTRUCTLITARRAYLITSLICELITPTRLITCONVCONVIFACECONVNOPCOPYDCLDCLFUNCDCLCONSTDCLTYPEDELETEDOTDOTPTRDOTMETHDOTINTERXDOTDOTTYPEDOTTYPE2EQNELTLEGEGTDEREFINDEXINDEXMAPKEYSTRUCTKEYLENMAKEMAKECHANMAKEMAPMAKESLICEMAKESLICECOPYMULDIVMODLSHRSHANDANDNOTNEWNOTBITNOTPLUSNEGORORPANICPRINTPRINTNPARENSENDSLICESLICEARRSLICESTRSLICE3SLICE3ARRSLICEHEADERRECOVERRECVRUNESTRSELRECV2IOTAREALIMAGCOMPLEXALIGNOFOFFSETOFSIZEOFMETHEXPRSTMTEXPRBLOCKBREAKCASECONTINUEDEFERFALLFORFORUNTILGOTOIFLABELGORANGERETURNSELECTSWITCHTYPESWTCHANTMAPTSTRUCTTINTERTFUNCTARRAYTSLICEINLCALLEFACEITABIDATASPTRCFUNCCHECKNILVARDEFVARKILLVARLIVERESULTINLMARKLINKSYMOFFSETTAILCALLGETGEND"
+const _Op_name = "XXXNAMENONAMETYPEPACKLITERALNILADDSUBORXORADDSTRADDRANDANDAPPENDBYTES2STRBYTES2STRTMPRUNES2STRSTR2BYTESSTR2BYTESTMPSTR2RUNESASAS2AS2DOTTYPEAS2FUNCAS2MAPRAS2RECVASOPCALLCALLFUNCCALLMETHCALLINTERCALLPARTCAPCLOSECLOSURECOMPLITMAPLITSTRUCTLITARRAYLITSLICELITPTRLITCONVCONVIFACECONVNOPCOPYDCLDCLFUNCDCLCONSTDCLTYPEDELETEDOTDOTPTRDOTMETHDOTINTERXDOTDOTTYPEDOTTYPE2EQNELTLEGEGTDEREFINDEXINDEXMAPKEYSTRUCTKEYLENMAKEMAKECHANMAKEMAPMAKESLICEMAKESLICECOPYMULDIVMODLSHRSHANDANDNOTNEWNOTBITNOTPLUSNEGORORPANICPRINTPRINTNPARENSENDSLICESLICEARRSLICESTRSLICE3SLICE3ARRSLICEHEADERRECOVERRECVRUNESTRSELRECV2IOTAREALIMAGCOMPLEXALIGNOFOFFSETOFSIZEOFMETHEXPRSTMTEXPRBLOCKBREAKCASECONTINUEDEFERFALLFORFORUNTILGOTOIFLABELGORANGERETURNSELECTSWITCHTYPESWFUNCINSTTYPEINSTTCHANTMAPTSTRUCTTINTERTFUNCTARRAYTSLICEINLCALLEFACEITABIDATASPTRCFUNCCHECKNILVARDEFVARKILLVARLIVERESULTINLMARKLINKSYMOFFSETTAILCALLGETGEND"
 
-var _Op_index = [...]uint16{0, 3, 7, 13, 17, 21, 28, 31, 34, 37, 39, 42, 48, 52, 58, 64, 73, 85, 94, 103, 115, 124, 126, 129, 139, 146, 153, 160, 164, 168, 176, 184, 193, 201, 204, 209, 216, 223, 229, 238, 246, 254, 260, 264, 273, 280, 284, 287, 294, 302, 309, 315, 318, 324, 331, 339, 343, 350, 358, 360, 362, 364, 366, 368, 370, 375, 380, 388, 391, 400, 403, 407, 415, 422, 431, 444, 447, 450, 453, 456, 459, 462, 468, 471, 474, 480, 484, 487, 491, 496, 501, 507, 512, 516, 521, 529, 537, 543, 552, 563, 570, 574, 581, 589, 593, 597, 601, 608, 615, 623, 629, 637, 645, 650, 655, 659, 667, 672, 676, 679, 687, 691, 693, 698, 700, 705, 711, 717, 723, 729, 734, 738, 745, 751, 756, 762, 768, 775, 780, 784, 789, 793, 798, 806, 812, 819, 826, 832, 839, 852, 860, 864, 867}
+var _Op_index = [...]uint16{0, 3, 7, 13, 17, 21, 28, 31, 34, 37, 39, 42, 48, 52, 58, 64, 73, 85, 94, 103, 115, 124, 126, 129, 139, 146, 153, 160, 164, 168, 176, 184, 193, 201, 204, 209, 216, 223, 229, 238, 246, 254, 260, 264, 273, 280, 284, 287, 294, 302, 309, 315, 318, 324, 331, 339, 343, 350, 358, 360, 362, 364, 366, 368, 370, 375, 380, 388, 391, 400, 403, 407, 415, 422, 431, 444, 447, 450, 453, 456, 459, 462, 468, 471, 474, 480, 484, 487, 491, 496, 501, 507, 512, 516, 521, 529, 537, 543, 552, 563, 570, 574, 581, 589, 593, 597, 601, 608, 615, 623, 629, 637, 645, 650, 655, 659, 667, 672, 676, 679, 687, 691, 693, 698, 700, 705, 711, 717, 723, 729, 737, 745, 750, 754, 761, 767, 772, 778, 784, 791, 796, 800, 805, 809, 814, 822, 828, 835, 842, 848, 855, 868, 876, 880, 883}
 
 func (i Op) String() string {
 	if i >= Op(len(_Op_index)-1) {

src/cmd/compile/internal/ir/package.go

@@ -32,4 +32,7 @@ type Package struct {
 
 	// Exported (or re-exported) symbols.
 	Exports []*Name
+
+	// Map from function names of stencils to already-created stencils.
+	Stencils map[*types.Sym]*Func
 }

src/cmd/compile/internal/noder/decl.go

@@ -0,0 +1,241 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"go/constant"
+
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
+)
+
+// TODO(mdempsky): Skip blank declarations? Probably only safe
+// for declarations without pragmas.
+
+func (g *irgen) decls(decls []syntax.Decl) []ir.Node {
+	var res ir.Nodes
+	for _, decl := range decls {
+		switch decl := decl.(type) {
+		case *syntax.ConstDecl:
+			g.constDecl(&res, decl)
+		case *syntax.FuncDecl:
+			g.funcDecl(&res, decl)
+		case *syntax.TypeDecl:
+			if ir.CurFunc == nil {
+				continue // already handled in irgen.generate
+			}
+			g.typeDecl(&res, decl)
+		case *syntax.VarDecl:
+			g.varDecl(&res, decl)
+		default:
+			g.unhandled("declaration", decl)
+		}
+	}
+	return res
+}
+
+func (g *irgen) importDecl(p *noder, decl *syntax.ImportDecl) {
+	// TODO(mdempsky): Merge with gcimports so we don't have to import
+	// packages twice.
+
+	g.pragmaFlags(decl.Pragma, 0)
+
+	ipkg := importfile(decl)
+	if ipkg == ir.Pkgs.Unsafe {
+		p.importedUnsafe = true
+	}
+	if ipkg.Path == "embed" {
+		p.importedEmbed = true
+	}
+}
+
+func (g *irgen) constDecl(out *ir.Nodes, decl *syntax.ConstDecl) {
+	g.pragmaFlags(decl.Pragma, 0)
+
+	for _, name := range decl.NameList {
+		name, obj := g.def(name)
+
+		// For untyped numeric constants, make sure the value
+		// representation matches what the rest of the
+		// compiler (really just iexport) expects.
+		// TODO(mdempsky): Revisit after #43891 is resolved.
+		val := obj.(*types2.Const).Val()
+		switch name.Type() {
+		case types.UntypedInt, types.UntypedRune:
+			val = constant.ToInt(val)
+		case types.UntypedFloat:
+			val = constant.ToFloat(val)
+		case types.UntypedComplex:
+			val = constant.ToComplex(val)
+		}
+		name.SetVal(val)
+
+		out.Append(ir.NewDecl(g.pos(decl), ir.ODCLCONST, name))
+	}
+}
+
+func (g *irgen) funcDecl(out *ir.Nodes, decl *syntax.FuncDecl) {
+	fn := ir.NewFunc(g.pos(decl))
+	fn.Nname, _ = g.def(decl.Name)
+	fn.Nname.Func = fn
+	fn.Nname.Defn = fn
+
+	fn.Pragma = g.pragmaFlags(decl.Pragma, funcPragmas)
+	if fn.Pragma&ir.Systemstack != 0 && fn.Pragma&ir.Nosplit != 0 {
+		base.ErrorfAt(fn.Pos(), "go:nosplit and go:systemstack cannot be combined")
+	}
+
+	if decl.Name.Value == "init" && decl.Recv == nil {
+		g.target.Inits = append(g.target.Inits, fn)
+	}
+
+	g.funcBody(fn, decl.Recv, decl.Type, decl.Body)
+
+	out.Append(fn)
+}
+
+func (g *irgen) typeDecl(out *ir.Nodes, decl *syntax.TypeDecl) {
+	if decl.Alias {
+		name, _ := g.def(decl.Name)
+		g.pragmaFlags(decl.Pragma, 0)
+
+		// TODO(mdempsky): This matches how typecheckdef marks aliases for
+		// export, but this won't generalize to exporting function-scoped
+		// type aliases. We should maybe just use n.Alias() instead.
+		if ir.CurFunc == nil {
+			name.Sym().Def = ir.TypeNode(name.Type())
+		}
+
+		out.Append(ir.NewDecl(g.pos(decl), ir.ODCLTYPE, name))
+		return
+	}
+
+	// Prevent size calculations until we set the underlying type.
+	types.DeferCheckSize()
+
+	name, obj := g.def(decl.Name)
+	ntyp, otyp := name.Type(), obj.Type()
+	if ir.CurFunc != nil {
+		typecheck.TypeGen++
+		ntyp.Vargen = typecheck.TypeGen
+	}
+
+	pragmas := g.pragmaFlags(decl.Pragma, typePragmas)
+	name.SetPragma(pragmas) // TODO(mdempsky): Is this still needed?
+
+	if pragmas&ir.NotInHeap != 0 {
+		ntyp.SetNotInHeap(true)
+	}
+
+	// We need to use g.typeExpr(decl.Type) here to ensure that for
+	// chained, defined-type declarations like
+	//
+	//	type T U
+	//
+	//	//go:notinheap
+	//	type U struct { … }
+	//
+	// that we mark both T and U as NotInHeap. If we instead used just
+	// g.typ(otyp.Underlying()), then we'd instead set T's underlying
+	// type directly to the struct type (which is not marked NotInHeap)
+	// and fail to mark T as NotInHeap.
+	//
+	// Also, we rely here on Type.SetUnderlying allowing passing a
+	// defined type and handling forward references like from T to U
+	// above. Contrast with go/types's Named.SetUnderlying, which
+	// disallows this.
+	//
+	// [mdempsky: Subtleties like these are why I always vehemently
+	// object to new type pragmas.]
+	ntyp.SetUnderlying(g.typeExpr(decl.Type))
+	types.ResumeCheckSize()
+
+	if otyp, ok := otyp.(*types2.Named); ok && otyp.NumMethods() != 0 {
+		methods := make([]*types.Field, otyp.NumMethods())
+		for i := range methods {
+			m := otyp.Method(i)
+			meth := g.obj(m)
+			methods[i] = types.NewField(meth.Pos(), g.selector(m), meth.Type())
+			methods[i].Nname = meth
+		}
+		ntyp.Methods().Set(methods)
+	}
+
+	out.Append(ir.NewDecl(g.pos(decl), ir.ODCLTYPE, name))
+}
+
+func (g *irgen) varDecl(out *ir.Nodes, decl *syntax.VarDecl) {
+	pos := g.pos(decl)
+	names := make([]*ir.Name, len(decl.NameList))
+	for i, name := range decl.NameList {
+		names[i], _ = g.def(name)
+	}
+	values := g.exprList(decl.Values)
+
+	if decl.Pragma != nil {
+		pragma := decl.Pragma.(*pragmas)
+		// TODO(mdempsky): Plumb noder.importedEmbed through to here.
+		varEmbed(g.makeXPos, names[0], decl, pragma, true)
+		g.reportUnused(pragma)
+	}
+
+	var as2 *ir.AssignListStmt
+	if len(values) != 0 && len(names) != len(values) {
+		as2 = ir.NewAssignListStmt(pos, ir.OAS2, make([]ir.Node, len(names)), values)
+	}
+
+	for i, name := range names {
+		if ir.CurFunc != nil {
+			out.Append(ir.NewDecl(pos, ir.ODCL, name))
+		}
+		if as2 != nil {
+			as2.Lhs[i] = name
+			name.Defn = as2
+		} else {
+			as := ir.NewAssignStmt(pos, name, nil)
+			if len(values) != 0 {
+				as.Y = values[i]
+				name.Defn = as
+			} else if ir.CurFunc == nil {
+				name.Defn = as
+			}
+			out.Append(typecheck.Stmt(as))
+		}
+	}
+	if as2 != nil {
+		out.Append(typecheck.Stmt(as2))
+	}
+}
+
+// pragmaFlags returns any specified pragma flags included in allowed,
+// and reports errors about any other, unexpected pragmas.
+func (g *irgen) pragmaFlags(pragma syntax.Pragma, allowed ir.PragmaFlag) ir.PragmaFlag {
+	if pragma == nil {
+		return 0
+	}
+	p := pragma.(*pragmas)
+	present := p.Flag & allowed
+	p.Flag &^= allowed
+	g.reportUnused(p)
+	return present
+}
+
+// reportUnused reports errors about any unused pragmas.
+func (g *irgen) reportUnused(pragma *pragmas) {
+	for _, pos := range pragma.Pos {
+		if pos.Flag&pragma.Flag != 0 {
+			base.ErrorfAt(g.makeXPos(pos.Pos), "misplaced compiler directive")
+		}
+	}
+	if len(pragma.Embeds) > 0 {
+		for _, e := range pragma.Embeds {
+			base.ErrorfAt(g.makeXPos(e.Pos), "misplaced go:embed directive")
+		}
+	}
+}

src/cmd/compile/internal/noder/expr.go

@@ -0,0 +1,365 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
+	"cmd/internal/src"
+)
+
+func (g *irgen) expr(expr syntax.Expr) ir.Node {
+	// TODO(mdempsky): Change callers to not call on nil?
+	if expr == nil {
+		return nil
+	}
+
+	if expr, ok := expr.(*syntax.Name); ok && expr.Value == "_" {
+		return ir.BlankNode
+	}
+
+	tv, ok := g.info.Types[expr]
+	if !ok {
+		base.FatalfAt(g.pos(expr), "missing type for %v (%T)", expr, expr)
+	}
+	switch {
+	case tv.IsBuiltin():
+		// TODO(mdempsky): Handle in CallExpr?
+		return g.use(expr.(*syntax.Name))
+	case tv.IsType():
+		return ir.TypeNode(g.typ(tv.Type))
+	case tv.IsValue(), tv.IsVoid():
+		// ok
+	default:
+		base.FatalfAt(g.pos(expr), "unrecognized type-checker result")
+	}
+
+	// The gc backend expects all expressions to have a concrete type, and
+	// types2 mostly satisfies this expectation already. But there are a few
+	// cases where the Go spec doesn't require converting to concrete type,
+	// and so types2 leaves them untyped. So we need to fix those up here.
+	typ := tv.Type
+	if basic, ok := typ.(*types2.Basic); ok && basic.Info()&types2.IsUntyped != 0 {
+		switch basic.Kind() {
+		case types2.UntypedNil:
+			// ok; can appear in type switch case clauses
+			// TODO(mdempsky): Handle as part of type switches instead?
+		case types2.UntypedBool:
+			typ = types2.Typ[types2.Bool] // expression in "if" or "for" condition
+		case types2.UntypedString:
+			typ = types2.Typ[types2.String] // argument to "append" or "copy" calls
+		default:
+			base.FatalfAt(g.pos(expr), "unexpected untyped type: %v", basic)
+		}
+	}
+
+	// Constant expression.
+	if tv.Value != nil {
+		return Const(g.pos(expr), g.typ(typ), tv.Value)
+	}
+
+	n := g.expr0(typ, expr)
+	if n.Typecheck() != 1 {
+		base.FatalfAt(g.pos(expr), "missed typecheck: %+v", n)
+	}
+	if !g.match(n.Type(), typ, tv.HasOk()) {
+		base.FatalfAt(g.pos(expr), "expected %L to have type %v", n, typ)
+	}
+	return n
+}
+
+func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
+	pos := g.pos(expr)
+
+	switch expr := expr.(type) {
+	case *syntax.Name:
+		if _, isNil := g.info.Uses[expr].(*types2.Nil); isNil {
+			return Nil(pos, g.typ(typ))
+		}
+		// TODO(mdempsky): Remove dependency on typecheck.Expr.
+		return typecheck.Expr(g.use(expr))
+
+	case *syntax.CompositeLit:
+		return g.compLit(typ, expr)
+
+	case *syntax.FuncLit:
+		return g.funcLit(typ, expr)
+
+	case *syntax.AssertExpr:
+		return Assert(pos, g.expr(expr.X), g.typeExpr(expr.Type))
+
+	case *syntax.CallExpr:
+		fun := g.expr(expr.Fun)
+		if inferred, ok := g.info.Inferred[expr]; ok && len(inferred.Targs) > 0 {
+			targs := make([]ir.Node, len(inferred.Targs))
+			for i, targ := range inferred.Targs {
+				targs[i] = ir.TypeNode(g.typ(targ))
+			}
+			if fun.Op() == ir.OFUNCINST {
+				// Replace explicit type args with the full list that
+				// includes the additional inferred type args
+				fun.(*ir.InstExpr).Targs = targs
+			} else {
+				// Create a function instantiation here, given
+				// there are only inferred type args (e.g.
+				// min(5,6), where min is a generic function)
+				inst := ir.NewInstExpr(pos, ir.OFUNCINST, fun, targs)
+				typed(fun.Type(), inst)
+				fun = inst
+			}
+
+		}
+		return Call(pos, g.typ(typ), fun, g.exprs(expr.ArgList), expr.HasDots)
+
+	case *syntax.IndexExpr:
+		var targs []ir.Node
+		if _, ok := expr.Index.(*syntax.ListExpr); ok {
+			targs = g.exprList(expr.Index)
+		} else {
+			index := g.expr(expr.Index)
+			if index.Op() != ir.OTYPE {
+				// This is just a normal index expression
+				return Index(pos, g.expr(expr.X), index)
+			}
+			// This is generic function instantiation with a single type
+			targs = []ir.Node{index}
+		}
+		// This is a generic function instantiation (e.g. min[int])
+		x := g.expr(expr.X)
+		if x.Op() != ir.ONAME || x.Type().Kind() != types.TFUNC {
+			panic("Incorrect argument for generic func instantiation")
+		}
+		// This could also be an OTYPEINST once we can handle those examples.
+		n := ir.NewInstExpr(pos, ir.OFUNCINST, x, targs)
+		typed(g.typ(typ), n)
+		return n
+
+	case *syntax.ParenExpr:
+		return g.expr(expr.X) // skip parens; unneeded after parse+typecheck
+
+	case *syntax.SelectorExpr:
+		// Qualified identifier.
+		if name, ok := expr.X.(*syntax.Name); ok {
+			if _, ok := g.info.Uses[name].(*types2.PkgName); ok {
+				// TODO(mdempsky): Remove dependency on typecheck.Expr.
+				return typecheck.Expr(g.use(expr.Sel))
+			}
+		}
+		return g.selectorExpr(pos, typ, expr)
+
+	case *syntax.SliceExpr:
+		return Slice(pos, g.expr(expr.X), g.expr(expr.Index[0]), g.expr(expr.Index[1]), g.expr(expr.Index[2]))
+
+	case *syntax.Operation:
+		if expr.Y == nil {
+			return Unary(pos, g.op(expr.Op, unOps[:]), g.expr(expr.X))
+		}
+		switch op := g.op(expr.Op, binOps[:]); op {
+		case ir.OEQ, ir.ONE, ir.OLT, ir.OLE, ir.OGT, ir.OGE:
+			return Compare(pos, g.typ(typ), op, g.expr(expr.X), g.expr(expr.Y))
+		default:
+			return Binary(pos, op, g.expr(expr.X), g.expr(expr.Y))
+		}
+
+	default:
+		g.unhandled("expression", expr)
+		panic("unreachable")
+	}
+}
+
+// selectorExpr resolves the choice of ODOT, ODOTPTR, OCALLPART (eventually
+// ODOTMETH & ODOTINTER), and OMETHEXPR and deals with embedded fields here rather
+// than in typecheck.go.
+func (g *irgen) selectorExpr(pos src.XPos, typ types2.Type, expr *syntax.SelectorExpr) ir.Node {
+	x := g.expr(expr.X)
+	if x.Type().Kind() == types.TTYPEPARAM {
+		// Leave a method call on a type param as an OXDOT, since it can
+		// only be fully transformed once it has an instantiated type.
+		n := ir.NewSelectorExpr(pos, ir.OXDOT, x, typecheck.Lookup(expr.Sel.Value))
+		typed(g.typ(typ), n)
+		return n
+	}
+
+	selinfo := g.info.Selections[expr]
+	// Everything up to the last selection is an implicit embedded field access,
+	// and the last selection is determined by selinfo.Kind().
+	index := selinfo.Index()
+	embeds, last := index[:len(index)-1], index[len(index)-1]
+
+	origx := x
+	for _, ix := range embeds {
+		x = Implicit(DotField(pos, x, ix))
+	}
+
+	kind := selinfo.Kind()
+	if kind == types2.FieldVal {
+		return DotField(pos, x, last)
+	}
+
+	// TODO(danscales,mdempsky): Interface method sets are not sorted the
+	// same between types and types2. In particular, using "last" here
+	// without conversion will likely fail if an interface contains
+	// unexported methods from two different packages (due to cross-package
+	// interface embedding).
+
+	var n ir.Node
+	method2 := selinfo.Obj().(*types2.Func)
+
+	if kind == types2.MethodExpr {
+		// OMETHEXPR is unusual in using directly the node and type of the
+		// original OTYPE node (origx) before passing through embedded
+		// fields, even though the method is selected from the type
+		// (x.Type()) reached after following the embedded fields. We will
+		// actually drop any ODOT nodes we created due to the embedded
+		// fields.
+		n = MethodExpr(pos, origx, x.Type(), last)
+	} else {
+		// Add implicit addr/deref for method values, if needed.
+		if x.Type().IsInterface() {
+			n = DotMethod(pos, x, last)
+		} else {
+			recvType2 := method2.Type().(*types2.Signature).Recv().Type()
+			_, wantPtr := recvType2.(*types2.Pointer)
+			havePtr := x.Type().IsPtr()
+
+			if havePtr != wantPtr {
+				if havePtr {
+					x = Implicit(Deref(pos, x))
+				} else {
+					x = Implicit(Addr(pos, x))
+				}
+			}
+			recvType2Base := recvType2
+			if wantPtr {
+				recvType2Base = types2.AsPointer(recvType2).Elem()
+			}
+			if len(types2.AsNamed(recvType2Base).TParams()) > 0 {
+				// recvType2 is the original generic type that is
+				// instantiated for this method call.
+				// selinfo.Recv() is the instantiated type
+				recvType2 = recvType2Base
+				// method is the generic method associated with the gen type
+				method := g.obj(types2.AsNamed(recvType2).Method(last))
+				n = ir.NewSelectorExpr(pos, ir.OCALLPART, x, method.Sym())
+				n.(*ir.SelectorExpr).Selection = types.NewField(pos, method.Sym(), method.Type())
+				n.(*ir.SelectorExpr).Selection.Nname = method
+				typed(method.Type(), n)
+
+				// selinfo.Targs() are the types used to
+				// instantiate the type of receiver
+				targs2 := selinfo.TArgs()
+				targs := make([]ir.Node, len(targs2))
+				for i, targ2 := range targs2 {
+					targs[i] = ir.TypeNode(g.typ(targ2))
+				}
+
+				// Create function instantiation with the type
+				// args for the receiver type for the method call.
+				n = ir.NewInstExpr(pos, ir.OFUNCINST, n, targs)
+				typed(g.typ(typ), n)
+				return n
+			}
+
+			if !g.match(x.Type(), recvType2, false) {
+				base.FatalfAt(pos, "expected %L to have type %v", x, recvType2)
+			} else {
+				n = DotMethod(pos, x, last)
+			}
+		}
+	}
+	if have, want := n.Sym(), g.selector(method2); have != want {
+		base.FatalfAt(pos, "bad Sym: have %v, want %v", have, want)
+	}
+	return n
+}
+
+func (g *irgen) exprList(expr syntax.Expr) []ir.Node {
+	switch expr := expr.(type) {
+	case nil:
+		return nil
+	case *syntax.ListExpr:
+		return g.exprs(expr.ElemList)
+	default:
+		return []ir.Node{g.expr(expr)}
+	}
+}
+
+func (g *irgen) exprs(exprs []syntax.Expr) []ir.Node {
+	nodes := make([]ir.Node, len(exprs))
+	for i, expr := range exprs {
+		nodes[i] = g.expr(expr)
+	}
+	return nodes
+}
+
+func (g *irgen) compLit(typ types2.Type, lit *syntax.CompositeLit) ir.Node {
+	if ptr, ok := typ.Underlying().(*types2.Pointer); ok {
+		n := ir.NewAddrExpr(g.pos(lit), g.compLit(ptr.Elem(), lit))
+		n.SetOp(ir.OPTRLIT)
+		return typed(g.typ(typ), n)
+	}
+
+	_, isStruct := typ.Underlying().(*types2.Struct)
+
+	exprs := make([]ir.Node, len(lit.ElemList))
+	for i, elem := range lit.ElemList {
+		switch elem := elem.(type) {
+		case *syntax.KeyValueExpr:
+			if isStruct {
+				exprs[i] = ir.NewStructKeyExpr(g.pos(elem), g.name(elem.Key.(*syntax.Name)), g.expr(elem.Value))
+			} else {
+				exprs[i] = ir.NewKeyExpr(g.pos(elem), g.expr(elem.Key), g.expr(elem.Value))
+			}
+		default:
+			exprs[i] = g.expr(elem)
+		}
+	}
+
+	// TODO(mdempsky): Remove dependency on typecheck.Expr.
+	return typecheck.Expr(ir.NewCompLitExpr(g.pos(lit), ir.OCOMPLIT, ir.TypeNode(g.typ(typ)), exprs))
+}
+
+func (g *irgen) funcLit(typ types2.Type, expr *syntax.FuncLit) ir.Node {
+	fn := ir.NewFunc(g.pos(expr))
+	fn.SetIsHiddenClosure(ir.CurFunc != nil)
+
+	fn.Nname = ir.NewNameAt(g.pos(expr), typecheck.ClosureName(ir.CurFunc))
+	ir.MarkFunc(fn.Nname)
+	fn.Nname.SetType(g.typ(typ))
+	fn.Nname.Func = fn
+	fn.Nname.Defn = fn
+
+	fn.OClosure = ir.NewClosureExpr(g.pos(expr), fn)
+	fn.OClosure.SetType(fn.Nname.Type())
+	fn.OClosure.SetTypecheck(1)
+
+	g.funcBody(fn, nil, expr.Type, expr.Body)
+
+	ir.FinishCaptureNames(fn.Pos(), ir.CurFunc, fn)
+
+	// TODO(mdempsky): ir.CaptureName should probably handle
+	// copying these fields from the canonical variable.
+	for _, cv := range fn.ClosureVars {
+		cv.SetType(cv.Canonical().Type())
+		cv.SetTypecheck(1)
+		cv.SetWalkdef(1)
+	}
+
+	g.target.Decls = append(g.target.Decls, fn)
+
+	return fn.OClosure
+}
+
+func (g *irgen) typeExpr(typ syntax.Expr) *types.Type {
+	n := g.expr(typ)
+	if n.Op() != ir.OTYPE {
+		base.FatalfAt(g.pos(typ), "expected type: %L", n)
+	}
+	return n.Type()
+}

src/cmd/compile/internal/noder/func.go

@@ -0,0 +1,74 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/internal/src"
+)
+
+func (g *irgen) funcBody(fn *ir.Func, recv *syntax.Field, sig *syntax.FuncType, block *syntax.BlockStmt) {
+	typecheck.Func(fn)
+
+	// TODO(mdempsky): Remove uses of ir.CurFunc and
+	// typecheck.DeclContext after we stop relying on typecheck
+	// for desugaring.
+	outerfn, outerctxt := ir.CurFunc, typecheck.DeclContext
+	ir.CurFunc = fn
+
+	typ := fn.Type()
+	if param := typ.Recv(); param != nil {
+		g.defParam(param, recv, ir.PPARAM)
+	}
+	for i, param := range typ.Params().FieldSlice() {
+		g.defParam(param, sig.ParamList[i], ir.PPARAM)
+	}
+	for i, result := range typ.Results().FieldSlice() {
+		g.defParam(result, sig.ResultList[i], ir.PPARAMOUT)
+	}
+
+	// We may have type-checked a call to this function already and
+	// calculated its size, including parameter offsets. Now that we've
+	// created the parameter Names, force a recalculation to ensure
+	// their offsets are correct.
+	typ.Align = 0
+	types.CalcSize(typ)
+
+	if block != nil {
+		typecheck.DeclContext = ir.PAUTO
+
+		fn.Body = g.stmts(block.List)
+		if fn.Body == nil {
+			fn.Body = []ir.Node{ir.NewBlockStmt(src.NoXPos, nil)}
+		}
+		fn.Endlineno = g.makeXPos(block.Rbrace)
+
+		if base.Flag.Dwarf {
+			g.recordScopes(fn, sig)
+		}
+	}
+
+	ir.CurFunc, typecheck.DeclContext = outerfn, outerctxt
+}
+
+func (g *irgen) defParam(param *types.Field, decl *syntax.Field, class ir.Class) {
+	typecheck.DeclContext = class
+
+	var name *ir.Name
+	if decl.Name != nil {
+		name, _ = g.def(decl.Name)
+	} else if class == ir.PPARAMOUT {
+		name = g.obj(g.info.Implicits[decl])
+	}
+
+	if name != nil {
+		param.Nname = name
+		param.Sym = name.Sym() // in case it was renamed
+	}
+}

src/cmd/compile/internal/noder/helpers.go

@@ -0,0 +1,277 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"go/constant"
+
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/internal/src"
+)
+
+// Helpers for constructing typed IR nodes.
+//
+// TODO(mdempsky): Move into their own package so they can be easily
+// reused by iimport and frontend optimizations.
+//
+// TODO(mdempsky): Update to consistently return already typechecked
+// results, rather than leaving the caller responsible for using
+// typecheck.Expr or typecheck.Stmt.
+
+type ImplicitNode interface {
+	ir.Node
+	SetImplicit(x bool)
+}
+
+// Implicit returns n after marking it as Implicit.
+func Implicit(n ImplicitNode) ImplicitNode {
+	n.SetImplicit(true)
+	return n
+}
+
+// typed returns n after setting its type to typ.
+func typed(typ *types.Type, n ir.Node) ir.Node {
+	n.SetType(typ)
+	n.SetTypecheck(1)
+	return n
+}
+
+// Values
+
+func Const(pos src.XPos, typ *types.Type, val constant.Value) ir.Node {
+	return typed(typ, ir.NewBasicLit(pos, val))
+}
+
+func Nil(pos src.XPos, typ *types.Type) ir.Node {
+	return typed(typ, ir.NewNilExpr(pos))
+}
+
+// Expressions
+
+func Addr(pos src.XPos, x ir.Node) *ir.AddrExpr {
+	// TODO(mdempsky): Avoid typecheck.Expr. Probably just need to set OPTRLIT when appropriate.
+	n := typecheck.Expr(typecheck.NodAddrAt(pos, x)).(*ir.AddrExpr)
+	typed(types.NewPtr(x.Type()), n)
+	return n
+}
+
+func Assert(pos src.XPos, x ir.Node, typ *types.Type) ir.Node {
+	return typed(typ, ir.NewTypeAssertExpr(pos, x, nil))
+}
+
+func Binary(pos src.XPos, op ir.Op, x, y ir.Node) ir.Node {
+	switch op {
+	case ir.OANDAND, ir.OOROR:
+		return typed(x.Type(), ir.NewLogicalExpr(pos, op, x, y))
+	case ir.OADD:
+		if x.Type().IsString() {
+			// TODO(mdempsky): Construct OADDSTR directly.
+			return typecheck.Expr(ir.NewBinaryExpr(pos, op, x, y))
+		}
+		fallthrough
+	default:
+		return typed(x.Type(), ir.NewBinaryExpr(pos, op, x, y))
+	}
+}
+
+func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool) ir.Node {
+	// TODO(mdempsky): This should not be so difficult.
+	if fun.Op() == ir.OTYPE {
+		// Actually a type conversion, not a function call.
+		n := ir.NewCallExpr(pos, ir.OCALL, fun, args)
+		if fun.Type().Kind() == types.TTYPEPARAM {
+			// For type params, don't typecheck until we actually know
+			// the type.
+			return typed(typ, n)
+		}
+		return typecheck.Expr(n)
+	}
+
+	if fun, ok := fun.(*ir.Name); ok && fun.BuiltinOp != 0 {
+		// Call to a builtin function.
+		n := ir.NewCallExpr(pos, ir.OCALL, fun, args)
+		n.IsDDD = dots
+		switch fun.BuiltinOp {
+		case ir.OCLOSE, ir.ODELETE, ir.OPANIC, ir.OPRINT, ir.OPRINTN:
+			return typecheck.Stmt(n)
+		default:
+			return typecheck.Expr(n)
+		}
+	}
+
+	// Add information, now that we know that fun is actually being called.
+	switch fun := fun.(type) {
+	case *ir.ClosureExpr:
+		fun.Func.SetClosureCalled(true)
+	case *ir.SelectorExpr:
+		if fun.Op() == ir.OCALLPART {
+			op := ir.ODOTMETH
+			if fun.X.Type().IsInterface() {
+				op = ir.ODOTINTER
+			}
+			fun.SetOp(op)
+			// Set the type to include the receiver, since that's what
+			// later parts of the compiler expect
+			fun.SetType(fun.Selection.Type)
+		}
+	}
+
+	n := ir.NewCallExpr(pos, ir.OCALL, fun, args)
+	n.IsDDD = dots
+
+	if fun.Op() == ir.OXDOT {
+		if fun.(*ir.SelectorExpr).X.Type().Kind() != types.TTYPEPARAM {
+			base.FatalfAt(pos, "Expecting type param receiver in %v", fun)
+		}
+		// For methods called in a generic function, don't do any extra
+		// transformations. We will do those later when we create the
+		// instantiated function and have the correct receiver type.
+		typed(typ, n)
+		return n
+	}
+	if fun.Op() != ir.OFUNCINST {
+		// If no type params, still do normal typechecking, since we're
+		// still missing some things done by tcCall below (mainly
+		// typecheckargs and typecheckaste).
+		typecheck.Call(n)
+		return n
+	}
+
+	n.Use = ir.CallUseExpr
+	if fun.Type().NumResults() == 0 {
+		n.Use = ir.CallUseStmt
+	}
+
+	// Rewrite call node depending on use.
+	switch fun.Op() {
+	case ir.ODOTINTER:
+		n.SetOp(ir.OCALLINTER)
+
+	case ir.ODOTMETH:
+		n.SetOp(ir.OCALLMETH)
+
+	default:
+		n.SetOp(ir.OCALLFUNC)
+	}
+
+	typed(typ, n)
+	return n
+}
+
+func Compare(pos src.XPos, typ *types.Type, op ir.Op, x, y ir.Node) ir.Node {
+	n := ir.NewBinaryExpr(pos, op, x, y)
+	if !types.Identical(x.Type(), y.Type()) {
+		// TODO(mdempsky): Handle subtleties of constructing mixed-typed comparisons.
+		n = typecheck.Expr(n).(*ir.BinaryExpr)
+	}
+	return typed(typ, n)
+}
+
+func Deref(pos src.XPos, x ir.Node) *ir.StarExpr {
+	n := ir.NewStarExpr(pos, x)
+	typed(x.Type().Elem(), n)
+	return n
+}
+
+func DotField(pos src.XPos, x ir.Node, index int) *ir.SelectorExpr {
+	op, typ := ir.ODOT, x.Type()
+	if typ.IsPtr() {
+		op, typ = ir.ODOTPTR, typ.Elem()
+	}
+	if !typ.IsStruct() {
+		base.FatalfAt(pos, "DotField of non-struct: %L", x)
+	}
+
+	// TODO(mdempsky): This is the backend's responsibility.
+	types.CalcSize(typ)
+
+	field := typ.Field(index)
+	return dot(pos, field.Type, op, x, field)
+}
+
+func DotMethod(pos src.XPos, x ir.Node, index int) *ir.SelectorExpr {
+	method := method(x.Type(), index)
+
+	// Method value.
+	typ := typecheck.NewMethodType(method.Type, nil)
+	return dot(pos, typ, ir.OCALLPART, x, method)
+}
+
+// MethodExpr returns a OMETHEXPR node with the indicated index into the methods
+// of typ. The receiver type is set from recv, which is different from typ if the
+// method was accessed via embedded fields. Similarly, the X value of the
+// ir.SelectorExpr is recv, the original OTYPE node before passing through the
+// embedded fields.
+func MethodExpr(pos src.XPos, recv ir.Node, embed *types.Type, index int) *ir.SelectorExpr {
+	method := method(embed, index)
+	typ := typecheck.NewMethodType(method.Type, recv.Type())
+	// The method expression T.m requires a wrapper when T
+	// is different from m's declared receiver type. We
+	// normally generate these wrappers while writing out
+	// runtime type descriptors, which is always done for
+	// types declared at package scope. However, we need
+	// to make sure to generate wrappers for anonymous
+	// receiver types too.
+	if recv.Sym() == nil {
+		typecheck.NeedRuntimeType(recv.Type())
+	}
+	return dot(pos, typ, ir.OMETHEXPR, recv, method)
+}
+
+func dot(pos src.XPos, typ *types.Type, op ir.Op, x ir.Node, selection *types.Field) *ir.SelectorExpr {
+	n := ir.NewSelectorExpr(pos, op, x, selection.Sym)
+	n.Selection = selection
+	typed(typ, n)
+	return n
+}
+
+// TODO(mdempsky): Move to package types.
+func method(typ *types.Type, index int) *types.Field {
+	if typ.IsInterface() {
+		return typ.Field(index)
+	}
+	return types.ReceiverBaseType(typ).Methods().Index(index)
+}
+
+func Index(pos src.XPos, x, index ir.Node) ir.Node {
+	// TODO(mdempsky): Avoid typecheck.Expr (which will call tcIndex)
+	return typecheck.Expr(ir.NewIndexExpr(pos, x, index))
+}
+
+func Slice(pos src.XPos, x, low, high, max ir.Node) ir.Node {
+	op := ir.OSLICE
+	if max != nil {
+		op = ir.OSLICE3
+	}
+	// TODO(mdempsky): Avoid typecheck.Expr.
+	return typecheck.Expr(ir.NewSliceExpr(pos, op, x, low, high, max))
+}
+
+func Unary(pos src.XPos, op ir.Op, x ir.Node) ir.Node {
+	switch op {
+	case ir.OADDR:
+		return Addr(pos, x)
+	case ir.ODEREF:
+		return Deref(pos, x)
+	}
+
+	typ := x.Type()
+	if op == ir.ORECV {
+		typ = typ.Elem()
+	}
+	return typed(typ, ir.NewUnaryExpr(pos, op, x))
+}
+
+// Statements
+
+var one = constant.MakeInt64(1)
+
+func IncDec(pos src.XPos, op ir.Op, x ir.Node) ir.Node {
+	x = typecheck.AssignExpr(x)
+	return ir.NewAssignOpStmt(pos, op, x, typecheck.DefaultLit(ir.NewBasicLit(pos, one), x.Type()))
+}

src/cmd/compile/internal/noder/import.go

@@ -7,6 +7,7 @@ package noder
 import (
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	pathpkg "path"
 	"runtime"
@@ -17,10 +18,12 @@ import (
 	"unicode/utf8"
 
 	"cmd/compile/internal/base"
+	"cmd/compile/internal/importer"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/syntax"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
 	"cmd/internal/archive"
 	"cmd/internal/bio"
 	"cmd/internal/goobj"
@@ -28,6 +31,29 @@ import (
 	"cmd/internal/src"
 )
 
+// Temporary import helper to get type2-based type-checking going.
+type gcimports struct {
+	packages map[string]*types2.Package
+}
+
+func (m *gcimports) Import(path string) (*types2.Package, error) {
+	return m.ImportFrom(path, "" /* no vendoring */, 0)
+}
+
+func (m *gcimports) ImportFrom(path, srcDir string, mode types2.ImportMode) (*types2.Package, error) {
+	if mode != 0 {
+		panic("mode must be 0")
+	}
+
+	path, err := resolveImportPath(path)
+	if err != nil {
+		return nil, err
+	}
+
+	lookup := func(path string) (io.ReadCloser, error) { return openPackage(path) }
+	return importer.Import(m.packages, path, srcDir, lookup)
+}
+
 func isDriveLetter(b byte) bool {
 	return 'a' <= b && b <= 'z' || 'A' <= b && b <= 'Z'
 }

src/cmd/compile/internal/noder/irgen.go

@@ -0,0 +1,209 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"fmt"
+	"os"
+
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/dwarfgen"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
+	"cmd/internal/src"
+)
+
+// check2 type checks a Go package using types2, and then generates IR
+// using the results.
+func check2(noders []*noder) {
+	if base.SyntaxErrors() != 0 {
+		base.ErrorExit()
+	}
+
+	// setup and syntax error reporting
+	var m posMap
+	files := make([]*syntax.File, len(noders))
+	for i, p := range noders {
+		m.join(&p.posMap)
+		files[i] = p.file
+	}
+
+	// typechecking
+	conf := types2.Config{
+		GoVersion:             base.Flag.Lang,
+		InferFromConstraints:  true,
+		IgnoreLabels:          true, // parser already checked via syntax.CheckBranches mode
+		CompilerErrorMessages: true, // use error strings matching existing compiler errors
+		Error: func(err error) {
+			terr := err.(types2.Error)
+			if len(terr.Msg) > 0 && terr.Msg[0] == '\t' {
+				// types2 reports error clarifications via separate
+				// error messages which are indented with a tab.
+				// Ignore them to satisfy tools and tests that expect
+				// only one error in such cases.
+				// TODO(gri) Need to adjust error reporting in types2.
+				return
+			}
+			base.ErrorfAt(m.makeXPos(terr.Pos), "%s", terr.Msg)
+		},
+		Importer: &gcimports{
+			packages: make(map[string]*types2.Package),
+		},
+		Sizes: &gcSizes{},
+	}
+	info := types2.Info{
+		Types:      make(map[syntax.Expr]types2.TypeAndValue),
+		Defs:       make(map[*syntax.Name]types2.Object),
+		Uses:       make(map[*syntax.Name]types2.Object),
+		Selections: make(map[*syntax.SelectorExpr]*types2.Selection),
+		Implicits:  make(map[syntax.Node]types2.Object),
+		Scopes:     make(map[syntax.Node]*types2.Scope),
+		Inferred:   make(map[syntax.Expr]types2.Inferred),
+		// expand as needed
+	}
+	pkg, err := conf.Check(base.Ctxt.Pkgpath, files, &info)
+	files = nil
+	if err != nil {
+		base.FatalfAt(src.NoXPos, "conf.Check error: %v", err)
+	}
+	base.ExitIfErrors()
+	if base.Flag.G < 2 {
+		os.Exit(0)
+	}
+
+	g := irgen{
+		target: typecheck.Target,
+		self:   pkg,
+		info:   &info,
+		posMap: m,
+		objs:   make(map[types2.Object]*ir.Name),
+		typs:   make(map[types2.Type]*types.Type),
+	}
+	g.generate(noders)
+
+	if base.Flag.G < 3 {
+		os.Exit(0)
+	}
+}
+
+type irgen struct {
+	target *ir.Package
+	self   *types2.Package
+	info   *types2.Info
+
+	posMap
+	objs   map[types2.Object]*ir.Name
+	typs   map[types2.Type]*types.Type
+	marker dwarfgen.ScopeMarker
+}
+
+func (g *irgen) generate(noders []*noder) {
+	types.LocalPkg.Name = g.self.Name()
+	typecheck.TypecheckAllowed = true
+
+	// Prevent size calculations until we set the underlying type
+	// for all package-block defined types.
+	types.DeferCheckSize()
+
+	// At this point, types2 has already handled name resolution and
+	// type checking. We just need to map from its object and type
+	// representations to those currently used by the rest of the
+	// compiler. This happens mostly in 3 passes.
+
+	// 1. Process all import declarations. We use the compiler's own
+	// importer for this, rather than types2's gcimporter-derived one,
+	// to handle extensions and inline function bodies correctly.
+	//
+	// Also, we need to do this in a separate pass, because mappings are
+	// instantiated on demand. If we interleaved processing import
+	// declarations with other declarations, it's likely we'd end up
+	// wanting to map an object/type from another source file, but not
+	// yet have the import data it relies on.
+	declLists := make([][]syntax.Decl, len(noders))
+Outer:
+	for i, p := range noders {
+		g.pragmaFlags(p.file.Pragma, ir.GoBuildPragma)
+		for j, decl := range p.file.DeclList {
+			switch decl := decl.(type) {
+			case *syntax.ImportDecl:
+				g.importDecl(p, decl)
+			default:
+				declLists[i] = p.file.DeclList[j:]
+				continue Outer // no more ImportDecls
+			}
+		}
+	}
+	types.LocalPkg.Height = myheight
+
+	// 2. Process all package-block type declarations. As with imports,
+	// we need to make sure all types are properly instantiated before
+	// trying to map any expressions that utilize them. In particular,
+	// we need to make sure type pragmas are already known (see comment
+	// in irgen.typeDecl).
+	//
+	// We could perhaps instead defer processing of package-block
+	// variable initializers and function bodies, like noder does, but
+	// special-casing just package-block type declarations minimizes the
+	// differences between processing package-block and function-scoped
+	// declarations.
+	for _, declList := range declLists {
+		for _, decl := range declList {
+			switch decl := decl.(type) {
+			case *syntax.TypeDecl:
+				g.typeDecl((*ir.Nodes)(&g.target.Decls), decl)
+			}
+		}
+	}
+	types.ResumeCheckSize()
+
+	// 3. Process all remaining declarations.
+	for _, declList := range declLists {
+		g.target.Decls = append(g.target.Decls, g.decls(declList)...)
+	}
+
+	if base.Flag.W > 1 {
+		for _, n := range g.target.Decls {
+			s := fmt.Sprintf("\nafter noder2 %v", n)
+			ir.Dump(s, n)
+		}
+	}
+
+	typecheck.DeclareUniverse()
+
+	for _, p := range noders {
+		// Process linkname and cgo pragmas.
+		p.processPragmas()
+
+		// Double check for any type-checking inconsistencies. This can be
+		// removed once we're confident in IR generation results.
+		syntax.Walk(p.file, func(n syntax.Node) bool {
+			g.validate(n)
+			return false
+		})
+	}
+
+	// Create any needed stencils of generic functions
+	g.stencil()
+
+	// For now, remove all generic functions from g.target.Decl, since they
+	// have been used for stenciling, but don't compile. TODO: We will
+	// eventually export any exportable generic functions.
+	j := 0
+	for i, decl := range g.target.Decls {
+		if decl.Op() != ir.ODCLFUNC || !decl.Type().HasTParam() {
+			g.target.Decls[j] = g.target.Decls[i]
+			j++
+		}
+	}
+	g.target.Decls = g.target.Decls[:j]
+}
+
+func (g *irgen) unhandled(what string, p poser) {
+	base.FatalfAt(g.pos(p), "unhandled %s: %T", what, p)
+	panic("unreachable")
+}

src/cmd/compile/internal/noder/noder.go

@@ -30,6 +30,9 @@ func LoadPackage(filenames []string) {
 	base.Timer.Start("fe", "parse")
 
 	mode := syntax.CheckBranches
+	if base.Flag.G != 0 {
+		mode |= syntax.AllowGenerics
+	}
 
 	// Limit the number of simultaneously open files.
 	sem := make(chan struct{}, runtime.GOMAXPROCS(0)+10)
@@ -65,10 +68,16 @@ func LoadPackage(filenames []string) {
 		for e := range p.err {
 			p.errorAt(e.Pos, "%s", e.Msg)
 		}
-		lines += p.file.Lines
+		lines += p.file.EOF.Line()
 	}
 	base.Timer.AddEvent(int64(lines), "lines")
 
+	if base.Flag.G != 0 {
+		// Use types2 to type-check and possibly generate IR.
+		check2(noders)
+		return
+	}
+
 	for _, p := range noders {
 		p.node()
 		p.file = nil // release memory
@@ -1031,8 +1040,7 @@ func (p *noder) stmtFall(stmt syntax.Stmt, fallOK bool) ir.Node {
 	case *syntax.DeclStmt:
 		return ir.NewBlockStmt(src.NoXPos, p.decls(stmt.DeclList))
 	case *syntax.AssignStmt:
-		if stmt.Rhs == syntax.ImplicitOne {
+		if stmt.Rhs == nil {
-			one := constant.MakeInt64(1)
 			pos := p.pos(stmt)
 			n := ir.NewAssignOpStmt(pos, p.binOp(stmt.Op), p.expr(stmt.Lhs), ir.NewBasicLit(pos, one))
 			n.IncDec = true
@@ -1447,6 +1455,20 @@ func (p *noder) basicLit(lit *syntax.BasicLit) constant.Value {
 	switch lit.Kind {
 	case syntax.IntLit, syntax.FloatLit, syntax.ImagLit:
 		checkLangCompat(lit)
+		// The max. mantissa precision for untyped numeric values
+		// is 512 bits, or 4048 bits for each of the two integer
+		// parts of a fraction for floating-point numbers that are
+		// represented accurately in the go/constant package.
+		// Constant literals that are longer than this many bits
+		// are not meaningful; and excessively long constants may
+		// consume a lot of space and time for a useless conversion.
+		// Cap constant length with a generous upper limit that also
+		// allows for separators between all digits.
+		const limit = 10000
+		if len(lit.Value) > limit {
+			p.errorAt(lit.Pos(), "excessively long constant: %s... (%d chars)", lit.Value[:10], len(lit.Value))
+			return constant.MakeUnknown()
+		}
 	}
 
 	v := constant.MakeFromLiteral(lit.Value, tokenForLitKind[lit.Kind], 0)

src/cmd/compile/internal/noder/object.go

@@ -0,0 +1,175 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
+	"cmd/internal/src"
+)
+
+func (g *irgen) def(name *syntax.Name) (*ir.Name, types2.Object) {
+	obj, ok := g.info.Defs[name]
+	if !ok {
+		base.FatalfAt(g.pos(name), "unknown name %v", name)
+	}
+	return g.obj(obj), obj
+}
+
+func (g *irgen) use(name *syntax.Name) *ir.Name {
+	obj, ok := g.info.Uses[name]
+	if !ok {
+		base.FatalfAt(g.pos(name), "unknown name %v", name)
+	}
+	return ir.CaptureName(g.pos(obj), ir.CurFunc, g.obj(obj))
+}
+
+// obj returns the Name that represents the given object. If no such
+// Name exists yet, it will be implicitly created.
+//
+// For objects declared at function scope, ir.CurFunc must already be
+// set to the respective function when the Name is created.
+func (g *irgen) obj(obj types2.Object) *ir.Name {
+	// For imported objects, we use iimport directly instead of mapping
+	// the types2 representation.
+	if obj.Pkg() != g.self {
+		sym := g.sym(obj)
+		if sym.Def != nil {
+			return sym.Def.(*ir.Name)
+		}
+		n := typecheck.Resolve(ir.NewIdent(src.NoXPos, sym))
+		if n, ok := n.(*ir.Name); ok {
+			return n
+		}
+		base.FatalfAt(g.pos(obj), "failed to resolve %v", obj)
+	}
+
+	if name, ok := g.objs[obj]; ok {
+		return name // previously mapped
+	}
+
+	var name *ir.Name
+	pos := g.pos(obj)
+
+	class := typecheck.DeclContext
+	if obj.Parent() == g.self.Scope() {
+		class = ir.PEXTERN // forward reference to package-block declaration
+	}
+
+	// "You are in a maze of twisting little passages, all different."
+	switch obj := obj.(type) {
+	case *types2.Const:
+		name = g.objCommon(pos, ir.OLITERAL, g.sym(obj), class, g.typ(obj.Type()))
+
+	case *types2.Func:
+		sig := obj.Type().(*types2.Signature)
+		var sym *types.Sym
+		var typ *types.Type
+		if recv := sig.Recv(); recv == nil {
+			if obj.Name() == "init" {
+				sym = renameinit()
+			} else {
+				sym = g.sym(obj)
+			}
+			typ = g.typ(sig)
+		} else {
+			sym = g.selector(obj)
+			if !sym.IsBlank() {
+				sym = ir.MethodSym(g.typ(recv.Type()), sym)
+			}
+			typ = g.signature(g.param(recv), sig)
+		}
+		name = g.objCommon(pos, ir.ONAME, sym, ir.PFUNC, typ)
+
+	case *types2.TypeName:
+		if obj.IsAlias() {
+			name = g.objCommon(pos, ir.OTYPE, g.sym(obj), class, g.typ(obj.Type()))
+		} else {
+			name = ir.NewDeclNameAt(pos, ir.OTYPE, g.sym(obj))
+			g.objFinish(name, class, types.NewNamed(name))
+		}
+
+	case *types2.Var:
+		var sym *types.Sym
+		if class == ir.PPARAMOUT {
+			// Backend needs names for result parameters,
+			// even if they're anonymous or blank.
+			switch obj.Name() {
+			case "":
+				sym = typecheck.LookupNum("~r", len(ir.CurFunc.Dcl)) // 'r' for "result"
+			case "_":
+				sym = typecheck.LookupNum("~b", len(ir.CurFunc.Dcl)) // 'b' for "blank"
+			}
+		}
+		if sym == nil {
+			sym = g.sym(obj)
+		}
+		name = g.objCommon(pos, ir.ONAME, sym, class, g.typ(obj.Type()))
+
+	default:
+		g.unhandled("object", obj)
+	}
+
+	g.objs[obj] = name
+	return name
+}
+
+func (g *irgen) objCommon(pos src.XPos, op ir.Op, sym *types.Sym, class ir.Class, typ *types.Type) *ir.Name {
+	name := ir.NewDeclNameAt(pos, op, sym)
+	g.objFinish(name, class, typ)
+	return name
+}
+
+func (g *irgen) objFinish(name *ir.Name, class ir.Class, typ *types.Type) {
+	sym := name.Sym()
+
+	name.SetType(typ)
+	name.Class = class
+	if name.Class == ir.PFUNC {
+		sym.SetFunc(true)
+	}
+
+	// We already know name's type, but typecheck is really eager to try
+	// recomputing it later. This appears to prevent that at least.
+	name.Ntype = ir.TypeNode(typ)
+	name.SetTypecheck(1)
+	name.SetWalkdef(1)
+
+	if ir.IsBlank(name) {
+		return
+	}
+
+	switch class {
+	case ir.PEXTERN:
+		g.target.Externs = append(g.target.Externs, name)
+		fallthrough
+	case ir.PFUNC:
+		sym.Def = name
+		if name.Class == ir.PFUNC && name.Type().Recv() != nil {
+			break // methods are exported with their receiver type
+		}
+		if types.IsExported(sym.Name) {
+			if name.Class == ir.PFUNC && name.Type().NumTParams() > 0 {
+				base.FatalfAt(name.Pos(), "Cannot export a generic function (yet): %v", name)
+			}
+			typecheck.Export(name)
+		}
+		if base.Flag.AsmHdr != "" && !name.Sym().Asm() {
+			name.Sym().SetAsm(true)
+			g.target.Asms = append(g.target.Asms, name)
+		}
+
+	default:
+		// Function-scoped declaration.
+		name.Curfn = ir.CurFunc
+		if name.Op() == ir.ONAME {
+			ir.CurFunc.Dcl = append(ir.CurFunc.Dcl, name)
+		}
+	}
+}

src/cmd/compile/internal/noder/scopes.go

@@ -0,0 +1,64 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"strings"
+
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/types2"
+)
+
+// recordScopes populates fn.Parents and fn.Marks based on the scoping
+// information provided by types2.
+func (g *irgen) recordScopes(fn *ir.Func, sig *syntax.FuncType) {
+	scope, ok := g.info.Scopes[sig]
+	if !ok {
+		base.FatalfAt(fn.Pos(), "missing scope for %v", fn)
+	}
+
+	for i, n := 0, scope.NumChildren(); i < n; i++ {
+		g.walkScope(scope.Child(i))
+	}
+
+	g.marker.WriteTo(fn)
+}
+
+func (g *irgen) walkScope(scope *types2.Scope) bool {
+	// types2 doesn't provide a proper API for determining the
+	// lexical element a scope represents, so we have to resort to
+	// string matching. Conveniently though, this allows us to
+	// skip both function types and function literals, neither of
+	// which are interesting to us here.
+	if strings.HasPrefix(scope.String(), "function scope ") {
+		return false
+	}
+
+	g.marker.Push(g.pos(scope))
+
+	haveVars := false
+	for _, name := range scope.Names() {
+		if obj, ok := scope.Lookup(name).(*types2.Var); ok && obj.Name() != "_" {
+			haveVars = true
+			break
+		}
+	}
+
+	for i, n := 0, scope.NumChildren(); i < n; i++ {
+		if g.walkScope(scope.Child(i)) {
+			haveVars = true
+		}
+	}
+
+	if haveVars {
+		g.marker.Pop(g.end(scope))
+	} else {
+		g.marker.Unpush()
+	}
+
+	return haveVars
+}

src/cmd/compile/internal/noder/sizes.go

@@ -0,0 +1,150 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"fmt"
+
+	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
+)
+
+// Code below based on go/types.StdSizes.
+// Intentional differences are marked with "gc:".
+
+type gcSizes struct{}
+
+func (s *gcSizes) Alignof(T types2.Type) int64 {
+	// For arrays and structs, alignment is defined in terms
+	// of alignment of the elements and fields, respectively.
+	switch t := T.Underlying().(type) {
+	case *types2.Array:
+		// spec: "For a variable x of array type: unsafe.Alignof(x)
+		// is the same as unsafe.Alignof(x[0]), but at least 1."
+		return s.Alignof(t.Elem())
+	case *types2.Struct:
+		// spec: "For a variable x of struct type: unsafe.Alignof(x)
+		// is the largest of the values unsafe.Alignof(x.f) for each
+		// field f of x, but at least 1."
+		max := int64(1)
+		for i, nf := 0, t.NumFields(); i < nf; i++ {
+			if a := s.Alignof(t.Field(i).Type()); a > max {
+				max = a
+			}
+		}
+		return max
+	case *types2.Slice, *types2.Interface:
+		// Multiword data structures are effectively structs
+		// in which each element has size PtrSize.
+		return int64(types.PtrSize)
+	case *types2.Basic:
+		// Strings are like slices and interfaces.
+		if t.Info()&types2.IsString != 0 {
+			return int64(types.PtrSize)
+		}
+	}
+	a := s.Sizeof(T) // may be 0
+	// spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1."
+	if a < 1 {
+		return 1
+	}
+	// complex{64,128} are aligned like [2]float{32,64}.
+	if isComplex(T) {
+		a /= 2
+	}
+	if a > int64(types.RegSize) {
+		return int64(types.RegSize)
+	}
+	return a
+}
+
+func isComplex(T types2.Type) bool {
+	basic, ok := T.Underlying().(*types2.Basic)
+	return ok && basic.Info()&types2.IsComplex != 0
+}
+
+func (s *gcSizes) Offsetsof(fields []*types2.Var) []int64 {
+	offsets := make([]int64, len(fields))
+	var o int64
+	for i, f := range fields {
+		typ := f.Type()
+		a := s.Alignof(typ)
+		o = types.Rnd(o, a)
+		offsets[i] = o
+		o += s.Sizeof(typ)
+	}
+	return offsets
+}
+
+func (s *gcSizes) Sizeof(T types2.Type) int64 {
+	switch t := T.Underlying().(type) {
+	case *types2.Basic:
+		k := t.Kind()
+		if int(k) < len(basicSizes) {
+			if s := basicSizes[k]; s > 0 {
+				return int64(s)
+			}
+		}
+		switch k {
+		case types2.String:
+			return int64(types.PtrSize) * 2
+		case types2.Int, types2.Uint, types2.Uintptr, types2.UnsafePointer:
+			return int64(types.PtrSize)
+		}
+		panic(fmt.Sprintf("unimplemented basic: %v (kind %v)", T, k))
+	case *types2.Array:
+		n := t.Len()
+		if n <= 0 {
+			return 0
+		}
+		// n > 0
+		// gc: Size includes alignment padding.
+		return s.Sizeof(t.Elem()) * n
+	case *types2.Slice:
+		return int64(types.PtrSize) * 3
+	case *types2.Struct:
+		n := t.NumFields()
+		if n == 0 {
+			return 0
+		}
+		fields := make([]*types2.Var, n)
+		for i := range fields {
+			fields[i] = t.Field(i)
+		}
+		offsets := s.Offsetsof(fields)
+
+		// gc: The last field of a struct is not allowed to
+		// have size 0.
+		last := s.Sizeof(fields[n-1].Type())
+		if last == 0 {
+			last = 1
+		}
+
+		// gc: Size includes alignment padding.
+		return types.Rnd(offsets[n-1]+last, s.Alignof(t))
+	case *types2.Interface:
+		return int64(types.PtrSize) * 2
+	case *types2.Chan, *types2.Map, *types2.Pointer, *types2.Signature:
+		return int64(types.PtrSize)
+	default:
+		panic(fmt.Sprintf("unimplemented type: %T", t))
+	}
+}
+
+var basicSizes = [...]byte{
+	types2.Bool:       1,
+	types2.Int8:       1,
+	types2.Int16:      2,
+	types2.Int32:      4,
+	types2.Int64:      8,
+	types2.Uint8:      1,
+	types2.Uint16:     2,
+	types2.Uint32:     4,
+	types2.Uint64:     8,
+	types2.Float32:    4,
+	types2.Float64:    8,
+	types2.Complex64:  8,
+	types2.Complex128: 16,
+}

src/cmd/compile/internal/noder/stencil.go

@@ -0,0 +1,460 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file will evolve, since we plan to do a mix of stenciling and passing
+// around dictionaries.
+
+package noder
+
+import (
+	"bytes"
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/internal/src"
+	"fmt"
+	"strings"
+)
+
+// stencil scans functions for instantiated generic function calls and
+// creates the required stencils for simple generic functions.
+func (g *irgen) stencil() {
+	g.target.Stencils = make(map[*types.Sym]*ir.Func)
+	// Don't use range(g.target.Decls) - we also want to process any new instantiated
+	// functions that are created during this loop, in order to handle generic
+	// functions calling other generic functions.
+	for i := 0; i < len(g.target.Decls); i++ {
+		decl := g.target.Decls[i]
+		if decl.Op() != ir.ODCLFUNC || decl.Type().NumTParams() > 0 {
+			// Skip any non-function declarations and skip generic functions
+			continue
+		}
+
+		// For each non-generic function, search for any function calls using
+		// generic function instantiations. (We don't yet handle generic
+		// function instantiations that are not immediately called.)
+		// Then create the needed instantiated function if it hasn't been
+		// created yet, and change to calling that function directly.
+		f := decl.(*ir.Func)
+		modified := false
+		ir.VisitList(f.Body, func(n ir.Node) {
+			if n.Op() != ir.OCALLFUNC || n.(*ir.CallExpr).X.Op() != ir.OFUNCINST {
+				return
+			}
+			// We have found a function call using a generic function
+			// instantiation.
+			call := n.(*ir.CallExpr)
+			inst := call.X.(*ir.InstExpr)
+			sym := makeInstName(inst)
+			//fmt.Printf("Found generic func call in %v to %v\n", f, s)
+			st := g.target.Stencils[sym]
+			if st == nil {
+				// If instantiation doesn't exist yet, create it and add
+				// to the list of decls.
+				st = genericSubst(sym, inst)
+				g.target.Stencils[sym] = st
+				g.target.Decls = append(g.target.Decls, st)
+				if base.Flag.W > 1 {
+					ir.Dump(fmt.Sprintf("\nstenciled %v", st), st)
+				}
+			}
+			// Replace the OFUNCINST with a direct reference to the
+			// new stenciled function
+			call.X = st.Nname
+			if inst.X.Op() == ir.OCALLPART {
+				// When we create an instantiation of a method
+				// call, we make it a function. So, move the
+				// receiver to be the first arg of the function
+				// call.
+				withRecv := make([]ir.Node, len(call.Args)+1)
+				dot := inst.X.(*ir.SelectorExpr)
+				withRecv[0] = dot.X
+				copy(withRecv[1:], call.Args)
+				call.Args = withRecv
+			}
+			modified = true
+		})
+		if base.Flag.W > 1 && modified {
+			ir.Dump(fmt.Sprintf("\nmodified %v", decl), decl)
+		}
+	}
+
+}
+
+// makeInstName makes the unique name for a stenciled generic function, based on
+// the name of the function and the types of the type params.
+func makeInstName(inst *ir.InstExpr) *types.Sym {
+	b := bytes.NewBufferString("#")
+	if meth, ok := inst.X.(*ir.SelectorExpr); ok {
+		// Write the name of the generic method, including receiver type
+		b.WriteString(meth.Selection.Nname.Sym().Name)
+	} else {
+		b.WriteString(inst.X.(*ir.Name).Name().Sym().Name)
+	}
+	b.WriteString("[")
+	for i, targ := range inst.Targs {
+		if i > 0 {
+			b.WriteString(",")
+		}
+		b.WriteString(targ.Type().String())
+	}
+	b.WriteString("]")
+	return typecheck.Lookup(b.String())
+}
+
+// Struct containing info needed for doing the substitution as we create the
+// instantiation of a generic function with specified type arguments.
+type subster struct {
+	newf    *ir.Func // Func node for the new stenciled function
+	tparams []*types.Field
+	targs   []ir.Node
+	// The substitution map from name nodes in the generic function to the
+	// name nodes in the new stenciled function.
+	vars map[*ir.Name]*ir.Name
+	seen map[*types.Type]*types.Type
+}
+
+// genericSubst returns a new function with the specified name. The function is an
+// instantiation of a generic function or method with type params, as specified by
+// inst. For a method with a generic receiver, it returns an instantiated function
+// type where the receiver becomes the first parameter. Otherwise the instantiated
+// method would still need to be transformed by later compiler phases.
+func genericSubst(name *types.Sym, inst *ir.InstExpr) *ir.Func {
+	var nameNode *ir.Name
+	var tparams []*types.Field
+	if selExpr, ok := inst.X.(*ir.SelectorExpr); ok {
+		// Get the type params from the method receiver (after skipping
+		// over any pointer)
+		nameNode = ir.AsNode(selExpr.Selection.Nname).(*ir.Name)
+		recvType := selExpr.Type().Recv().Type
+		if recvType.IsPtr() {
+			recvType = recvType.Elem()
+		}
+		tparams = make([]*types.Field, len(recvType.RParams))
+		for i, rparam := range recvType.RParams {
+			tparams[i] = types.NewField(src.NoXPos, nil, rparam)
+		}
+	} else {
+		nameNode = inst.X.(*ir.Name)
+		tparams = nameNode.Type().TParams().Fields().Slice()
+	}
+	gf := nameNode.Func
+	newf := ir.NewFunc(inst.Pos())
+	newf.Nname = ir.NewNameAt(inst.Pos(), name)
+	newf.Nname.Func = newf
+	newf.Nname.Defn = newf
+	name.Def = newf.Nname
+
+	subst := &subster{
+		newf:    newf,
+		tparams: tparams,
+		targs:   inst.Targs,
+		vars:    make(map[*ir.Name]*ir.Name),
+		seen:    make(map[*types.Type]*types.Type),
+	}
+
+	newf.Dcl = make([]*ir.Name, len(gf.Dcl))
+	for i, n := range gf.Dcl {
+		newf.Dcl[i] = subst.node(n).(*ir.Name)
+	}
+	newf.Body = subst.list(gf.Body)
+
+	// Ugly: we have to insert the Name nodes of the parameters/results into
+	// the function type. The current function type has no Nname fields set,
+	// because it came via conversion from the types2 type.
+	oldt := inst.X.Type()
+	// We also transform a generic method type to the corresponding
+	// instantiated function type where the receiver is the first parameter.
+	newt := types.NewSignature(oldt.Pkg(), nil, nil,
+		subst.fields(ir.PPARAM, append(oldt.Recvs().FieldSlice(), oldt.Params().FieldSlice()...), newf.Dcl),
+		subst.fields(ir.PPARAMOUT, oldt.Results().FieldSlice(), newf.Dcl))
+
+	newf.Nname.Ntype = ir.TypeNode(newt)
+	newf.Nname.SetType(newt)
+	ir.MarkFunc(newf.Nname)
+	newf.SetTypecheck(1)
+	newf.Nname.SetTypecheck(1)
+	// TODO(danscales) - remove later, but avoid confusion for now.
+	newf.Pragma = ir.Noinline
+	return newf
+}
+
+// node is like DeepCopy(), but creates distinct ONAME nodes, and also descends
+// into closures. It substitutes type arguments for type parameters in all the new
+// nodes.
+func (subst *subster) node(n ir.Node) ir.Node {
+	// Use closure to capture all state needed by the ir.EditChildren argument.
+	var edit func(ir.Node) ir.Node
+	edit = func(x ir.Node) ir.Node {
+		switch x.Op() {
+		case ir.OTYPE:
+			return ir.TypeNode(subst.typ(x.Type()))
+
+		case ir.ONAME:
+			name := x.(*ir.Name)
+			if v := subst.vars[name]; v != nil {
+				return v
+			}
+			m := ir.NewNameAt(name.Pos(), name.Sym())
+			t := x.Type()
+			newt := subst.typ(t)
+			m.SetType(newt)
+			m.Curfn = subst.newf
+			m.Class = name.Class
+			m.Func = name.Func
+			subst.vars[name] = m
+			m.SetTypecheck(1)
+			return m
+		case ir.OLITERAL, ir.ONIL:
+			if x.Sym() != nil {
+				return x
+			}
+		}
+		m := ir.Copy(x)
+		if _, isExpr := m.(ir.Expr); isExpr {
+			t := x.Type()
+			if t == nil {
+				// t can be nil only if this is a call that has no
+				// return values, so allow that and otherwise give
+				// an error.
+				if _, isCallExpr := m.(*ir.CallExpr); !isCallExpr {
+					base.Fatalf(fmt.Sprintf("Nil type for %v", x))
+				}
+			} else {
+				m.SetType(subst.typ(x.Type()))
+			}
+		}
+		ir.EditChildren(m, edit)
+
+		if x.Op() == ir.OXDOT {
+			// A method value/call via a type param will have been left as an
+			// OXDOT. When we see this during stenciling, finish the
+			// typechecking, now that we have the instantiated receiver type.
+			// We need to do this now, since the access/selection to the
+			// method for the real type is very different from the selection
+			// for the type param.
+			m.SetTypecheck(0)
+			// m will transform to an OCALLPART
+			typecheck.Expr(m)
+		}
+		if x.Op() == ir.OCALL {
+			call := m.(*ir.CallExpr)
+			if call.X.Op() == ir.OTYPE {
+				// Do typechecking on a conversion, now that we
+				// know the type argument.
+				m.SetTypecheck(0)
+				m = typecheck.Expr(m)
+			} else if call.X.Op() == ir.OCALLPART {
+				// Redo the typechecking, now that we know the method
+				// value is being called.
+				call.X.(*ir.SelectorExpr).SetOp(ir.OXDOT)
+				call.X.SetTypecheck(0)
+				call.X.SetType(nil)
+				typecheck.Callee(call.X)
+				m.SetTypecheck(0)
+				typecheck.Call(m.(*ir.CallExpr))
+			} else {
+				base.FatalfAt(call.Pos(), "Expecting OCALLPART or OTYPE with CALL")
+			}
+		}
+
+		if x.Op() == ir.OCLOSURE {
+			x := x.(*ir.ClosureExpr)
+			// Need to save/duplicate x.Func.Nname,
+			// x.Func.Nname.Ntype, x.Func.Dcl, x.Func.ClosureVars, and
+			// x.Func.Body.
+			oldfn := x.Func
+			newfn := ir.NewFunc(oldfn.Pos())
+			if oldfn.ClosureCalled() {
+				newfn.SetClosureCalled(true)
+			}
+			m.(*ir.ClosureExpr).Func = newfn
+			newfn.Nname = ir.NewNameAt(oldfn.Nname.Pos(), oldfn.Nname.Sym())
+			newfn.Nname.SetType(oldfn.Nname.Type())
+			newfn.Nname.Ntype = subst.node(oldfn.Nname.Ntype).(ir.Ntype)
+			newfn.Body = subst.list(oldfn.Body)
+			// Make shallow copy of the Dcl and ClosureVar slices
+			newfn.Dcl = append([]*ir.Name(nil), oldfn.Dcl...)
+			newfn.ClosureVars = append([]*ir.Name(nil), oldfn.ClosureVars...)
+		}
+		return m
+	}
+
+	return edit(n)
+}
+
+func (subst *subster) list(l []ir.Node) []ir.Node {
+	s := make([]ir.Node, len(l))
+	for i, n := range l {
+		s[i] = subst.node(n)
+	}
+	return s
+}
+
+// tstruct substitutes type params in a structure type
+func (subst *subster) tstruct(t *types.Type) *types.Type {
+	if t.NumFields() == 0 {
+		return t
+	}
+	var newfields []*types.Field
+	for i, f := range t.Fields().Slice() {
+		t2 := subst.typ(f.Type)
+		if t2 != f.Type && newfields == nil {
+			newfields = make([]*types.Field, t.NumFields())
+			for j := 0; j < i; j++ {
+				newfields[j] = t.Field(j)
+			}
+		}
+		if newfields != nil {
+			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
+		}
+	}
+	if newfields != nil {
+		return types.NewStruct(t.Pkg(), newfields)
+	}
+	return t
+
+}
+
+// instTypeName creates a name for an instantiated type, based on the type args
+func instTypeName(name string, targs []ir.Node) string {
+	b := bytes.NewBufferString(name)
+	b.WriteByte('[')
+	for i, targ := range targs {
+		if i > 0 {
+			b.WriteByte(',')
+		}
+		b.WriteString(targ.Type().String())
+	}
+	b.WriteByte(']')
+	return b.String()
+}
+
+// typ computes the type obtained by substituting any type parameter in t with the
+// corresponding type argument in subst. If t contains no type parameters, the
+// result is t; otherwise the result is a new type.
+// It deals with recursive types by using a map and TFORW types.
+// TODO(danscales) deal with recursion besides ptr/struct cases.
+func (subst *subster) typ(t *types.Type) *types.Type {
+	if !t.HasTParam() {
+		return t
+	}
+	if subst.seen[t] != nil {
+		// We've hit a recursive type
+		return subst.seen[t]
+	}
+
+	var newt *types.Type
+	switch t.Kind() {
+	case types.TTYPEPARAM:
+		for i, tp := range subst.tparams {
+			if tp.Type == t {
+				return subst.targs[i].Type()
+			}
+		}
+		return t
+
+	case types.TARRAY:
+		elem := t.Elem()
+		newelem := subst.typ(elem)
+		if newelem != elem {
+			newt = types.NewArray(newelem, t.NumElem())
+		}
+
+	case types.TPTR:
+		elem := t.Elem()
+		// In order to deal with recursive generic types, create a TFORW
+		// type initially and store it in the seen map, so it can be
+		// accessed if this type appears recursively within the type.
+		forw := types.New(types.TFORW)
+		subst.seen[t] = forw
+		newelem := subst.typ(elem)
+		if newelem != elem {
+			forw.SetUnderlying(types.NewPtr(newelem))
+			newt = forw
+		}
+		delete(subst.seen, t)
+
+	case types.TSLICE:
+		elem := t.Elem()
+		newelem := subst.typ(elem)
+		if newelem != elem {
+			newt = types.NewSlice(newelem)
+		}
+
+	case types.TSTRUCT:
+		forw := types.New(types.TFORW)
+		subst.seen[t] = forw
+		newt = subst.tstruct(t)
+		if newt != t {
+			forw.SetUnderlying(newt)
+			newt = forw
+		}
+		delete(subst.seen, t)
+
+	case types.TFUNC:
+		newrecvs := subst.tstruct(t.Recvs())
+		newparams := subst.tstruct(t.Params())
+		newresults := subst.tstruct(t.Results())
+		if newrecvs != t.Recvs() || newparams != t.Params() || newresults != t.Results() {
+			var newrecv *types.Field
+			if newrecvs.NumFields() > 0 {
+				newrecv = newrecvs.Field(0)
+			}
+			newt = types.NewSignature(t.Pkg(), newrecv, nil, newparams.FieldSlice(), newresults.FieldSlice())
+		}
+
+		// TODO: case TCHAN
+		// TODO: case TMAP
+		// TODO: case TINTER
+	}
+	if newt != nil {
+		if t.Sym() != nil {
+			// Since we've substituted types, we also need to change
+			// the defined name of the type, by removing the old types
+			// (in brackets) from the name, and adding the new types.
+			oldname := t.Sym().Name
+			i := strings.Index(oldname, "[")
+			oldname = oldname[:i]
+			sym := t.Sym().Pkg.Lookup(instTypeName(oldname, subst.targs))
+			if sym.Def != nil {
+				// We've already created this instantiated defined type.
+				return sym.Def.Type()
+			}
+			newt.SetSym(sym)
+			sym.Def = ir.TypeNode(newt)
+		}
+		return newt
+	}
+
+	return t
+}
+
+// fields sets the Nname field for the Field nodes inside a type signature, based
+// on the corresponding in/out parameters in dcl. It depends on the in and out
+// parameters being in order in dcl.
+func (subst *subster) fields(class ir.Class, oldfields []*types.Field, dcl []*ir.Name) []*types.Field {
+	newfields := make([]*types.Field, len(oldfields))
+	var i int
+
+	// Find the starting index in dcl of declarations of the class (either
+	// PPARAM or PPARAMOUT).
+	for i = range dcl {
+		if dcl[i].Class == class {
+			break
+		}
+	}
+
+	// Create newfields nodes that are copies of the oldfields nodes, but
+	// with substitution for any type params, and with Nname set to be the node in
+	// Dcl for the corresponding PPARAM or PPARAMOUT.
+	for j := range oldfields {
+		newfields[j] = oldfields[j].Copy()
+		newfields[j].Type = subst.typ(oldfields[j].Type)
+		newfields[j].Nname = dcl[i]
+		i++
+	}
+	return newfields
+}

src/cmd/compile/internal/noder/stmt.go

@@ -0,0 +1,303 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/internal/src"
+)
+
+func (g *irgen) stmts(stmts []syntax.Stmt) []ir.Node {
+	var nodes []ir.Node
+	for _, stmt := range stmts {
+		switch s := g.stmt(stmt).(type) {
+		case nil: // EmptyStmt
+		case *ir.BlockStmt:
+			nodes = append(nodes, s.List...)
+		default:
+			nodes = append(nodes, s)
+		}
+	}
+	return nodes
+}
+
+func (g *irgen) stmt(stmt syntax.Stmt) ir.Node {
+	// TODO(mdempsky): Remove dependency on typecheck.
+	return typecheck.Stmt(g.stmt0(stmt))
+}
+
+func (g *irgen) stmt0(stmt syntax.Stmt) ir.Node {
+	switch stmt := stmt.(type) {
+	case nil, *syntax.EmptyStmt:
+		return nil
+	case *syntax.LabeledStmt:
+		return g.labeledStmt(stmt)
+	case *syntax.BlockStmt:
+		return ir.NewBlockStmt(g.pos(stmt), g.blockStmt(stmt))
+	case *syntax.ExprStmt:
+		x := g.expr(stmt.X)
+		if call, ok := x.(*ir.CallExpr); ok {
+			call.Use = ir.CallUseStmt
+		}
+		return x
+	case *syntax.SendStmt:
+		return ir.NewSendStmt(g.pos(stmt), g.expr(stmt.Chan), g.expr(stmt.Value))
+	case *syntax.DeclStmt:
+		return ir.NewBlockStmt(g.pos(stmt), g.decls(stmt.DeclList))
+
+	case *syntax.AssignStmt:
+		if stmt.Op != 0 && stmt.Op != syntax.Def {
+			op := g.op(stmt.Op, binOps[:])
+			if stmt.Rhs == nil {
+				return IncDec(g.pos(stmt), op, g.expr(stmt.Lhs))
+			}
+			return ir.NewAssignOpStmt(g.pos(stmt), op, g.expr(stmt.Lhs), g.expr(stmt.Rhs))
+		}
+
+		names, lhs := g.assignList(stmt.Lhs, stmt.Op == syntax.Def)
+		rhs := g.exprList(stmt.Rhs)
+
+		if len(lhs) == 1 && len(rhs) == 1 {
+			n := ir.NewAssignStmt(g.pos(stmt), lhs[0], rhs[0])
+			n.Def = initDefn(n, names)
+			return n
+		}
+
+		n := ir.NewAssignListStmt(g.pos(stmt), ir.OAS2, lhs, rhs)
+		n.Def = initDefn(n, names)
+		return n
+
+	case *syntax.BranchStmt:
+		return ir.NewBranchStmt(g.pos(stmt), g.tokOp(int(stmt.Tok), branchOps[:]), g.name(stmt.Label))
+	case *syntax.CallStmt:
+		return ir.NewGoDeferStmt(g.pos(stmt), g.tokOp(int(stmt.Tok), callOps[:]), g.expr(stmt.Call))
+	case *syntax.ReturnStmt:
+		return ir.NewReturnStmt(g.pos(stmt), g.exprList(stmt.Results))
+	case *syntax.IfStmt:
+		return g.ifStmt(stmt)
+	case *syntax.ForStmt:
+		return g.forStmt(stmt)
+	case *syntax.SelectStmt:
+		return g.selectStmt(stmt)
+	case *syntax.SwitchStmt:
+		return g.switchStmt(stmt)
+
+	default:
+		g.unhandled("statement", stmt)
+		panic("unreachable")
+	}
+}
+
+// TODO(mdempsky): Investigate replacing with switch statements or dense arrays.
+
+var branchOps = [...]ir.Op{
+	syntax.Break:       ir.OBREAK,
+	syntax.Continue:    ir.OCONTINUE,
+	syntax.Fallthrough: ir.OFALL,
+	syntax.Goto:        ir.OGOTO,
+}
+
+var callOps = [...]ir.Op{
+	syntax.Defer: ir.ODEFER,
+	syntax.Go:    ir.OGO,
+}
+
+func (g *irgen) tokOp(tok int, ops []ir.Op) ir.Op {
+	// TODO(mdempsky): Validate.
+	return ops[tok]
+}
+
+func (g *irgen) op(op syntax.Operator, ops []ir.Op) ir.Op {
+	// TODO(mdempsky): Validate.
+	return ops[op]
+}
+
+func (g *irgen) assignList(expr syntax.Expr, def bool) ([]*ir.Name, []ir.Node) {
+	if !def {
+		return nil, g.exprList(expr)
+	}
+
+	var exprs []syntax.Expr
+	if list, ok := expr.(*syntax.ListExpr); ok {
+		exprs = list.ElemList
+	} else {
+		exprs = []syntax.Expr{expr}
+	}
+
+	var names []*ir.Name
+	res := make([]ir.Node, len(exprs))
+	for i, expr := range exprs {
+		expr := expr.(*syntax.Name)
+		if expr.Value == "_" {
+			res[i] = ir.BlankNode
+			continue
+		}
+
+		if obj, ok := g.info.Uses[expr]; ok {
+			res[i] = g.obj(obj)
+			continue
+		}
+
+		name, _ := g.def(expr)
+		names = append(names, name)
+		res[i] = name
+	}
+
+	return names, res
+}
+
+// initDefn marks the given names as declared by defn and populates
+// its Init field with ODCL nodes. It then reports whether any names
+// were so declared, which can be used to initialize defn.Def.
+func initDefn(defn ir.InitNode, names []*ir.Name) bool {
+	if len(names) == 0 {
+		return false
+	}
+
+	init := make([]ir.Node, len(names))
+	for i, name := range names {
+		name.Defn = defn
+		init[i] = ir.NewDecl(name.Pos(), ir.ODCL, name)
+	}
+	defn.SetInit(init)
+	return true
+}
+
+func (g *irgen) blockStmt(stmt *syntax.BlockStmt) []ir.Node {
+	return g.stmts(stmt.List)
+}
+
+func (g *irgen) ifStmt(stmt *syntax.IfStmt) ir.Node {
+	init := g.stmt(stmt.Init)
+	n := ir.NewIfStmt(g.pos(stmt), g.expr(stmt.Cond), g.blockStmt(stmt.Then), nil)
+	if stmt.Else != nil {
+		e := g.stmt(stmt.Else)
+		if e.Op() == ir.OBLOCK {
+			e := e.(*ir.BlockStmt)
+			n.Else = e.List
+		} else {
+			n.Else = []ir.Node{e}
+		}
+	}
+	return g.init(init, n)
+}
+
+// unpackTwo returns the first two nodes in list. If list has fewer
+// than 2 nodes, then the missing nodes are replaced with nils.
+func unpackTwo(list []ir.Node) (fst, snd ir.Node) {
+	switch len(list) {
+	case 0:
+		return nil, nil
+	case 1:
+		return list[0], nil
+	default:
+		return list[0], list[1]
+	}
+}
+
+func (g *irgen) forStmt(stmt *syntax.ForStmt) ir.Node {
+	if r, ok := stmt.Init.(*syntax.RangeClause); ok {
+		names, lhs := g.assignList(r.Lhs, r.Def)
+		key, value := unpackTwo(lhs)
+		n := ir.NewRangeStmt(g.pos(r), key, value, g.expr(r.X), g.blockStmt(stmt.Body))
+		n.Def = initDefn(n, names)
+		return n
+	}
+
+	return ir.NewForStmt(g.pos(stmt), g.stmt(stmt.Init), g.expr(stmt.Cond), g.stmt(stmt.Post), g.blockStmt(stmt.Body))
+}
+
+func (g *irgen) selectStmt(stmt *syntax.SelectStmt) ir.Node {
+	body := make([]*ir.CommClause, len(stmt.Body))
+	for i, clause := range stmt.Body {
+		body[i] = ir.NewCommStmt(g.pos(clause), g.stmt(clause.Comm), g.stmts(clause.Body))
+	}
+	return ir.NewSelectStmt(g.pos(stmt), body)
+}
+
+func (g *irgen) switchStmt(stmt *syntax.SwitchStmt) ir.Node {
+	pos := g.pos(stmt)
+	init := g.stmt(stmt.Init)
+
+	var expr ir.Node
+	switch tag := stmt.Tag.(type) {
+	case *syntax.TypeSwitchGuard:
+		var ident *ir.Ident
+		if tag.Lhs != nil {
+			ident = ir.NewIdent(g.pos(tag.Lhs), g.name(tag.Lhs))
+		}
+		expr = ir.NewTypeSwitchGuard(pos, ident, g.expr(tag.X))
+	default:
+		expr = g.expr(tag)
+	}
+
+	body := make([]*ir.CaseClause, len(stmt.Body))
+	for i, clause := range stmt.Body {
+		// Check for an implicit clause variable before
+		// visiting body, because it may contain function
+		// literals that reference it, and then it'll be
+		// associated to the wrong function.
+		//
+		// Also, override its position to the clause's colon, so that
+		// dwarfgen can find the right scope for it later.
+		// TODO(mdempsky): We should probably just store the scope
+		// directly in the ir.Name.
+		var cv *ir.Name
+		if obj, ok := g.info.Implicits[clause]; ok {
+			cv = g.obj(obj)
+			cv.SetPos(g.makeXPos(clause.Colon))
+		}
+		body[i] = ir.NewCaseStmt(g.pos(clause), g.exprList(clause.Cases), g.stmts(clause.Body))
+		body[i].Var = cv
+	}
+
+	return g.init(init, ir.NewSwitchStmt(pos, expr, body))
+}
+
+func (g *irgen) labeledStmt(label *syntax.LabeledStmt) ir.Node {
+	sym := g.name(label.Label)
+	lhs := ir.NewLabelStmt(g.pos(label), sym)
+	ls := g.stmt(label.Stmt)
+
+	// Attach label directly to control statement too.
+	switch ls := ls.(type) {
+	case *ir.ForStmt:
+		ls.Label = sym
+	case *ir.RangeStmt:
+		ls.Label = sym
+	case *ir.SelectStmt:
+		ls.Label = sym
+	case *ir.SwitchStmt:
+		ls.Label = sym
+	}
+
+	l := []ir.Node{lhs}
+	if ls != nil {
+		if ls.Op() == ir.OBLOCK {
+			ls := ls.(*ir.BlockStmt)
+			l = append(l, ls.List...)
+		} else {
+			l = append(l, ls)
+		}
+	}
+	return ir.NewBlockStmt(src.NoXPos, l)
+}
+
+func (g *irgen) init(init ir.Node, stmt ir.InitNode) ir.InitNode {
+	if init != nil {
+		stmt.SetInit([]ir.Node{init})
+	}
+	return stmt
+}
+
+func (g *irgen) name(name *syntax.Name) *types.Sym {
+	if name == nil {
+		return nil
+	}
+	return typecheck.Lookup(name.Value)
+}

src/cmd/compile/internal/noder/types.go

@@ -0,0 +1,328 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"bytes"
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
+	"cmd/internal/src"
+)
+
+func (g *irgen) pkg(pkg *types2.Package) *types.Pkg {
+	switch pkg {
+	case nil:
+		return types.BuiltinPkg
+	case g.self:
+		return types.LocalPkg
+	case types2.Unsafe:
+		return ir.Pkgs.Unsafe
+	}
+	return types.NewPkg(pkg.Path(), pkg.Name())
+}
+
+// typ converts a types2.Type to a types.Type, including caching of previously
+// translated types.
+func (g *irgen) typ(typ types2.Type) *types.Type {
+	// Caching type mappings isn't strictly needed, because typ0 preserves
+	// type identity; but caching minimizes memory blow-up from mapping the
+	// same composite type multiple times, and also plays better with the
+	// current state of cmd/compile (e.g., haphazard calculation of type
+	// sizes).
+	res, ok := g.typs[typ]
+	if !ok {
+		res = g.typ0(typ)
+		g.typs[typ] = res
+
+		// Ensure we calculate the size for all concrete types seen by
+		// the frontend. This is another heavy hammer for something that
+		// should really be the backend's responsibility instead.
+		if res != nil && !res.IsUntyped() && !res.IsFuncArgStruct() {
+			types.CheckSize(res)
+		}
+	}
+	return res
+}
+
+// instTypeName2 creates a name for an instantiated type, base on the type args
+// (given as types2 types).
+func instTypeName2(name string, targs []types2.Type) string {
+	b := bytes.NewBufferString(name)
+	b.WriteByte('[')
+	for i, targ := range targs {
+		if i > 0 {
+			b.WriteByte(',')
+		}
+		b.WriteString(types2.TypeString(targ,
+			func(*types2.Package) string { return "" }))
+	}
+	b.WriteByte(']')
+	return b.String()
+}
+
+// typ0 converts a types2.Type to a types.Type, but doesn't do the caching check
+// at the top level.
+func (g *irgen) typ0(typ types2.Type) *types.Type {
+	switch typ := typ.(type) {
+	case *types2.Basic:
+		return g.basic(typ)
+	case *types2.Named:
+		if typ.TParams() != nil {
+			// typ is an instantiation of a defined (named) generic type.
+			// This instantiation should also be a defined (named) type.
+			// types2 gives us the substituted type in t.Underlying()
+			// The substituted type may or may not still have type
+			// params. We might, for example, be substituting one type
+			// param for another type param.
+
+			if typ.TArgs() == nil {
+				base.Fatalf("In typ0, Targs should be set if TParams is set")
+			}
+
+			// When converted to types.Type, typ must have a name,
+			// based on the names of the type arguments. We need a
+			// name to deal with recursive generic types (and it also
+			// looks better when printing types).
+			instName := instTypeName2(typ.Obj().Name(), typ.TArgs())
+			s := g.pkg(typ.Obj().Pkg()).Lookup(instName)
+			if s.Def != nil {
+				// We have already encountered this instantiation,
+				// so use the type we previously created, since there
+				// must be exactly one instance of a defined type.
+				return s.Def.Type()
+			}
+
+			// Create a forwarding type first and put it in the g.typs
+			// map, in order to deal with recursive generic types.
+			ntyp := types.New(types.TFORW)
+			g.typs[typ] = ntyp
+			ntyp.SetUnderlying(g.typ(typ.Underlying()))
+			ntyp.SetSym(s)
+
+			if ntyp.HasTParam() {
+				// If ntyp still has type params, then we must be
+				// referencing something like 'value[T2]', as when
+				// specifying the generic receiver of a method,
+				// where value was defined as "type value[T any]
+				// ...". Save the type args, which will now be the
+				// new type params of the current type.
+				ntyp.RParams = make([]*types.Type, len(typ.TArgs()))
+				for i, targ := range typ.TArgs() {
+					ntyp.RParams[i] = g.typ(targ)
+				}
+			}
+
+			// Make sure instantiated type can be uniquely found from
+			// the sym
+			s.Def = ir.TypeNode(ntyp)
+			return ntyp
+		}
+		obj := g.obj(typ.Obj())
+		if obj.Op() != ir.OTYPE {
+			base.FatalfAt(obj.Pos(), "expected type: %L", obj)
+		}
+		return obj.Type()
+
+	case *types2.Array:
+		return types.NewArray(g.typ(typ.Elem()), typ.Len())
+	case *types2.Chan:
+		return types.NewChan(g.typ(typ.Elem()), dirs[typ.Dir()])
+	case *types2.Map:
+		return types.NewMap(g.typ(typ.Key()), g.typ(typ.Elem()))
+	case *types2.Pointer:
+		return types.NewPtr(g.typ(typ.Elem()))
+	case *types2.Signature:
+		return g.signature(nil, typ)
+	case *types2.Slice:
+		return types.NewSlice(g.typ(typ.Elem()))
+
+	case *types2.Struct:
+		fields := make([]*types.Field, typ.NumFields())
+		for i := range fields {
+			v := typ.Field(i)
+			f := types.NewField(g.pos(v), g.selector(v), g.typ(v.Type()))
+			f.Note = typ.Tag(i)
+			if v.Embedded() {
+				f.Embedded = 1
+			}
+			fields[i] = f
+		}
+		return types.NewStruct(g.tpkg(typ), fields)
+
+	case *types2.Interface:
+		embeddeds := make([]*types.Field, typ.NumEmbeddeds())
+		for i := range embeddeds {
+			// TODO(mdempsky): Get embedding position.
+			e := typ.EmbeddedType(i)
+			embeddeds[i] = types.NewField(src.NoXPos, nil, g.typ(e))
+		}
+
+		methods := make([]*types.Field, typ.NumExplicitMethods())
+		for i := range methods {
+			m := typ.ExplicitMethod(i)
+			mtyp := g.signature(typecheck.FakeRecv(), m.Type().(*types2.Signature))
+			methods[i] = types.NewField(g.pos(m), g.selector(m), mtyp)
+		}
+
+		return types.NewInterface(g.tpkg(typ), append(embeddeds, methods...))
+
+	case *types2.TypeParam:
+		tp := types.NewTypeParam(g.tpkg(typ), g.typ(typ.Bound()))
+		// Save the name of the type parameter in the sym of the type.
+		tp.SetSym(g.sym(typ.Obj()))
+		return tp
+
+	case *types2.Tuple:
+		// Tuples are used for the type of a function call (i.e. the
+		// return value of the function).
+		if typ == nil {
+			return (*types.Type)(nil)
+		}
+		fields := make([]*types.Field, typ.Len())
+		for i := range fields {
+			fields[i] = g.param(typ.At(i))
+		}
+		t := types.NewStruct(types.LocalPkg, fields)
+		types.CheckSize(t)
+		// Can only set after doing the types.CheckSize()
+		t.StructType().Funarg = types.FunargResults
+		return t
+
+	default:
+		base.FatalfAt(src.NoXPos, "unhandled type: %v (%T)", typ, typ)
+		panic("unreachable")
+	}
+}
+
+func (g *irgen) signature(recv *types.Field, sig *types2.Signature) *types.Type {
+	tparams2 := sig.TParams()
+	tparams := make([]*types.Field, len(tparams2))
+	for i := range tparams {
+		tp := tparams2[i]
+		tparams[i] = types.NewField(g.pos(tp), g.sym(tp), g.typ(tp.Type()))
+	}
+
+	do := func(typ *types2.Tuple) []*types.Field {
+		fields := make([]*types.Field, typ.Len())
+		for i := range fields {
+			fields[i] = g.param(typ.At(i))
+		}
+		return fields
+	}
+	params := do(sig.Params())
+	results := do(sig.Results())
+	if sig.Variadic() {
+		params[len(params)-1].SetIsDDD(true)
+	}
+
+	return types.NewSignature(g.tpkg(sig), recv, tparams, params, results)
+}
+
+func (g *irgen) param(v *types2.Var) *types.Field {
+	return types.NewField(g.pos(v), g.sym(v), g.typ(v.Type()))
+}
+
+func (g *irgen) sym(obj types2.Object) *types.Sym {
+	if name := obj.Name(); name != "" {
+		return g.pkg(obj.Pkg()).Lookup(obj.Name())
+	}
+	return nil
+}
+
+func (g *irgen) selector(obj types2.Object) *types.Sym {
+	pkg, name := g.pkg(obj.Pkg()), obj.Name()
+	if types.IsExported(name) {
+		pkg = types.LocalPkg
+	}
+	return pkg.Lookup(name)
+}
+
+// tpkg returns the package that a function, interface, or struct type
+// expression appeared in.
+//
+// Caveat: For the degenerate types "func()", "interface{}", and
+// "struct{}", tpkg always returns LocalPkg. However, we only need the
+// package information so that go/types can report it via its API, and
+// the reason we fail to return the original package for these
+// particular types is because go/types does *not* report it for
+// them. So in practice this limitation is probably moot.
+func (g *irgen) tpkg(typ types2.Type) *types.Pkg {
+	anyObj := func() types2.Object {
+		switch typ := typ.(type) {
+		case *types2.Signature:
+			if recv := typ.Recv(); recv != nil {
+				return recv
+			}
+			if params := typ.Params(); params.Len() > 0 {
+				return params.At(0)
+			}
+			if results := typ.Results(); results.Len() > 0 {
+				return results.At(0)
+			}
+		case *types2.Struct:
+			if typ.NumFields() > 0 {
+				return typ.Field(0)
+			}
+		case *types2.Interface:
+			if typ.NumExplicitMethods() > 0 {
+				return typ.ExplicitMethod(0)
+			}
+		}
+		return nil
+	}
+
+	if obj := anyObj(); obj != nil {
+		return g.pkg(obj.Pkg())
+	}
+	return types.LocalPkg
+}
+
+func (g *irgen) basic(typ *types2.Basic) *types.Type {
+	switch typ.Name() {
+	case "byte":
+		return types.ByteType
+	case "rune":
+		return types.RuneType
+	}
+	return *basics[typ.Kind()]
+}
+
+var basics = [...]**types.Type{
+	types2.Invalid:        new(*types.Type),
+	types2.Bool:           &types.Types[types.TBOOL],
+	types2.Int:            &types.Types[types.TINT],
+	types2.Int8:           &types.Types[types.TINT8],
+	types2.Int16:          &types.Types[types.TINT16],
+	types2.Int32:          &types.Types[types.TINT32],
+	types2.Int64:          &types.Types[types.TINT64],
+	types2.Uint:           &types.Types[types.TUINT],
+	types2.Uint8:          &types.Types[types.TUINT8],
+	types2.Uint16:         &types.Types[types.TUINT16],
+	types2.Uint32:         &types.Types[types.TUINT32],
+	types2.Uint64:         &types.Types[types.TUINT64],
+	types2.Uintptr:        &types.Types[types.TUINTPTR],
+	types2.Float32:        &types.Types[types.TFLOAT32],
+	types2.Float64:        &types.Types[types.TFLOAT64],
+	types2.Complex64:      &types.Types[types.TCOMPLEX64],
+	types2.Complex128:     &types.Types[types.TCOMPLEX128],
+	types2.String:         &types.Types[types.TSTRING],
+	types2.UnsafePointer:  &types.Types[types.TUNSAFEPTR],
+	types2.UntypedBool:    &types.UntypedBool,
+	types2.UntypedInt:     &types.UntypedInt,
+	types2.UntypedRune:    &types.UntypedRune,
+	types2.UntypedFloat:   &types.UntypedFloat,
+	types2.UntypedComplex: &types.UntypedComplex,
+	types2.UntypedString:  &types.UntypedString,
+	types2.UntypedNil:     &types.Types[types.TNIL],
+}
+
+var dirs = [...]types.ChanDir{
+	types2.SendRecv: types.Cboth,
+	types2.SendOnly: types.Csend,
+	types2.RecvOnly: types.Crecv,
+}

src/cmd/compile/internal/noder/validate.go

@@ -0,0 +1,113 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package noder
+
+import (
+	"go/constant"
+
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
+)
+
+// match reports whether types t1 and t2 are consistent
+// representations for a given expression's type.
+func (g *irgen) match(t1 *types.Type, t2 types2.Type, hasOK bool) bool {
+	tuple, ok := t2.(*types2.Tuple)
+	if !ok {
+		// Not a tuple; can use simple type identity comparison.
+		return types.Identical(t1, g.typ(t2))
+	}
+
+	if hasOK {
+		// For has-ok values, types2 represents the expression's type as
+		// a 2-element tuple, whereas ir just uses the first type and
+		// infers that the second type is boolean.
+		return tuple.Len() == 2 && types.Identical(t1, g.typ(tuple.At(0).Type()))
+	}
+
+	if t1 == nil || tuple == nil {
+		return t1 == nil && tuple == nil
+	}
+	if !t1.IsFuncArgStruct() {
+		return false
+	}
+	if t1.NumFields() != tuple.Len() {
+		return false
+	}
+	for i, result := range t1.FieldSlice() {
+		if !types.Identical(result.Type, g.typ(tuple.At(i).Type())) {
+			return false
+		}
+	}
+	return true
+}
+
+func (g *irgen) validate(n syntax.Node) {
+	switch n := n.(type) {
+	case *syntax.CallExpr:
+		tv := g.info.Types[n.Fun]
+		if tv.IsBuiltin() {
+			switch builtin := n.Fun.(type) {
+			case *syntax.Name:
+				g.validateBuiltin(builtin.Value, n)
+			case *syntax.SelectorExpr:
+				g.validateBuiltin(builtin.Sel.Value, n)
+			default:
+				g.unhandled("builtin", n)
+			}
+		}
+	}
+}
+
+func (g *irgen) validateBuiltin(name string, call *syntax.CallExpr) {
+	switch name {
+	case "Alignof", "Offsetof", "Sizeof":
+		// Check that types2+gcSizes calculates sizes the same
+		// as cmd/compile does.
+
+		got, ok := constant.Int64Val(g.info.Types[call].Value)
+		if !ok {
+			base.FatalfAt(g.pos(call), "expected int64 constant value")
+		}
+
+		want := g.unsafeExpr(name, call.ArgList[0])
+		if got != want {
+			base.FatalfAt(g.pos(call), "got %v from types2, but want %v", got, want)
+		}
+	}
+}
+
+// unsafeExpr evaluates the given unsafe builtin function on arg.
+func (g *irgen) unsafeExpr(name string, arg syntax.Expr) int64 {
+	switch name {
+	case "Alignof":
+		return g.typ(g.info.Types[arg].Type).Alignment()
+	case "Sizeof":
+		return g.typ(g.info.Types[arg].Type).Size()
+	}
+
+	// Offsetof
+
+	sel := arg.(*syntax.SelectorExpr)
+	selection := g.info.Selections[sel]
+
+	typ := g.typ(g.info.Types[sel.X].Type)
+	if typ.IsPtr() {
+		typ = typ.Elem()
+	}
+
+	var offset int64
+	for _, i := range selection.Index() {
+		// Ensure field offsets have been calculated.
+		types.CalcSize(typ)
+
+		f := typ.Field(i)
+		offset += f.Offset
+		typ = f.Type
+	}
+	return offset
+}

src/cmd/compile/internal/reflectdata/alg.go

@@ -289,7 +289,7 @@ func hashfor(t *types.Type) ir.Node {
 
 	n := typecheck.NewName(sym)
 	ir.MarkFunc(n)
-	n.SetType(types.NewSignature(types.NoPkg, nil, []*types.Field{
+	n.SetType(types.NewSignature(types.NoPkg, nil, nil, []*types.Field{
 		types.NewField(base.Pos, nil, types.NewPtr(t)),
 		types.NewField(base.Pos, nil, types.Types[types.TUINTPTR]),
 	}, []*types.Field{
@@ -777,7 +777,7 @@ func hashmem(t *types.Type) ir.Node {
 
 	n := typecheck.NewName(sym)
 	ir.MarkFunc(n)
-	n.SetType(types.NewSignature(types.NoPkg, nil, []*types.Field{
+	n.SetType(types.NewSignature(types.NoPkg, nil, nil, []*types.Field{
 		types.NewField(base.Pos, nil, types.NewPtr(t)),
 		types.NewField(base.Pos, nil, types.Types[types.TUINTPTR]),
 		types.NewField(base.Pos, nil, types.Types[types.TUINTPTR]),

src/cmd/compile/internal/reflectdata/reflect.go

@@ -1288,6 +1288,11 @@ func ITabSym(it *obj.LSym, offset int64) *obj.LSym {
 
 // NeedRuntimeType ensures that a runtime type descriptor is emitted for t.
 func NeedRuntimeType(t *types.Type) {
+	if t.HasTParam() {
+		// Generic types don't have a runtime type descriptor (but will
+		// have a dictionary)
+		return
+	}
 	if _, ok := signatset[t]; !ok {
 		signatset[t] = struct{}{}
 		signatslice = append(signatslice, t)
@@ -1399,7 +1404,7 @@ func WriteBasicTypes() {
 		// The latter is the type of an auto-generated wrapper.
 		writeType(types.NewPtr(types.ErrorType))
 
-		writeType(types.NewSignature(types.NoPkg, nil, []*types.Field{
+		writeType(types.NewSignature(types.NoPkg, nil, nil, []*types.Field{
 			types.NewField(base.Pos, nil, types.ErrorType),
 		}, []*types.Field{
 			types.NewField(base.Pos, nil, types.Types[types.TSTRING]),

src/cmd/compile/internal/syntax/dumper.go

@@ -26,7 +26,7 @@ func Fdump(w io.Writer, n Node) (err error) {
 
 	defer func() {
 		if e := recover(); e != nil {
-			err = e.(localError).err // re-panics if it's not a localError
+			err = e.(writeError).err // re-panics if it's not a writeError
 		}
 	}()
 
@@ -82,16 +82,16 @@ func (p *dumper) Write(data []byte) (n int, err error) {
 	return
 }
 
-// localError wraps locally caught errors so we can distinguish
+// writeError wraps locally caught write errors so we can distinguish
 // them from genuine panics which we don't want to return as errors.
-type localError struct {
+type writeError struct {
 	err error
 }
 
 // printf is a convenience wrapper that takes care of print errors.
 func (p *dumper) printf(format string, args ...interface{}) {
 	if _, err := fmt.Fprintf(p, format, args...); err != nil {
-		panic(localError{err})
+		panic(writeError{err})
 	}
 }
 

src/cmd/compile/internal/syntax/error_test.go

@@ -128,6 +128,10 @@ func testSyntaxErrors(t *testing.T, filename string) {
 	}
 	defer f.Close()
 
+	var mode Mode
+	if strings.HasSuffix(filename, ".go2") {
+		mode = AllowGenerics
+	}
 	ParseFile(filename, func(err error) {
 		e, ok := err.(Error)
 		if !ok {
@@ -162,7 +166,7 @@ func testSyntaxErrors(t *testing.T, filename string) {
 		} else {
 			t.Errorf("%s: unexpected error: %s", orig, e.Msg)
 		}
-	}, nil, 0)
+	}, nil, mode)
 
 	if *print {
 		fmt.Println()

src/cmd/compile/internal/syntax/nodes.go

@@ -37,7 +37,7 @@ type File struct {
 	Pragma   Pragma
 	PkgName  *Name
 	DeclList []Decl
-	Lines    uint
+	EOF      Pos
 	node
 }
 
@@ -56,7 +56,7 @@ type (
 		Group        *Group // nil means not part of a group
 		Pragma       Pragma
 		LocalPkgName *Name     // including "."; nil means no rename present
-		Path         *BasicLit
+		Path         *BasicLit // Path.Bad || Path.Kind == StringLit; nil means no path
 		decl
 	}
 
@@ -77,6 +77,7 @@ type (
 		Group      *Group // nil means not part of a group
 		Pragma     Pragma
 		Name       *Name
+		TParamList []*Field // nil means no type parameters
 		Alias      bool
 		Type       Expr
 		decl
@@ -102,6 +103,7 @@ type (
 		Pragma     Pragma
 		Recv       *Field // nil means regular function
 		Name       *Name
+		TParamList []*Field // nil means no type parameters
 		Type       *FuncType
 		Body       *BlockStmt // nil means no body (forward declaration)
 		decl
@@ -120,6 +122,13 @@ type Group struct {
 // ----------------------------------------------------------------------------
 // Expressions
 
+func NewName(pos Pos, value string) *Name {
+	n := new(Name)
+	n.pos = pos
+	n.Value = value
+	return n
+}
+
 type (
 	Expr interface {
 		Node
@@ -182,6 +191,7 @@ type (
 	}
 
 	// X[Index]
+	// X[T1, T2, ...] (with Ti = Index.(*ListExpr).ElemList[i])
 	IndexExpr struct {
 		X     Expr
 		Index Expr
@@ -272,7 +282,7 @@ type (
 
 	// interface { MethodList[0]; MethodList[1]; ... }
 	InterfaceType struct {
-		MethodList []*Field
+		MethodList []*Field // a field named "type" means a type constraint
 		expr
 	}
 
@@ -357,7 +367,7 @@ type (
 
 	AssignStmt struct {
 		Op       Operator // 0 means no operation
-		Lhs, Rhs Expr     // Rhs == ImplicitOne means Lhs++ (Op == Add) or Lhs-- (Op == Sub)
+		Lhs, Rhs Expr     // Rhs == nil means Lhs++ (Op == Add) or Lhs-- (Op == Sub)
 		simpleStmt
 	}
 

src/cmd/compile/internal/syntax/parser_test.go

@@ -26,10 +26,35 @@ var (
 )
 
 func TestParse(t *testing.T) {
-	ParseFile(*src_, func(err error) { t.Error(err) }, nil, 0)
+	ParseFile(*src_, func(err error) { t.Error(err) }, nil, AllowGenerics)
 }
 
-func TestStdLib(t *testing.T) {
+func TestVerify(t *testing.T) {
+	ast, err := ParseFile(*src_, func(err error) { t.Error(err) }, nil, AllowGenerics)
+	if err != nil {
+		return // error already reported
+	}
+	verifyPrint(t, *src_, ast)
+}
+
+func TestParseGo2(t *testing.T) {
+	dir := filepath.Join(testdata, "go2")
+	list, err := ioutil.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, fi := range list {
+		name := fi.Name()
+		if !fi.IsDir() && !strings.HasPrefix(name, ".") {
+			ParseFile(filepath.Join(dir, name), func(err error) { t.Error(err) }, nil, AllowGenerics)
+		}
+	}
+}
+
+func TestStdLib(t *testing.T)        { testStdLib(t, 0) }
+func TestStdLibGeneric(t *testing.T) { testStdLib(t, AllowGenerics) }
+
+func testStdLib(t *testing.T, mode Mode) {
 	if testing.Short() {
 		t.Skip("skipping test in short mode")
 	}
@@ -68,15 +93,15 @@ func TestStdLib(t *testing.T) {
 				if debug {
 					fmt.Printf("parsing %s\n", filename)
 				}
-				ast, err := ParseFile(filename, nil, nil, 0)
+				ast, err := ParseFile(filename, nil, nil, mode)
 				if err != nil {
 					t.Error(err)
 					return
 				}
 				if *verify {
-					verifyPrint(filename, ast)
+					verifyPrint(t, filename, ast)
 				}
-				results <- parseResult{filename, ast.Lines}
+				results <- parseResult{filename, ast.EOF.Line()}
 			})
 		}
 	}()
@@ -142,12 +167,13 @@ func walkDirs(t *testing.T, dir string, action func(string)) {
 	}
 }
 
-func verifyPrint(filename string, ast1 *File) {
+func verifyPrint(t *testing.T, filename string, ast1 *File) {
 	var buf1 bytes.Buffer
-	_, err := Fprint(&buf1, ast1, true)
+	_, err := Fprint(&buf1, ast1, LineForm)
 	if err != nil {
 		panic(err)
 	}
+	bytes1 := buf1.Bytes()
 
 	ast2, err := Parse(NewFileBase(filename), &buf1, nil, nil, 0)
 	if err != nil {
@@ -155,20 +181,22 @@ func verifyPrint(filename string, ast1 *File) {
 	}
 
 	var buf2 bytes.Buffer
-	_, err = Fprint(&buf2, ast2, true)
+	_, err = Fprint(&buf2, ast2, LineForm)
 	if err != nil {
 		panic(err)
 	}
+	bytes2 := buf2.Bytes()
 
-	if bytes.Compare(buf1.Bytes(), buf2.Bytes()) != 0 {
+	if bytes.Compare(bytes1, bytes2) != 0 {
 		fmt.Printf("--- %s ---\n", filename)
-		fmt.Printf("%s\n", buf1.Bytes())
+		fmt.Printf("%s\n", bytes1)
 		fmt.Println()
 
 		fmt.Printf("--- %s ---\n", filename)
-		fmt.Printf("%s\n", buf2.Bytes())
+		fmt.Printf("%s\n", bytes2)
 		fmt.Println()
-		panic("not equal")
+
+		t.Error("printed syntax trees do not match")
 	}
 }
 

src/cmd/compile/internal/syntax/pos.go

@@ -26,6 +26,7 @@ func MakePos(base *PosBase, line, col uint) Pos { return Pos{base, sat32(line),
 // TODO(gri) IsKnown makes an assumption about linebase < 1.
 //           Maybe we should check for Base() != nil instead.
 
+func (pos Pos) Pos() Pos       { return pos }
 func (pos Pos) IsKnown() bool  { return pos.line > 0 }
 func (pos Pos) Base() *PosBase { return pos.base }
 func (pos Pos) Line() uint     { return uint(pos.line) }

src/cmd/compile/internal/syntax/printer.go

@@ -13,19 +13,28 @@ import (
 	"strings"
 )
 
-// TODO(gri) Consider removing the linebreaks flag from this signature.
+// Form controls print formatting.
-// Its likely rarely used in common cases.
+type Form uint
 
-func Fprint(w io.Writer, x Node, linebreaks bool) (n int, err error) {
+const (
+	_         Form = iota // default
+	LineForm              // use spaces instead of linebreaks where possible
+	ShortForm             // like LineForm but print "…" for non-empty function or composite literal bodies
+)
+
+// Fprint prints node x to w in the specified form.
+// It returns the number of bytes written, and whether there was an error.
+func Fprint(w io.Writer, x Node, form Form) (n int, err error) {
 	p := printer{
 		output:     w,
-		linebreaks: linebreaks,
+		form:       form,
+		linebreaks: form == 0,
 	}
 
 	defer func() {
 		n = p.written
 		if e := recover(); e != nil {
-			err = e.(localError).err // re-panics if it's not a localError
+			err = e.(writeError).err // re-panics if it's not a writeError
 		}
 	}()
 
@@ -35,11 +44,13 @@ func Fprint(w io.Writer, x Node, linebreaks bool) (n int, err error) {
 	return
 }
 
+// String is a convenience functions that prints n in ShortForm
+// and returns the printed string.
 func String(n Node) string {
 	var buf bytes.Buffer
-	_, err := Fprint(&buf, n, false)
+	_, err := Fprint(&buf, n, ShortForm)
 	if err != nil {
-		panic(err) // TODO(gri) print something sensible into buf instead
+		fmt.Fprintf(&buf, "<<< ERROR: %s", err)
 	}
 	return buf.String()
 }
@@ -66,6 +77,7 @@ type whitespace struct {
 type printer struct {
 	output     io.Writer
 	written    int // number of bytes written
+	form       Form
 	linebreaks bool // print linebreaks instead of semis
 
 	indent  int // current indentation level
@@ -81,7 +93,7 @@ func (p *printer) write(data []byte) {
 	n, err := p.output.Write(data)
 	p.written += n
 	if err != nil {
-		panic(localError{err})
+		panic(writeError{err})
 	}
 }
 
@@ -355,18 +367,35 @@ func (p *printer) printRawNode(n Node) {
 		p.print(_Name, n.Value) // _Name requires actual value following immediately
 
 	case *FuncLit:
-		p.print(n.Type, blank, n.Body)
+		p.print(n.Type, blank)
+		if n.Body != nil {
+			if p.form == ShortForm {
+				p.print(_Lbrace)
+				if len(n.Body.List) > 0 {
+					p.print(_Name, "…")
+				}
+				p.print(_Rbrace)
+			} else {
+				p.print(n.Body)
+			}
+		}
 
 	case *CompositeLit:
 		if n.Type != nil {
 			p.print(n.Type)
 		}
 		p.print(_Lbrace)
+		if p.form == ShortForm {
+			if len(n.ElemList) > 0 {
+				p.print(_Name, "…")
+			}
+		} else {
 			if n.NKeys > 0 && n.NKeys == len(n.ElemList) {
 				p.printExprLines(n.ElemList)
 			} else {
 				p.printExprList(n.ElemList)
 			}
+		}
 		p.print(_Rbrace)
 
 	case *ParenExpr:
@@ -450,9 +479,13 @@ func (p *printer) printRawNode(n Node) {
 		}
 		p.print(_Lbrace)
 		if len(n.FieldList) > 0 {
+			if p.linebreaks {
 				p.print(newline, indent)
 				p.printFieldList(n.FieldList, n.TagList)
 				p.print(outdent, newline)
+			} else {
+				p.printFieldList(n.FieldList, n.TagList)
+			}
 		}
 		p.print(_Rbrace)
 
@@ -467,9 +500,13 @@ func (p *printer) printRawNode(n Node) {
 		}
 		p.print(_Lbrace)
 		if len(n.MethodList) > 0 {
+			if p.linebreaks {
 				p.print(newline, indent)
 				p.printMethodList(n.MethodList)
 				p.print(outdent, newline)
+			} else {
+				p.printMethodList(n.MethodList)
+			}
 		}
 		p.print(_Rbrace)
 
@@ -484,7 +521,15 @@ func (p *printer) printRawNode(n Node) {
 		if n.Dir == SendOnly {
 			p.print(_Arrow)
 		}
-		p.print(blank, n.Elem)
+		p.print(blank)
+		if e, _ := n.Elem.(*ChanType); n.Dir == 0 && e != nil && e.Dir == RecvOnly {
+			// don't print chan (<-chan T) as chan <-chan T
+			p.print(_Lparen)
+			p.print(n.Elem)
+			p.print(_Rparen)
+		} else {
+			p.print(n.Elem)
+		}
 
 	// statements
 	case *DeclStmt:
@@ -504,7 +549,7 @@ func (p *printer) printRawNode(n Node) {
 
 	case *AssignStmt:
 		p.print(n.Lhs)
-		if n.Rhs == ImplicitOne {
+		if n.Rhs == nil {
 			// TODO(gri) This is going to break the mayCombine
 			//           check once we enable that again.
 			p.print(n.Op, n.Op) // ++ or --

src/cmd/compile/internal/syntax/printer_test.go

@@ -25,17 +25,60 @@ func TestPrint(t *testing.T) {
 	}
 
 	if ast != nil {
-		Fprint(testOut(), ast, true)
+		Fprint(testOut(), ast, LineForm)
 		fmt.Println()
 	}
 }
 
-func TestPrintString(t *testing.T) {
+type shortBuffer struct {
-	for _, want := range []string{
+	buf []byte
+}
+
+func (w *shortBuffer) Write(data []byte) (n int, err error) {
+	w.buf = append(w.buf, data...)
+	n = len(data)
+	if len(w.buf) > 10 {
+		err = io.ErrShortBuffer
+	}
+	return
+}
+
+func TestPrintError(t *testing.T) {
+	const src = "package p; var x int"
+	ast, err := Parse(nil, strings.NewReader(src), nil, nil, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf shortBuffer
+	_, err = Fprint(&buf, ast, 0)
+	if err == nil || err != io.ErrShortBuffer {
+		t.Errorf("got err = %s, want %s", err, io.ErrShortBuffer)
+	}
+}
+
+var stringTests = []string{
 	"package p",
-		"package p; type _ = int; type T1 = struct{}; type ( _ = *struct{}; T2 = float32 )",
+	"package p; type _ int; type T1 = struct{}; type ( _ *struct{}; T2 = float32 )",
+
+	// channels
+	"package p; type _ chan chan int",
+	"package p; type _ chan (<-chan int)",
+	"package p; type _ chan chan<- int",
+
+	"package p; type _ <-chan chan int",
+	"package p; type _ <-chan <-chan int",
+	"package p; type _ <-chan chan<- int",
+
+	"package p; type _ chan<- chan int",
+	"package p; type _ chan<- <-chan int",
+	"package p; type _ chan<- chan<- int",
+
 	// TODO(gri) expand
-	} {
+}
+
+func TestPrintString(t *testing.T) {
+	for _, want := range stringTests {
 		ast, err := Parse(nil, strings.NewReader(want), nil, nil, 0)
 		if err != nil {
 			t.Error(err)
@@ -53,3 +96,90 @@ func testOut() io.Writer {
 	}
 	return ioutil.Discard
 }
+
+func dup(s string) [2]string { return [2]string{s, s} }
+
+var exprTests = [][2]string{
+	// basic type literals
+	dup("x"),
+	dup("true"),
+	dup("42"),
+	dup("3.1415"),
+	dup("2.71828i"),
+	dup(`'a'`),
+	dup(`"foo"`),
+	dup("`bar`"),
+
+	// func and composite literals
+	dup("func() {}"),
+	dup("[]int{}"),
+	{"func(x int) complex128 { return 0 }", "func(x int) complex128 {…}"},
+	{"[]int{1, 2, 3}", "[]int{…}"},
+
+	// non-type expressions
+	dup("(x)"),
+	dup("x.f"),
+	dup("a[i]"),
+
+	dup("s[:]"),
+	dup("s[i:]"),
+	dup("s[:j]"),
+	dup("s[i:j]"),
+	dup("s[:j:k]"),
+	dup("s[i:j:k]"),
+
+	dup("x.(T)"),
+
+	dup("x.([10]int)"),
+	dup("x.([...]int)"),
+
+	dup("x.(struct{})"),
+	dup("x.(struct{x int; y, z float32; E})"),
+
+	dup("x.(func())"),
+	dup("x.(func(x int))"),
+	dup("x.(func() int)"),
+	dup("x.(func(x, y int, z float32) (r int))"),
+	dup("x.(func(a, b, c int))"),
+	dup("x.(func(x ...T))"),
+
+	dup("x.(interface{})"),
+	dup("x.(interface{m(); n(x int); E})"),
+	dup("x.(interface{m(); n(x int) T; E; F})"),
+
+	dup("x.(map[K]V)"),
+
+	dup("x.(chan E)"),
+	dup("x.(<-chan E)"),
+	dup("x.(chan<- chan int)"),
+	dup("x.(chan<- <-chan int)"),
+	dup("x.(<-chan chan int)"),
+	dup("x.(chan (<-chan int))"),
+
+	dup("f()"),
+	dup("f(x)"),
+	dup("int(x)"),
+	dup("f(x, x + y)"),
+	dup("f(s...)"),
+	dup("f(a, s...)"),
+
+	dup("*x"),
+	dup("&x"),
+	dup("x + y"),
+	dup("x + y << (2 * s)"),
+}
+
+func TestShortString(t *testing.T) {
+	for _, test := range exprTests {
+		src := "package p; var _ = " + test[0]
+		ast, err := Parse(nil, strings.NewReader(src), nil, nil, 0)
+		if err != nil {
+			t.Errorf("%s: %s", test[0], err)
+			continue
+		}
+		x := ast.DeclList[0].(*VarDecl).Values
+		if got := String(x); got != test[1] {
+			t.Errorf("%s: got %s, want %s", test[0], got, test[1])
+		}
+	}
+}

src/cmd/compile/internal/syntax/syntax.go

@@ -16,6 +16,7 @@ type Mode uint
 // Modes supported by the parser.
 const (
 	CheckBranches Mode = 1 << iota // check correct use of labels, break, continue, and goto statements
+	AllowGenerics
 )
 
 // Error describes a syntax error. Error implements the error interface.

src/cmd/compile/internal/syntax/testdata/go2/chans.go2

@@ -0,0 +1,62 @@
+package chans
+
+import "runtime"
+
+// Ranger returns a Sender and a Receiver. The Receiver provides a
+// Next method to retrieve values. The Sender provides a Send method
+// to send values and a Close method to stop sending values. The Next
+// method indicates when the Sender has been closed, and the Send
+// method indicates when the Receiver has been freed.
+//
+// This is a convenient way to exit a goroutine sending values when
+// the receiver stops reading them.
+func Ranger[T any]() (*Sender[T], *Receiver[T]) {
+	c := make(chan T)
+	d := make(chan bool)
+	s := &Sender[T]{values: c, done: d}
+	r := &Receiver[T]{values: c, done: d}
+	runtime.SetFinalizer(r, r.finalize)
+	return s, r
+}
+
+// A sender is used to send values to a Receiver.
+type Sender[T any] struct {
+	values chan<- T
+	done <-chan bool
+}
+
+// Send sends a value to the receiver. It returns whether any more
+// values may be sent; if it returns false the value was not sent.
+func (s *Sender[T]) Send(v T) bool {
+	select {
+	case s.values <- v:
+		return true
+	case <-s.done:
+		return false
+	}
+}
+
+// Close tells the receiver that no more values will arrive.
+// After Close is called, the Sender may no longer be used.
+func (s *Sender[T]) Close() {
+	close(s.values)
+}
+
+// A Receiver receives values from a Sender.
+type Receiver[T any] struct {
+	values <-chan T
+	done chan<- bool
+}
+
+// Next returns the next value from the channel. The bool result
+// indicates whether the value is valid, or whether the Sender has
+// been closed and no more values will be received.
+func (r *Receiver[T]) Next() (T, bool) {
+	v, ok := <-r.values
+	return v, ok
+}
+
+// finalize is a finalizer for the receiver.
+func (r *Receiver[T]) finalize() {
+	close(r.done)
+}

src/cmd/compile/internal/syntax/testdata/go2/linalg.go2

@@ -0,0 +1,83 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linalg
+
+import "math"
+
+// Numeric is type bound that matches any numeric type.
+// It would likely be in a constraints package in the standard library.
+type Numeric interface {
+	type int, int8, int16, int32, int64,
+		uint, uint8, uint16, uint32, uint64, uintptr,
+		float32, float64,
+		complex64, complex128
+}
+
+func DotProduct[T Numeric](s1, s2 []T) T {
+	if len(s1) != len(s2) {
+		panic("DotProduct: slices of unequal length")
+	}
+	var r T
+	for i := range s1 {
+		r += s1[i] * s2[i]
+	}
+	return r
+}
+
+// NumericAbs matches numeric types with an Abs method.
+type NumericAbs[T any] interface {
+	Numeric
+
+	Abs() T
+}
+
+// AbsDifference computes the absolute value of the difference of
+// a and b, where the absolute value is determined by the Abs method.
+func AbsDifference[T NumericAbs[T]](a, b T) T {
+	d := a - b
+	return d.Abs()
+}
+
+// OrderedNumeric is a type bound that matches numeric types that support the < operator.
+type OrderedNumeric interface {
+	type int, int8, int16, int32, int64,
+		uint, uint8, uint16, uint32, uint64, uintptr,
+		float32, float64
+}
+
+// Complex is a type bound that matches the two complex types, which do not have a < operator.
+type Complex interface {
+	type complex64, complex128
+}
+
+// OrderedAbs is a helper type that defines an Abs method for
+// ordered numeric types.
+type OrderedAbs[T OrderedNumeric] T
+
+func (a OrderedAbs[T]) Abs() OrderedAbs[T] {
+	if a < 0 {
+		return -a
+	}
+	return a
+}
+
+// ComplexAbs is a helper type that defines an Abs method for
+// complex types.
+type ComplexAbs[T Complex] T
+
+func (a ComplexAbs[T]) Abs() ComplexAbs[T] {
+	r := float64(real(a))
+	i := float64(imag(a))
+	d := math.Sqrt(r * r + i * i)
+	return ComplexAbs[T](complex(d, 0))
+}
+
+func OrderedAbsDifference[T OrderedNumeric](a, b T) T {
+	return T(AbsDifference(OrderedAbs[T](a), OrderedAbs[T](b)))
+}
+
+func ComplexAbsDifference[T Complex](a, b T) T {
+	return T(AbsDifference(ComplexAbs[T](a), ComplexAbs[T](b)))
+}

src/cmd/compile/internal/syntax/testdata/go2/map.go2

@@ -0,0 +1,113 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package orderedmap provides an ordered map, implemented as a binary tree.
+package orderedmap
+
+// TODO(gri) fix imports for tests
+import "chans" // ERROR could not import
+
+// Map is an ordered map.
+type Map[K, V any] struct {
+	root    *node[K, V]
+	compare func(K, K) int
+}
+
+// node is the type of a node in the binary tree.
+type node[K, V any] struct {
+	key         K
+	val         V
+	left, right *node[K, V]
+}
+
+// New returns a new map.
+func New[K, V any](compare func(K, K) int) *Map[K, V] {
+        return &Map[K, V]{compare: compare}
+}
+
+// find looks up key in the map, and returns either a pointer
+// to the node holding key, or a pointer to the location where
+// such a node would go.
+func (m *Map[K, V]) find(key K) **node[K, V] {
+	pn := &m.root
+	for *pn != nil {
+		switch cmp := m.compare(key, (*pn).key); {
+		case cmp < 0:
+			pn = &(*pn).left
+		case cmp > 0:
+			pn = &(*pn).right
+		default:
+			return pn
+		}
+	}
+	return pn
+}
+
+// Insert inserts a new key/value into the map.
+// If the key is already present, the value is replaced.
+// Returns true if this is a new key, false if already present.
+func (m *Map[K, V]) Insert(key K, val V) bool {
+	pn := m.find(key)
+	if *pn != nil {
+		(*pn).val = val
+		return false
+	}
+        *pn = &node[K, V]{key: key, val: val}
+	return true
+}
+
+// Find returns the value associated with a key, or zero if not present.
+// The found result reports whether the key was found.
+func (m *Map[K, V]) Find(key K) (V, bool) {
+	pn := m.find(key)
+	if *pn == nil {
+		var zero V // see the discussion of zero values, above
+		return zero, false
+	}
+	return (*pn).val, true
+}
+
+// keyValue is a pair of key and value used when iterating.
+type keyValue[K, V any] struct {
+	key K
+	val V
+}
+
+// InOrder returns an iterator that does an in-order traversal of the map.
+func (m *Map[K, V]) InOrder() *Iterator[K, V] {
+	sender, receiver := chans.Ranger[keyValue[K, V]]()
+	var f func(*node[K, V]) bool
+	f = func(n *node[K, V]) bool {
+		if n == nil {
+			return true
+		}
+		// Stop sending values if sender.Send returns false,
+		// meaning that nothing is listening at the receiver end.
+		return f(n.left) &&
+                        sender.Send(keyValue[K, V]{n.key, n.val}) &&
+			f(n.right)
+	}
+	go func() {
+		f(m.root)
+		sender.Close()
+	}()
+	return &Iterator[K, V]{receiver}
+}
+
+// Iterator is used to iterate over the map.
+type Iterator[K, V any] struct {
+	r *chans.Receiver[keyValue[K, V]]
+}
+
+// Next returns the next key and value pair, and a boolean indicating
+// whether they are valid or whether we have reached the end.
+func (it *Iterator[K, V]) Next() (K, V, bool) {
+	keyval, ok := it.r.Next()
+	if !ok {
+		var zerok K
+		var zerov V
+		return zerok, zerov, false
+	}
+	return keyval.key, keyval.val, true
+}

src/cmd/compile/internal/syntax/testdata/go2/map2.go2

@@ -0,0 +1,146 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is like map.go2, but instead if importing chans, it contains
+// the necessary functionality at the end of the file.
+
+// Package orderedmap provides an ordered map, implemented as a binary tree.
+package orderedmap
+
+// Map is an ordered map.
+type Map[K, V any] struct {
+	root    *node[K, V]
+	compare func(K, K) int
+}
+
+// node is the type of a node in the binary tree.
+type node[K, V any] struct {
+	key         K
+	val         V
+	left, right *node[K, V]
+}
+
+// New returns a new map.
+func New[K, V any](compare func(K, K) int) *Map[K, V] {
+        return &Map[K, V]{compare: compare}
+}
+
+// find looks up key in the map, and returns either a pointer
+// to the node holding key, or a pointer to the location where
+// such a node would go.
+func (m *Map[K, V]) find(key K) **node[K, V] {
+	pn := &m.root
+	for *pn != nil {
+		switch cmp := m.compare(key, (*pn).key); {
+		case cmp < 0:
+			pn = &(*pn).left
+		case cmp > 0:
+			pn = &(*pn).right
+		default:
+			return pn
+		}
+	}
+	return pn
+}
+
+// Insert inserts a new key/value into the map.
+// If the key is already present, the value is replaced.
+// Returns true if this is a new key, false if already present.
+func (m *Map[K, V]) Insert(key K, val V) bool {
+	pn := m.find(key)
+	if *pn != nil {
+		(*pn).val = val
+		return false
+	}
+	*pn = &node[K, V]{key: key, val: val}
+	return true
+}
+
+// Find returns the value associated with a key, or zero if not present.
+// The found result reports whether the key was found.
+func (m *Map[K, V]) Find(key K) (V, bool) {
+	pn := m.find(key)
+	if *pn == nil {
+		var zero V // see the discussion of zero values, above
+		return zero, false
+	}
+	return (*pn).val, true
+}
+
+// keyValue is a pair of key and value used when iterating.
+type keyValue[K, V any] struct {
+	key K
+	val V
+}
+
+// InOrder returns an iterator that does an in-order traversal of the map.
+func (m *Map[K, V]) InOrder() *Iterator[K, V] {
+	sender, receiver := chans_Ranger[keyValue[K, V]]()
+	var f func(*node[K, V]) bool
+	f = func(n *node[K, V]) bool {
+		if n == nil {
+			return true
+		}
+		// Stop sending values if sender.Send returns false,
+		// meaning that nothing is listening at the receiver end.
+		return f(n.left) &&
+                        sender.Send(keyValue[K, V]{n.key, n.val}) &&
+			f(n.right)
+	}
+	go func() {
+		f(m.root)
+		sender.Close()
+	}()
+	return &Iterator[K, V]{receiver}
+}
+
+// Iterator is used to iterate over the map.
+type Iterator[K, V any] struct {
+	r *chans_Receiver[keyValue[K, V]]
+}
+
+// Next returns the next key and value pair, and a boolean indicating
+// whether they are valid or whether we have reached the end.
+func (it *Iterator[K, V]) Next() (K, V, bool) {
+	keyval, ok := it.r.Next()
+	if !ok {
+		var zerok K
+		var zerov V
+		return zerok, zerov, false
+	}
+	return keyval.key, keyval.val, true
+}
+
+// chans
+
+func chans_Ranger[T any]() (*chans_Sender[T], *chans_Receiver[T])
+
+// A sender is used to send values to a Receiver.
+type chans_Sender[T any] struct {
+	values chan<- T
+	done <-chan bool
+}
+
+func (s *chans_Sender[T]) Send(v T) bool {
+	select {
+	case s.values <- v:
+		return true
+	case <-s.done:
+		return false
+	}
+}
+
+func (s *chans_Sender[T]) Close() {
+	close(s.values)
+}
+
+type chans_Receiver[T any] struct {
+	values <-chan T
+	done chan<- bool
+}
+
+func (r *chans_Receiver[T]) Next() (T, bool) {
+	v, ok := <-r.values
+	return v, ok
+}

src/cmd/compile/internal/syntax/testdata/go2/slices.go2

@@ -0,0 +1,68 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package slices implements various slice algorithms.
+package slices
+
+// Map turns a []T1 to a []T2 using a mapping function.
+func Map[T1, T2 any](s []T1, f func(T1) T2) []T2 {
+	r := make([]T2, len(s))
+	for i, v := range s {
+		r[i] = f(v)
+	}
+	return r
+}
+
+// Reduce reduces a []T1 to a single value using a reduction function.
+func Reduce[T1, T2 any](s []T1, initializer T2, f func(T2, T1) T2) T2 {
+	r := initializer
+	for _, v := range s {
+		r = f(r, v)
+	}
+	return r
+}
+
+// Filter filters values from a slice using a filter function.
+func Filter[T any](s []T, f func(T) bool) []T {
+	var r []T
+	for _, v := range s {
+		if f(v) {
+			r = append(r, v)
+		}
+	}
+	return r
+}
+
+// Example uses
+
+func limiter(x int) byte {
+	switch {
+	case x < 0:
+		return 0
+	default:
+		return byte(x)
+	case x > 255:
+		return 255
+	}
+}
+
+var input = []int{-4, 68954, 7, 44, 0, -555, 6945}
+var limited1 = Map[int, byte](input, limiter)
+var limited2 = Map(input, limiter) // using type inference
+
+func reducer(x float64, y int) float64 {
+	return x + float64(y)
+}
+
+var reduced1 = Reduce[int, float64](input, 0, reducer)
+var reduced2 = Reduce(input, 1i /* ERROR overflows */, reducer) // using type inference
+var reduced3 = Reduce(input, 1, reducer) // using type inference
+
+func filter(x int) bool {
+	return x&1 != 0
+}
+
+var filtered1 = Filter[int](input, filter)
+var filtered2 = Filter(input, filter) // using type inference
+

src/cmd/compile/internal/syntax/testdata/go2/smoketest.go2

@@ -0,0 +1,83 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains basic generic code snippets.
+
+package p
+
+// type parameter lists
+type B[P any] struct{}
+type _[P interface{}] struct{}
+type _[P B] struct{}
+type _[P B[P]] struct{}
+
+type _[A, B, C any] struct{}
+type _[A, B, C B] struct{}
+type _[A, B, C B[A, B, C]] struct{}
+type _[A1, A2 B1, A3 B2, A4, A5, A6 B3] struct{}
+
+type _[A interface{}] struct{}
+type _[A, B interface{ m() }] struct{}
+
+type _[A, B, C any] struct{}
+
+// in functions
+func _[P any]()
+func _[P interface{}]()
+func _[P B]()
+func _[P B[P]]()
+
+// in methods
+func (T) _[P any]()
+func (T) _[P interface{}]()
+func (T) _[P B]()
+func (T) _[P B[P]]()
+
+// type instantiations
+type _ T[int]
+
+// in expressions
+var _ = T[int]{}
+
+// in embedded types
+type _ struct{ T[int] }
+
+// interfaces
+type _ interface{
+	m()
+	type int
+}
+
+type _ interface{
+	type int, float, string
+	type complex128
+	underlying(underlying underlying) underlying
+}
+
+type _ interface{
+	T
+	T[int]
+}
+
+// tricky cases
+func _(T[P], T[P1, P2])
+func _(a [N]T)
+
+type _ struct{
+	T[P]
+	T[P1, P2]
+	f [N]
+}
+type _ interface{
+	m()
+
+	// generic methods - disabled for now
+	// m[] /* ERROR empty type parameter list */ ()
+	// m[ /* ERROR cannot have type parameters */ P any](P)
+
+	// instantiated types
+	// T[] /* ERROR empty type argument list */ 
+	T[P]
+	T[P1, P2]
+}

src/cmd/compile/internal/syntax/testdata/go2/typeinst.go2

@@ -0,0 +1,60 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type myInt int
+
+// Parameterized type declarations
+
+type T1[P any] P
+
+type T2[P any] struct {
+        f P
+        g int // int should still be in scope chain
+}
+
+type List[P any] []P
+
+// Alias type declarations cannot have type parameters. Syntax error.
+// TODO(gri) Disabled for now as we don't check syntax error here.
+// type A1[P any] = /* ERROR cannot be alias */ P
+
+// But an alias may refer to a generic, uninstantiated type.
+type A2 = List
+var _ A2[int]
+var _ A2 /* ERROR without instantiation */
+
+type A3 = List[int]
+var _ A3
+
+// Parameterized type instantiations
+
+var x int
+type _ x /* ERROR not a type */ [int]
+
+type _ int /* ERROR not a generic type */ [int]
+type _ myInt /* ERROR not a generic type */ [int]
+
+// TODO(gri) better error messages
+type _ T1[int]
+type _ T1[x /* ERROR not a type */ ]
+type _ T1 /* ERROR got 2 arguments but 1 type parameters */ [int, float32]
+
+var _ T2[int] = T2[int]{}
+
+var _ List[int] = []int{1, 2, 3}
+var _ List[[]int] = [][]int{{1, 2, 3}}
+var _ List[List[List[int]]]
+
+// Parameterized types containing parameterized types
+
+type T3[P any] List[P]
+
+var _ T3[int] = T3[int](List[int]{1, 2, 3})
+
+// Self-recursive generic types are not permitted
+
+type self1[P any] self1 /* ERROR illegal cycle */ [P]
+type self2[P any] *self2[P] // this is ok

src/cmd/compile/internal/syntax/testdata/go2/typeinst2.go2

@@ -0,0 +1,256 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type List[E any] []E
+var _ List[List[List[int]]]
+var _ List[List[List[int]]] = []List[List[int]]{}
+
+type (
+	T1[P1 any] struct {
+		f1 T2[P1, float32]
+	}
+
+	T2[P2, P3 any] struct {
+		f2 P2
+		f3 P3
+	}
+)
+
+func _() {
+	var x1 T1[int]
+	var x2 T2[int, float32]
+
+	x1.f1.f2 = 0
+	x1.f1 = x2
+}
+
+type T3[P any] T1[T2[P, P]]
+
+func _() {
+	var x1 T3[int]
+	var x2 T2[int, int]
+	x1.f1.f2 = x2
+}
+
+func f[P any] (x P) List[P] {
+	return List[P]{x}
+}
+
+var (
+	_ []int = f(0)
+	_ []float32 = f[float32](10)
+	_ List[complex128] = f(1i)
+	_ []List[int] = f(List[int]{})
+        _ List[List[int]] = []List[int]{}
+        _ = []List[int]{}
+)
+
+// Parameterized types with methods
+
+func (l List[E]) Head() (_ E, _ bool) {
+	if len(l) > 0 {
+		return l[0], true
+	}
+	return
+}
+
+// A test case for instantiating types with other types (extracted from map.go2)
+
+type Pair[K any] struct {
+	key K
+}
+
+type Receiver[T any] struct {
+	values T
+}
+
+type Iterator[K any] struct {
+	r Receiver[Pair[K]]
+}
+
+func Values [T any] (r Receiver[T]) T {
+        return r.values
+}
+
+func (it Iterator[K]) Next() K {
+        return Values[Pair[K]](it.r).key
+}
+
+// A more complex test case testing type bounds (extracted from linalg.go2 and reduced to essence)
+
+type NumericAbs[T any] interface {
+	Abs() T
+}
+
+func AbsDifference[T NumericAbs[T]](x T)
+
+type OrderedAbs[T any] T
+
+func (a OrderedAbs[T]) Abs() OrderedAbs[T]
+
+func OrderedAbsDifference[T any](x T) {
+	AbsDifference(OrderedAbs[T](x))
+}
+
+// same code, reduced to essence
+
+func g[P interface{ m() P }](x P)
+
+type T4[P any] P
+
+func (_ T4[P]) m() T4[P]
+
+func _[Q any](x Q) {
+	g(T4[Q](x))
+}
+
+// Another test case that caused  problems in the past
+
+type T5[_ interface { a() }, _ interface{}] struct{}
+
+type A[P any] struct{ x P }
+
+func (_ A[P]) a() {}
+
+var _ T5[A[int], int]
+
+// Invoking methods with parameterized receiver types uses
+// type inference to determine the actual type arguments matching
+// the receiver type parameters from the actual receiver argument.
+// Go does implicit address-taking and dereferenciation depending
+// on the actual receiver and the method's receiver type. To make
+// type inference work, the type-checker matches "pointer-ness"
+// of the actual receiver and the method's receiver type.
+// The following code tests this mechanism.
+
+type R1[A any] struct{}
+func (_ R1[A]) vm()
+func (_ *R1[A]) pm()
+
+func _[T any](r R1[T], p *R1[T]) {
+	r.vm()
+	r.pm()
+	p.vm()
+	p.pm()
+}
+
+type R2[A, B any] struct{}
+func (_ R2[A, B]) vm()
+func (_ *R2[A, B]) pm()
+
+func _[T any](r R2[T, int], p *R2[string, T]) {
+	r.vm()
+	r.pm()
+	p.vm()
+	p.pm()
+}
+
+// An interface can (explicitly) declare at most one type list.
+type _ interface {
+	m0()
+	type int, string, bool
+	type /* ERROR multiple type lists */ float32, float64
+	m1()
+	m2()
+	type /* ERROR multiple type lists */ complex64, complex128
+	type /* ERROR multiple type lists */ rune
+}
+
+// Interface type lists may contain each type at most once.
+// (If there are multiple lists, we assume the author intended
+// for them to be all in a single list, and we report the error
+// as well.)
+type _ interface {
+	type int, int /* ERROR duplicate type int */
+	type /* ERROR multiple type lists */ int /* ERROR duplicate type int */
+}
+
+type _ interface {
+	type struct{f int}, struct{g int}, struct /* ERROR duplicate type */ {f int}
+}
+
+// Interface type lists can contain any type, incl. *Named types.
+// Verify that we use the underlying type to compute the operational type.
+type MyInt int
+func add1[T interface{type MyInt}](x T) T {
+	return x + 1
+}
+
+type MyString string
+func double[T interface{type MyInt, MyString}](x T) T {
+	return x + x
+}
+
+// Embedding of interfaces with type lists leads to interfaces
+// with type lists that are the intersection of the embedded
+// type lists.
+
+type E0 interface {
+	type int, bool, string
+}
+
+type E1 interface {
+	type int, float64, string
+}
+
+type E2 interface {
+	type float64
+}
+
+type I0 interface {
+	E0
+}
+
+func f0[T I0]()
+var _ = f0[int]
+var _ = f0[bool]
+var _ = f0[string]
+var _ = f0[float64 /* ERROR does not satisfy I0 */ ]
+
+type I01 interface {
+	E0
+	E1
+}
+
+func f01[T I01]()
+var _ = f01[int]
+var _ = f01[bool /* ERROR does not satisfy I0 */ ]
+var _ = f01[string]
+var _ = f01[float64 /* ERROR does not satisfy I0 */ ]
+
+type I012 interface {
+	E0
+	E1
+	E2
+}
+
+func f012[T I012]()
+var _ = f012[int /* ERROR does not satisfy I012 */ ]
+var _ = f012[bool /* ERROR does not satisfy I012 */ ]
+var _ = f012[string /* ERROR does not satisfy I012 */ ]
+var _ = f012[float64 /* ERROR does not satisfy I012 */ ]
+
+type I12 interface {
+	E1
+	E2
+}
+
+func f12[T I12]()
+var _ = f12[int /* ERROR does not satisfy I12 */ ]
+var _ = f12[bool /* ERROR does not satisfy I12 */ ]
+var _ = f12[string /* ERROR does not satisfy I12 */ ]
+var _ = f12[float64]
+
+type I0_ interface {
+	E0
+	type int
+}
+
+func f0_[T I0_]()
+var _ = f0_[int]
+var _ = f0_[bool /* ERROR does not satisfy I0_ */ ]
+var _ = f0_[string /* ERROR does not satisfy I0_ */ ]
+var _ = f0_[float64 /* ERROR does not satisfy I0_ */ ]

src/cmd/compile/internal/syntax/testdata/go2/typeparams.go2

@@ -0,0 +1,451 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// import "io" // for type assertion tests
+
+// The predeclared identifier "any" is only visible as a constraint
+// in a type parameter list.
+var _ any // ERROR undeclared
+func _[_ any /* ok here */ , _ interface{any /* ERROR undeclared */ }](any /* ERROR undeclared */ ) {
+        var _ any /* ERROR undeclared */
+}
+
+func identity[T any](x T) T { return x }
+
+func _[_ any](x int) int
+func _[T any](T /* ERROR redeclared */ T)()
+func _[T, T /* ERROR redeclared */ any]()
+
+func reverse[T any](list []T) []T {
+        rlist := make([]T, len(list))
+        i := len(list)
+        for _, x := range list {
+                i--
+                rlist[i] = x
+        }
+        return rlist
+}
+
+var _ = reverse /* ERROR cannot use generic function reverse */
+var _ = reverse[int, float32 /* ERROR got 2 type arguments */ ] ([]int{1, 2, 3})
+var _ = reverse[int]([ /* ERROR cannot use */ ]float32{1, 2, 3})
+var f = reverse[chan int]
+var _ = f(0 /* ERROR cannot convert 0 .* to \[\]chan int */ )
+
+func swap[A, B any](a A, b B) (B, A) { return b, a }
+
+var _ = swap /* ERROR single value is expected */ [int, float32](1, 2)
+var f32, i = swap[int, float32](swap(float32, int)(1, 2))
+var _ float32 = f32
+var _ int = i
+
+func swapswap[A, B any](a A, b B) (A, B) {
+        return swap[B, A](b, a)
+}
+
+type F[A, B any] func(A, B) (B, A)
+
+func min[T interface{ type int }](x, y T) T {
+        if x < y {
+                return x
+        }
+        return y
+}
+
+func _[T interface{type int, float32}](x, y T) bool { return x < y }
+func _[T any](x, y T) bool { return x /* ERROR cannot compare */ < y }
+func _[T interface{type int, float32, bool}](x, y T) bool { return x /* ERROR cannot compare */ < y }
+
+func _[T C1[T]](x, y T) bool { return x /* ERROR cannot compare */ < y }
+func _[T C2[T]](x, y T) bool { return x < y }
+
+type C1[T any] interface{}
+type C2[T any] interface{ type int, float32 }
+
+func new[T any]() *T {
+        var x T
+        return &x
+}
+
+var _ = new /* ERROR cannot use generic function new */
+var _ *int = new[int]()
+
+func _[T any](map[T /* ERROR invalid map key type T \(missing comparable constraint\) */]int) // w/o constraint we don't know if T is comparable
+
+func f1[T1 any](struct{T1}) int
+var _ = f1(int)(struct{T1}{})
+type T1 = int
+
+func f2[t1 any](struct{t1; x float32}) int
+var _ = f2(t1)(struct{t1; x float32}{})
+type t1 = int
+
+
+func f3[A, B, C any](A, struct{x B}, func(A, struct{x B}, *C)) int
+
+var _ = f3[int, rune, bool](1, struct{x rune}{}, nil)
+
+// indexing
+
+func _[T any] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ type int }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ type string }] (x T, i int) { _ = x[i] }
+func _[T interface{ type []int }] (x T, i int) { _ = x[i] }
+func _[T interface{ type [10]int, *[20]int, map[string]int }] (x T, i int) { _ = x[i] }
+func _[T interface{ type string, []byte }] (x T, i int) { _ = x[i] }
+func _[T interface{ type []int, [1]rune }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ type string, []rune }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+
+// slicing
+// TODO(gri) implement this
+
+func _[T interface{ type string }] (x T, i, j, k int) { _ = x /* ERROR invalid operation */ [i:j:k] }
+
+// len/cap built-ins
+
+func _[T any](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ type int }](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ type string, []byte, int }](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ type string }](x T) { _ = len(x) }
+func _[T interface{ type [10]int }](x T) { _ = len(x) }
+func _[T interface{ type []byte }](x T) { _ = len(x) }
+func _[T interface{ type map[int]int }](x T) { _ = len(x) }
+func _[T interface{ type chan int }](x T) { _ = len(x) }
+func _[T interface{ type string, []byte, chan int }](x T) { _ = len(x) }
+
+func _[T any](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type string, []byte, int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type string }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type [10]int }](x T) { _ = cap(x) }
+func _[T interface{ type []byte }](x T) { _ = cap(x) }
+func _[T interface{ type map[int]int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type chan int }](x T) { _ = cap(x) }
+func _[T interface{ type []byte, chan int }](x T) { _ = cap(x) }
+
+// range iteration
+
+func _[T interface{}](x T) {
+        for range x /* ERROR cannot range */ {}
+}
+
+func _[T interface{ type string, []string }](x T) {
+        for range x {}
+        for i := range x { _ = i }
+        for i, _ := range x { _ = i }
+        for i, e := range x /* ERROR must have the same element type */ { _ = i }
+        for _, e := range x /* ERROR must have the same element type */ {}
+        var e rune
+        _ = e
+        for _, (e) = range x /* ERROR must have the same element type */ {}
+}
+
+
+func _[T interface{ type string, []rune, map[int]rune }](x T) {
+        for _, e := range x { _ = e }
+        for i, e := range x { _ = i; _ = e }
+}
+
+func _[T interface{ type string, []rune, map[string]rune }](x T) {
+        for _, e := range x { _ = e }
+        for i, e := range x /* ERROR must have the same key type */ { _ = e }
+}
+
+func _[T interface{ type string, chan int }](x T) {
+        for range x {}
+        for i := range x { _ = i }
+        for i, _ := range x { _ = i } // TODO(gri) should get an error here: channels only return one value
+}
+
+func _[T interface{ type string, chan<-int }](x T) {
+        for i := range x /* ERROR send-only channel */ { _ = i }
+}
+
+// type inference checks
+
+var _ = new() /* ERROR cannot infer T */
+
+func f4[A, B, C any](A, B) C
+
+var _ = f4(1, 2) /* ERROR cannot infer C */
+var _ = f4[int, float32, complex128](1, 2)
+
+func f5[A, B, C any](A, []*B, struct{f []C}) int
+
+var _ = f5[int, float32, complex128](0, nil, struct{f []complex128}{})
+var _ = f5(0, nil, struct{f []complex128}{}) // ERROR cannot infer
+var _ = f5(0, []*float32{new[float32]()}, struct{f []complex128}{})
+
+func f6[A any](A, []A) int
+
+var _ = f6(0, nil)
+
+func f6nil[A any](A) int
+
+var _ = f6nil(nil) // ERROR cannot infer
+
+// type inference with variadic functions
+
+func f7[T any](...T) T
+
+var _ int = f7() /* ERROR cannot infer T */
+var _ int = f7(1)
+var _ int = f7(1, 2)
+var _ int = f7([]int{}...)
+var _ int = f7 /* ERROR cannot use */ ([]float64{}...)
+var _ float64 = f7([]float64{}...)
+var _ = f7[float64](1, 2.3)
+var _ = f7(float64(1), 2.3)
+var _ = f7(1, 2.3 /* ERROR does not match */ )
+var _ = f7(1.2, 3 /* ERROR does not match */ )
+
+func f8[A, B any](A, B, ...B) int
+
+var _ = f8(1) /* ERROR not enough arguments */
+var _ = f8(1, 2.3)
+var _ = f8(1, 2.3, 3.4, 4.5)
+var _ = f8(1, 2.3, 3.4, 4 /* ERROR does not match */ )
+var _ = f8(int, float64)(1, 2.3, 3.4, 4)
+
+var _ = f8(int, float64)(0, 0, nil...) // test case for #18268
+
+// init functions cannot have type parameters
+
+func init() {}
+func init[/* ERROR func init must have no type parameters */ _ any]() {}
+func init[/* ERROR func init must have no type parameters */ P any]() {}
+
+type T struct {}
+
+func (T) m1() {}
+// The type checker accepts method type parameters if configured accordingly.
+func (T) m2[_ any]() {}
+func (T) m3[P any]() {}
+
+// type inference across parameterized types
+
+type S1[P any] struct { f P }
+
+func f9[P any](x S1[P])
+
+func _() {
+        f9[int](S1[int]{42})
+	f9(S1[int]{42})
+}
+
+type S2[A, B, C any] struct{}
+
+func f10[X, Y, Z any](a S2[X, int, Z], b S2[X, Y, bool])
+
+func _[P any]() {
+        f10[int, float32, string](S2[int, int, string]{}, S2[int, float32, bool]{})
+        f10(S2[int, int, string]{}, S2[int, float32, bool]{})
+        f10(S2[P, int, P]{}, S2[P, float32, bool]{})
+}
+
+// corner case for type inference
+// (was bug: after instanting f11, the type-checker didn't mark f11 as non-generic)
+
+func f11[T any]()
+
+func _() {
+	f11[int]()
+}
+
+// the previous example was extracted from
+
+func f12[T interface{m() T}]()
+
+type A[T any] T
+
+func (a A[T]) m() A[T]
+
+func _[T any]() {
+	f12(A[T])()
+}
+
+// method expressions
+
+func (_ S1[P]) m()
+
+func _() {
+	m := S1[int].m
+	m(struct { f int }{42})
+}
+
+func _[T any] (x T) {
+        m := S1[T].m
+        m(S1[T]{x})
+}
+
+// type parameters in methods (generalization)
+
+type R0 struct{}
+
+func (R0) _[T any](x T)
+func (R0 /* ERROR invalid receiver */ ) _[R0 any]() // scope of type parameters starts at "func"
+
+type R1[A, B any] struct{}
+
+func (_ R1[A, B]) m0(A, B)
+func (_ R1[A, B]) m1[T any](A, B, T) T
+func (_ R1 /* ERROR not a generic type */ [R1, _]) _()
+func (_ R1[A, B]) _[A /* ERROR redeclared */ any](B)
+
+func _() {
+        var r R1[int, string]
+        r.m1[rune](42, "foo", 'a')
+        r.m1[rune](42, "foo", 1.2 /* ERROR truncated to rune */)
+        r.m1(42, "foo", 1.2) // using type inference
+        var _ float64 = r.m1(42, "foo", 1.2)
+}
+
+type I1[A any] interface {
+        m1(A)
+}
+
+var _ I1[int] = r1[int]{}
+
+type r1[T any] struct{}
+
+func (_ r1[T]) m1(T)
+
+type I2[A, B any] interface {
+        m1(A)
+        m2(A) B
+}
+
+var _ I2[int, float32] = R2[int, float32]{}
+
+type R2[P, Q any] struct{}
+
+func (_ R2[X, Y]) m1(X)
+func (_ R2[X, Y]) m2(X) Y
+
+// type assertions and type switches over generic types
+// NOTE: These are currently disabled because it's unclear what the correct
+// approach is, and one can always work around by assigning the variable to
+// an interface first.
+
+// // ReadByte1 corresponds to the ReadByte example in the draft design.
+// func ReadByte1[T io.Reader](r T) (byte, error) {
+// 	if br, ok := r.(io.ByteReader); ok {
+// 		return br.ReadByte()
+// 	}
+// 	var b [1]byte
+// 	_, err := r.Read(b[:])
+// 	return b[0], err
+// }
+//
+// // ReadBytes2 is like ReadByte1 but uses a type switch instead.
+// func ReadByte2[T io.Reader](r T) (byte, error) {
+//         switch br := r.(type) {
+//         case io.ByteReader:
+//                 return br.ReadByte()
+//         }
+// 	var b [1]byte
+// 	_, err := r.Read(b[:])
+// 	return b[0], err
+// }
+//
+// // type assertions and type switches over generic types are strict
+// type I3 interface {
+//         m(int)
+// }
+//
+// type I4 interface {
+//         m() int // different signature from I3.m
+// }
+//
+// func _[T I3](x I3, p T) {
+//         // type assertions and type switches over interfaces are not strict
+//         _ = x.(I4)
+//         switch x.(type) {
+//         case I4:
+//         }
+// 
+//         // type assertions and type switches over generic types are strict
+//         _ = p /* ERROR cannot have dynamic type I4 */.(I4)
+//         switch p.(type) {
+//         case I4 /* ERROR cannot have dynamic type I4 */ :
+//         }
+// }
+
+// type assertions and type switches over generic types lead to errors for now
+
+func _[T any](x T) {
+	_ = x /* ERROR not an interface */ .(int)
+	switch x /* ERROR not an interface */ .(type) {
+	}
+
+	// work-around
+	var t interface{} = x
+	_ = t.(int)
+	switch t.(type) {
+	}
+}
+
+func _[T interface{type int}](x T) {
+	_ = x /* ERROR not an interface */ .(int)
+	switch x /* ERROR not an interface */ .(type) {
+	}
+
+	// work-around
+	var t interface{} = x
+	_ = t.(int)
+	switch t.(type) {
+	}
+}
+
+// error messages related to type bounds mention those bounds
+type C[P any] interface{}
+
+func _[P C[P]] (x P) {
+	x.m /* ERROR x.m undefined */ ()
+}
+
+type I interface {}
+
+func _[P I] (x P) {
+	x.m /* ERROR interface I has no method m */ ()
+}
+
+func _[P interface{}] (x P) {
+	x.m /* ERROR type bound for P has no method m */ ()
+}
+
+func _[P any] (x P) {
+	x.m /* ERROR type bound for P has no method m */ ()
+}
+
+// automatic distinguishing between array and generic types
+// NOTE: Disabled when using unified parameter list syntax.
+/*
+const P = 10
+type A1 [P]byte
+func _(a A1) {
+        assert(len(a) == 10)
+}
+
+type A2 [P]struct{
+        f [P]byte
+}
+func _(a A2) {
+        assert(len(a) == 10)
+        assert(len(a[0].f) == 10)
+}
+
+type A3 [P]func(x [P]A3)
+func _(a A3) {
+        assert(len(a) == 10)
+}
+
+type T2[P] struct{ P }
+var _ T2[int]
+
+type T3[P] func(P)
+var _ T3[int]
+*/

src/cmd/compile/internal/syntax/testdata/tparams.go2

@@ -0,0 +1,22 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type t[ /* ERROR type parameters must be named */ a, b] struct{}
+type t[a t, b t, /* ERROR type parameters must be named */ c] struct{}
+type t struct {
+	t [n]byte
+	t[a]
+	t[a, b]
+}
+type t interface {
+	t[a]
+	m /* ERROR method cannot have type parameters */ [_ _, /* ERROR mixed */ _]()
+	t[a, b]
+}
+
+func f[ /* ERROR empty type parameter list */ ]()
+func f[ /* ERROR type parameters must be named */ a, b]()
+func f[a t, b t, /* ERROR type parameters must be named */ c]()

src/cmd/compile/internal/syntax/testing.go

@@ -0,0 +1,72 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements testing support.
+
+package syntax
+
+import (
+	"io"
+	"regexp"
+	"strings"
+)
+
+// CommentsDo parses the given source and calls the provided handler for each
+// comment or error. If the text provided to handler starts with a '/' it is
+// the comment text; otherwise it is the error message.
+func CommentsDo(src io.Reader, handler func(line, col uint, text string)) {
+	var s scanner
+	s.init(src, handler, comments)
+	for s.tok != _EOF {
+		s.next()
+	}
+}
+
+// ERROR comments must start with text `ERROR "msg"` or `ERROR msg`.
+// Space around "msg" or msg is ignored.
+var errRx = regexp.MustCompile(`^ *ERROR *"?([^"]*)"?`)
+
+// ErrorMap collects all comments with comment text of the form
+// `ERROR "msg"` or `ERROR msg` from the given src and returns them
+// as []Error lists in a map indexed by line number. The position
+// for each Error is the position of the token immediately preceding
+// the comment, the Error message is the message msg extracted from
+// the comment, with all errors that are on the same line collected
+// in a slice. If there is no preceding token (the `ERROR` comment
+// appears in the beginning of the file), then the recorded position
+// is unknown (line, col = 0, 0). If there are no ERROR comments, the
+// result is nil.
+func ErrorMap(src io.Reader) (errmap map[uint][]Error) {
+	// position of previous token
+	var base *PosBase
+	var prev struct{ line, col uint }
+
+	var s scanner
+	s.init(src, func(_, _ uint, text string) {
+		if text[0] != '/' {
+			return // error, ignore
+		}
+		if text[1] == '*' {
+			text = text[:len(text)-2] // strip trailing */
+		}
+		if s := errRx.FindStringSubmatch(text[2:]); len(s) == 2 {
+			pos := MakePos(base, prev.line, prev.col)
+			err := Error{pos, strings.TrimSpace(s[1])}
+			if errmap == nil {
+				errmap = make(map[uint][]Error)
+			}
+			errmap[prev.line] = append(errmap[prev.line], err)
+		}
+	}, comments)
+
+	for s.tok != _EOF {
+		s.next()
+		if s.tok == _Semi && s.lit != "semicolon" {
+			continue // ignore automatically inserted semicolons
+		}
+		prev.line, prev.col = s.line, s.col
+	}
+
+	return
+}

src/cmd/compile/internal/syntax/testing_test.go

@@ -0,0 +1,45 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syntax
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestErrorMap(t *testing.T) {
+	const src = `/* ERROR 0:0 */ /* ERROR "0:0" */ // ERROR 0:0
+// ERROR "0:0"
+x /* ERROR 3:1 */                // ignore automatically inserted semicolon here
+/* ERROR 3:1 */                  // position of x on previous line
+   x /* ERROR 5:4 */ ;           // do not ignore this semicolon
+/* ERROR 5:22 */                 // position of ; on previous line
+	package /* ERROR 7:2 */  // indented with tab
+        import  /* ERROR 8:9 */  // indented with blanks
+`
+	m := ErrorMap(strings.NewReader(src))
+	got := 0 // number of errors found
+	for line, errlist := range m {
+		for _, err := range errlist {
+			if err.Pos.Line() != line {
+				t.Errorf("%v: got map line %d; want %d", err, err.Pos.Line(), line)
+				continue
+			}
+			// err.Pos.Line() == line
+			msg := fmt.Sprintf("%d:%d", line, err.Pos.Col())
+			if err.Msg != msg {
+				t.Errorf("%v: got msg %q; want %q", err, err.Msg, msg)
+				continue
+			}
+		}
+		got += len(errlist)
+	}
+
+	want := strings.Count(src, "ERROR")
+	if got != want {
+		t.Errorf("ErrorMap got %d errors; want %d", got, want)
+	}
+}

src/cmd/compile/internal/syntax/walk.go

@@ -0,0 +1,318 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements syntax tree walking.
+
+package syntax
+
+import "fmt"
+
+// Walk traverses a syntax in pre-order: It starts by calling f(root);
+// root must not be nil. If f returns false (== "continue"), Walk calls
+// f recursively for each of the non-nil children of that node; if f
+// returns true (== "stop"), Walk does not traverse the respective node's
+// children.
+// Some nodes may be shared among multiple parent nodes (e.g., types in
+// field lists such as type T in "a, b, c T"). Such shared nodes are
+// walked multiple times.
+// TODO(gri) Revisit this design. It may make sense to walk those nodes
+//           only once. A place where this matters is types2.TestResolveIdents.
+func Walk(root Node, f func(Node) bool) {
+	w := walker{f}
+	w.node(root)
+}
+
+type walker struct {
+	f func(Node) bool
+}
+
+func (w *walker) node(n Node) {
+	if n == nil {
+		panic("invalid syntax tree: nil node")
+	}
+
+	if w.f(n) {
+		return
+	}
+
+	switch n := n.(type) {
+	// packages
+	case *File:
+		w.node(n.PkgName)
+		w.declList(n.DeclList)
+
+	// declarations
+	case *ImportDecl:
+		if n.LocalPkgName != nil {
+			w.node(n.LocalPkgName)
+		}
+		w.node(n.Path)
+
+	case *ConstDecl:
+		w.nameList(n.NameList)
+		if n.Type != nil {
+			w.node(n.Type)
+		}
+		if n.Values != nil {
+			w.node(n.Values)
+		}
+
+	case *TypeDecl:
+		w.node(n.Name)
+		w.fieldList(n.TParamList)
+		w.node(n.Type)
+
+	case *VarDecl:
+		w.nameList(n.NameList)
+		if n.Type != nil {
+			w.node(n.Type)
+		}
+		if n.Values != nil {
+			w.node(n.Values)
+		}
+
+	case *FuncDecl:
+		if n.Recv != nil {
+			w.node(n.Recv)
+		}
+		w.node(n.Name)
+		w.fieldList(n.TParamList)
+		w.node(n.Type)
+		if n.Body != nil {
+			w.node(n.Body)
+		}
+
+	// expressions
+	case *BadExpr: // nothing to do
+	case *Name: // nothing to do
+	case *BasicLit: // nothing to do
+
+	case *CompositeLit:
+		if n.Type != nil {
+			w.node(n.Type)
+		}
+		w.exprList(n.ElemList)
+
+	case *KeyValueExpr:
+		w.node(n.Key)
+		w.node(n.Value)
+
+	case *FuncLit:
+		w.node(n.Type)
+		w.node(n.Body)
+
+	case *ParenExpr:
+		w.node(n.X)
+
+	case *SelectorExpr:
+		w.node(n.X)
+		w.node(n.Sel)
+
+	case *IndexExpr:
+		w.node(n.X)
+		w.node(n.Index)
+
+	case *SliceExpr:
+		w.node(n.X)
+		for _, x := range n.Index {
+			if x != nil {
+				w.node(x)
+			}
+		}
+
+	case *AssertExpr:
+		w.node(n.X)
+		w.node(n.Type)
+
+	case *TypeSwitchGuard:
+		if n.Lhs != nil {
+			w.node(n.Lhs)
+		}
+		w.node(n.X)
+
+	case *Operation:
+		w.node(n.X)
+		if n.Y != nil {
+			w.node(n.Y)
+		}
+
+	case *CallExpr:
+		w.node(n.Fun)
+		w.exprList(n.ArgList)
+
+	case *ListExpr:
+		w.exprList(n.ElemList)
+
+	// types
+	case *ArrayType:
+		if n.Len != nil {
+			w.node(n.Len)
+		}
+		w.node(n.Elem)
+
+	case *SliceType:
+		w.node(n.Elem)
+
+	case *DotsType:
+		w.node(n.Elem)
+
+	case *StructType:
+		w.fieldList(n.FieldList)
+		for _, t := range n.TagList {
+			if t != nil {
+				w.node(t)
+			}
+		}
+
+	case *Field:
+		if n.Name != nil {
+			w.node(n.Name)
+		}
+		w.node(n.Type)
+
+	case *InterfaceType:
+		w.fieldList(n.MethodList)
+
+	case *FuncType:
+		w.fieldList(n.ParamList)
+		w.fieldList(n.ResultList)
+
+	case *MapType:
+		w.node(n.Key)
+		w.node(n.Value)
+
+	case *ChanType:
+		w.node(n.Elem)
+
+	// statements
+	case *EmptyStmt: // nothing to do
+
+	case *LabeledStmt:
+		w.node(n.Label)
+		w.node(n.Stmt)
+
+	case *BlockStmt:
+		w.stmtList(n.List)
+
+	case *ExprStmt:
+		w.node(n.X)
+
+	case *SendStmt:
+		w.node(n.Chan)
+		w.node(n.Value)
+
+	case *DeclStmt:
+		w.declList(n.DeclList)
+
+	case *AssignStmt:
+		w.node(n.Lhs)
+		if n.Rhs != nil {
+			w.node(n.Rhs)
+		}
+
+	case *BranchStmt:
+		if n.Label != nil {
+			w.node(n.Label)
+		}
+		// Target points to nodes elsewhere in the syntax tree
+
+	case *CallStmt:
+		w.node(n.Call)
+
+	case *ReturnStmt:
+		if n.Results != nil {
+			w.node(n.Results)
+		}
+
+	case *IfStmt:
+		if n.Init != nil {
+			w.node(n.Init)
+		}
+		w.node(n.Cond)
+		w.node(n.Then)
+		if n.Else != nil {
+			w.node(n.Else)
+		}
+
+	case *ForStmt:
+		if n.Init != nil {
+			w.node(n.Init)
+		}
+		if n.Cond != nil {
+			w.node(n.Cond)
+		}
+		if n.Post != nil {
+			w.node(n.Post)
+		}
+		w.node(n.Body)
+
+	case *SwitchStmt:
+		if n.Init != nil {
+			w.node(n.Init)
+		}
+		if n.Tag != nil {
+			w.node(n.Tag)
+		}
+		for _, s := range n.Body {
+			w.node(s)
+		}
+
+	case *SelectStmt:
+		for _, s := range n.Body {
+			w.node(s)
+		}
+
+	// helper nodes
+	case *RangeClause:
+		if n.Lhs != nil {
+			w.node(n.Lhs)
+		}
+		w.node(n.X)
+
+	case *CaseClause:
+		if n.Cases != nil {
+			w.node(n.Cases)
+		}
+		w.stmtList(n.Body)
+
+	case *CommClause:
+		if n.Comm != nil {
+			w.node(n.Comm)
+		}
+		w.stmtList(n.Body)
+
+	default:
+		panic(fmt.Sprintf("internal error: unknown node type %T", n))
+	}
+}
+
+func (w *walker) declList(list []Decl) {
+	for _, n := range list {
+		w.node(n)
+	}
+}
+
+func (w *walker) exprList(list []Expr) {
+	for _, n := range list {
+		w.node(n)
+	}
+}
+
+func (w *walker) stmtList(list []Stmt) {
+	for _, n := range list {
+		w.node(n)
+	}
+}
+
+func (w *walker) nameList(list []*Name) {
+	for _, n := range list {
+		w.node(n)
+	}
+}
+
+func (w *walker) fieldList(list []*Field) {
+	for _, n := range list {
+		w.node(n)
+	}
+}

src/cmd/compile/internal/test/abiutilsaux_test.go

@@ -57,7 +57,7 @@ func mkFuncType(rcvr *types.Type, ins []*types.Type, outs []*types.Type) *types.
 	if rcvr != nil {
 		rf = mkParamResultField(rcvr, q, ir.PPARAM)
 	}
-	return types.NewSignature(types.LocalPkg, rf, inf, outf)
+	return types.NewSignature(types.LocalPkg, rf, nil, inf, outf)
 }
 
 type expectedDump struct {

src/cmd/compile/internal/test/dep_test.go

@@ -19,7 +19,12 @@ func TestDeps(t *testing.T) {
 	for _, dep := range strings.Fields(strings.Trim(string(out), "[]")) {
 		switch dep {
 		case "go/build", "go/scanner":
-			t.Errorf("undesired dependency on %q", dep)
+			// cmd/compile/internal/importer introduces a dependency
+			// on go/build and go/token; cmd/compile/internal/ uses
+			// go/constant which uses go/token in its API. Once we
+			// got rid of those dependencies, enable this check again.
+			// TODO(gri) fix this
+			// t.Errorf("undesired dependency on %q", dep)
 		}
 	}
 }

src/cmd/compile/internal/typecheck/builtin.go

@@ -205,139 +205,153 @@ var runtimeDecls = [...]struct {
 	{"arm64HasATOMICS", varTag, 6},
 }
 
+// Not inlining this function removes a significant chunk of init code.
+//go:noinline
+func newSig(params, results []*types.Field) *types.Type {
+	return types.NewSignature(types.NoPkg, nil, nil, params, results)
+}
+
+func params(tlist ...*types.Type) []*types.Field {
+	flist := make([]*types.Field, len(tlist))
+	for i, typ := range tlist {
+		flist[i] = types.NewField(src.NoXPos, nil, typ)
+	}
+	return flist
+}
+
 func runtimeTypes() []*types.Type {
 	var typs [132]*types.Type
 	typs[0] = types.ByteType
 	typs[1] = types.NewPtr(typs[0])
 	typs[2] = types.Types[types.TANY]
 	typs[3] = types.NewPtr(typs[2])
-	typs[4] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[3])})
+	typs[4] = newSig(params(typs[1]), params(typs[3]))
 	typs[5] = types.Types[types.TUINTPTR]
 	typs[6] = types.Types[types.TBOOL]
 	typs[7] = types.Types[types.TUNSAFEPTR]
-	typs[8] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[5]), types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[6])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[7])})
+	typs[8] = newSig(params(typs[5], typs[1], typs[6]), params(typs[7]))
-	typs[9] = types.NewSignature(types.NoPkg, nil, nil, nil)
+	typs[9] = newSig(nil, nil)
 	typs[10] = types.Types[types.TINTER]
-	typs[11] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[10])}, nil)
+	typs[11] = newSig(params(typs[10]), nil)
 	typs[12] = types.Types[types.TINT32]
 	typs[13] = types.NewPtr(typs[12])
-	typs[14] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[13])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[10])})
+	typs[14] = newSig(params(typs[13]), params(typs[10]))
 	typs[15] = types.Types[types.TINT]
-	typs[16] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[15])}, nil)
+	typs[16] = newSig(params(typs[15], typs[15]), nil)
 	typs[17] = types.Types[types.TUINT]
-	typs[18] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[17]), types.NewField(src.NoXPos, nil, typs[15])}, nil)
+	typs[18] = newSig(params(typs[17], typs[15]), nil)
-	typs[19] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])}, nil)
+	typs[19] = newSig(params(typs[6]), nil)
 	typs[20] = types.Types[types.TFLOAT64]
-	typs[21] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[20])}, nil)
+	typs[21] = newSig(params(typs[20]), nil)
 	typs[22] = types.Types[types.TINT64]
-	typs[23] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[22])}, nil)
+	typs[23] = newSig(params(typs[22]), nil)
 	typs[24] = types.Types[types.TUINT64]
-	typs[25] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[24])}, nil)
+	typs[25] = newSig(params(typs[24]), nil)
 	typs[26] = types.Types[types.TCOMPLEX128]
-	typs[27] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[26])}, nil)
+	typs[27] = newSig(params(typs[26]), nil)
 	typs[28] = types.Types[types.TSTRING]
-	typs[29] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])}, nil)
+	typs[29] = newSig(params(typs[28]), nil)
-	typs[30] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[2])}, nil)
+	typs[30] = newSig(params(typs[2]), nil)
-	typs[31] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[5])}, nil)
+	typs[31] = newSig(params(typs[5]), nil)
 	typs[32] = types.NewArray(typs[0], 32)
 	typs[33] = types.NewPtr(typs[32])
-	typs[34] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[33]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[34] = newSig(params(typs[33], typs[28], typs[28]), params(typs[28]))
-	typs[35] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[33]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[35] = newSig(params(typs[33], typs[28], typs[28], typs[28]), params(typs[28]))
-	typs[36] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[33]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[36] = newSig(params(typs[33], typs[28], typs[28], typs[28], typs[28]), params(typs[28]))
-	typs[37] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[33]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[37] = newSig(params(typs[33], typs[28], typs[28], typs[28], typs[28], typs[28]), params(typs[28]))
 	typs[38] = types.NewSlice(typs[28])
-	typs[39] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[33]), types.NewField(src.NoXPos, nil, typs[38])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[39] = newSig(params(typs[33], typs[38]), params(typs[28]))
-	typs[40] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[28])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[15])})
+	typs[40] = newSig(params(typs[28], typs[28]), params(typs[15]))
 	typs[41] = types.NewArray(typs[0], 4)
 	typs[42] = types.NewPtr(typs[41])
-	typs[43] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[42]), types.NewField(src.NoXPos, nil, typs[22])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[43] = newSig(params(typs[42], typs[22]), params(typs[28]))
-	typs[44] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[33]), types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[15])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[44] = newSig(params(typs[33], typs[1], typs[15]), params(typs[28]))
-	typs[45] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[15])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[45] = newSig(params(typs[1], typs[15]), params(typs[28]))
 	typs[46] = types.RuneType
 	typs[47] = types.NewSlice(typs[46])
-	typs[48] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[33]), types.NewField(src.NoXPos, nil, typs[47])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])})
+	typs[48] = newSig(params(typs[33], typs[47]), params(typs[28]))
 	typs[49] = types.NewSlice(typs[0])
-	typs[50] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[33]), types.NewField(src.NoXPos, nil, typs[28])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[49])})
+	typs[50] = newSig(params(typs[33], typs[28]), params(typs[49]))
 	typs[51] = types.NewArray(typs[46], 32)
 	typs[52] = types.NewPtr(typs[51])
-	typs[53] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[52]), types.NewField(src.NoXPos, nil, typs[28])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[47])})
+	typs[53] = newSig(params(typs[52], typs[28]), params(typs[47]))
-	typs[54] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[5])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[15])})
+	typs[54] = newSig(params(typs[3], typs[15], typs[3], typs[15], typs[5]), params(typs[15]))
-	typs[55] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[28]), types.NewField(src.NoXPos, nil, typs[15])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[46]), types.NewField(src.NoXPos, nil, typs[15])})
+	typs[55] = newSig(params(typs[28], typs[15]), params(typs[46], typs[15]))
-	typs[56] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[28])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[15])})
+	typs[56] = newSig(params(typs[28]), params(typs[15]))
-	typs[57] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[2])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[2])})
+	typs[57] = newSig(params(typs[1], typs[2]), params(typs[2]))
-	typs[58] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[2])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[7])})
+	typs[58] = newSig(params(typs[2]), params(typs[7]))
-	typs[59] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[3])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[2])})
+	typs[59] = newSig(params(typs[1], typs[3]), params(typs[2]))
-	typs[60] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[2])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[2]), types.NewField(src.NoXPos, nil, typs[6])})
+	typs[60] = newSig(params(typs[1], typs[2]), params(typs[2], typs[6]))
-	typs[61] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[1])}, nil)
+	typs[61] = newSig(params(typs[1], typs[1], typs[1]), nil)
-	typs[62] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1])}, nil)
+	typs[62] = newSig(params(typs[1]), nil)
 	typs[63] = types.NewPtr(typs[5])
-	typs[64] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[63]), types.NewField(src.NoXPos, nil, typs[7]), types.NewField(src.NoXPos, nil, typs[7])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])})
+	typs[64] = newSig(params(typs[63], typs[7], typs[7]), params(typs[6]))
 	typs[65] = types.Types[types.TUINT32]
-	typs[66] = types.NewSignature(types.NoPkg, nil, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[65])})
+	typs[66] = newSig(nil, params(typs[65]))
 	typs[67] = types.NewMap(typs[2], typs[2])
-	typs[68] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[22]), types.NewField(src.NoXPos, nil, typs[3])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[67])})
+	typs[68] = newSig(params(typs[1], typs[22], typs[3]), params(typs[67]))
-	typs[69] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[3])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[67])})
+	typs[69] = newSig(params(typs[1], typs[15], typs[3]), params(typs[67]))
-	typs[70] = types.NewSignature(types.NoPkg, nil, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[67])})
+	typs[70] = newSig(nil, params(typs[67]))
-	typs[71] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67]), types.NewField(src.NoXPos, nil, typs[3])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[3])})
+	typs[71] = newSig(params(typs[1], typs[67], typs[3]), params(typs[3]))
-	typs[72] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67]), types.NewField(src.NoXPos, nil, typs[2])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[3])})
+	typs[72] = newSig(params(typs[1], typs[67], typs[2]), params(typs[3]))
-	typs[73] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67]), types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[1])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[3])})
+	typs[73] = newSig(params(typs[1], typs[67], typs[3], typs[1]), params(typs[3]))
-	typs[74] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67]), types.NewField(src.NoXPos, nil, typs[3])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[6])})
+	typs[74] = newSig(params(typs[1], typs[67], typs[3]), params(typs[3], typs[6]))
-	typs[75] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67]), types.NewField(src.NoXPos, nil, typs[2])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[6])})
+	typs[75] = newSig(params(typs[1], typs[67], typs[2]), params(typs[3], typs[6]))
-	typs[76] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67]), types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[1])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[6])})
+	typs[76] = newSig(params(typs[1], typs[67], typs[3], typs[1]), params(typs[3], typs[6]))
-	typs[77] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67]), types.NewField(src.NoXPos, nil, typs[3])}, nil)
+	typs[77] = newSig(params(typs[1], typs[67], typs[3]), nil)
-	typs[78] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67]), types.NewField(src.NoXPos, nil, typs[2])}, nil)
+	typs[78] = newSig(params(typs[1], typs[67], typs[2]), nil)
-	typs[79] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[3])}, nil)
+	typs[79] = newSig(params(typs[3]), nil)
-	typs[80] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[67])}, nil)
+	typs[80] = newSig(params(typs[1], typs[67]), nil)
 	typs[81] = types.NewChan(typs[2], types.Cboth)
-	typs[82] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[22])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[81])})
+	typs[82] = newSig(params(typs[1], typs[22]), params(typs[81]))
-	typs[83] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[15])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[81])})
+	typs[83] = newSig(params(typs[1], typs[15]), params(typs[81]))
 	typs[84] = types.NewChan(typs[2], types.Crecv)
-	typs[85] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[84]), types.NewField(src.NoXPos, nil, typs[3])}, nil)
+	typs[85] = newSig(params(typs[84], typs[3]), nil)
-	typs[86] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[84]), types.NewField(src.NoXPos, nil, typs[3])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])})
+	typs[86] = newSig(params(typs[84], typs[3]), params(typs[6]))
 	typs[87] = types.NewChan(typs[2], types.Csend)
-	typs[88] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[87]), types.NewField(src.NoXPos, nil, typs[3])}, nil)
+	typs[88] = newSig(params(typs[87], typs[3]), nil)
 	typs[89] = types.NewArray(typs[0], 3)
 	typs[90] = types.NewStruct(types.NoPkg, []*types.Field{types.NewField(src.NoXPos, Lookup("enabled"), typs[6]), types.NewField(src.NoXPos, Lookup("pad"), typs[89]), types.NewField(src.NoXPos, Lookup("needed"), typs[6]), types.NewField(src.NoXPos, Lookup("cgo"), typs[6]), types.NewField(src.NoXPos, Lookup("alignme"), typs[24])})
-	typs[91] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[3])}, nil)
+	typs[91] = newSig(params(typs[1], typs[3], typs[3]), nil)
-	typs[92] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[3])}, nil)
+	typs[92] = newSig(params(typs[1], typs[3]), nil)
-	typs[93] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[15])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[15])})
+	typs[93] = newSig(params(typs[1], typs[3], typs[15], typs[3], typs[15]), params(typs[15]))
-	typs[94] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[87]), types.NewField(src.NoXPos, nil, typs[3])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])})
+	typs[94] = newSig(params(typs[87], typs[3]), params(typs[6]))
-	typs[95] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[84])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])})
+	typs[95] = newSig(params(typs[3], typs[84]), params(typs[6]))
 	typs[96] = types.NewPtr(typs[6])
-	typs[97] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[96]), types.NewField(src.NoXPos, nil, typs[84])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])})
+	typs[97] = newSig(params(typs[3], typs[96], typs[84]), params(typs[6]))
-	typs[98] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[63])}, nil)
+	typs[98] = newSig(params(typs[63]), nil)
-	typs[99] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[63]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[6])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[6])})
+	typs[99] = newSig(params(typs[1], typs[1], typs[63], typs[15], typs[15], typs[6]), params(typs[15], typs[6]))
-	typs[100] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[15])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[7])})
+	typs[100] = newSig(params(typs[1], typs[15], typs[15]), params(typs[7]))
-	typs[101] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[22]), types.NewField(src.NoXPos, nil, typs[22])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[7])})
+	typs[101] = newSig(params(typs[1], typs[22], typs[22]), params(typs[7]))
-	typs[102] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[15]), types.NewField(src.NoXPos, nil, typs[7])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[7])})
+	typs[102] = newSig(params(typs[1], typs[15], typs[15], typs[7]), params(typs[7]))
 	typs[103] = types.NewSlice(typs[2])
-	typs[104] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[103]), types.NewField(src.NoXPos, nil, typs[15])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[103])})
+	typs[104] = newSig(params(typs[1], typs[103], typs[15]), params(typs[103]))
-	typs[105] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[5])}, nil)
+	typs[105] = newSig(params(typs[3], typs[3], typs[5]), nil)
-	typs[106] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[7]), types.NewField(src.NoXPos, nil, typs[5])}, nil)
+	typs[106] = newSig(params(typs[7], typs[5]), nil)
-	typs[107] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[5])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])})
+	typs[107] = newSig(params(typs[3], typs[3], typs[5]), params(typs[6]))
-	typs[108] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[3]), types.NewField(src.NoXPos, nil, typs[3])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])})
+	typs[108] = newSig(params(typs[3], typs[3]), params(typs[6]))
-	typs[109] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[7]), types.NewField(src.NoXPos, nil, typs[7])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[6])})
+	typs[109] = newSig(params(typs[7], typs[7]), params(typs[6]))
-	typs[110] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[7]), types.NewField(src.NoXPos, nil, typs[5]), types.NewField(src.NoXPos, nil, typs[5])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[5])})
+	typs[110] = newSig(params(typs[7], typs[5], typs[5]), params(typs[5]))
-	typs[111] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[7]), types.NewField(src.NoXPos, nil, typs[5])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[5])})
+	typs[111] = newSig(params(typs[7], typs[5]), params(typs[5]))
-	typs[112] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[22]), types.NewField(src.NoXPos, nil, typs[22])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[22])})
+	typs[112] = newSig(params(typs[22], typs[22]), params(typs[22]))
-	typs[113] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[24]), types.NewField(src.NoXPos, nil, typs[24])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[24])})
+	typs[113] = newSig(params(typs[24], typs[24]), params(typs[24]))
-	typs[114] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[20])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[22])})
+	typs[114] = newSig(params(typs[20]), params(typs[22]))
-	typs[115] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[20])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[24])})
+	typs[115] = newSig(params(typs[20]), params(typs[24]))
-	typs[116] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[20])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[65])})
+	typs[116] = newSig(params(typs[20]), params(typs[65]))
-	typs[117] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[22])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[20])})
+	typs[117] = newSig(params(typs[22]), params(typs[20]))
-	typs[118] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[24])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[20])})
+	typs[118] = newSig(params(typs[24]), params(typs[20]))
-	typs[119] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[65])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[20])})
+	typs[119] = newSig(params(typs[65]), params(typs[20]))
-	typs[120] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[26]), types.NewField(src.NoXPos, nil, typs[26])}, []*types.Field{types.NewField(src.NoXPos, nil, typs[26])})
+	typs[120] = newSig(params(typs[26], typs[26]), params(typs[26]))
-	typs[121] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[5]), types.NewField(src.NoXPos, nil, typs[5])}, nil)
+	typs[121] = newSig(params(typs[5], typs[5]), nil)
-	typs[122] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[5]), types.NewField(src.NoXPos, nil, typs[5]), types.NewField(src.NoXPos, nil, typs[5])}, nil)
+	typs[122] = newSig(params(typs[5], typs[5], typs[5]), nil)
-	typs[123] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[7]), types.NewField(src.NoXPos, nil, typs[1]), types.NewField(src.NoXPos, nil, typs[5])}, nil)
+	typs[123] = newSig(params(typs[7], typs[1], typs[5]), nil)
 	typs[124] = types.NewSlice(typs[7])
-	typs[125] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[7]), types.NewField(src.NoXPos, nil, typs[124])}, nil)
+	typs[125] = newSig(params(typs[7], typs[124]), nil)
 	typs[126] = types.Types[types.TUINT8]
-	typs[127] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[126]), types.NewField(src.NoXPos, nil, typs[126])}, nil)
+	typs[127] = newSig(params(typs[126], typs[126]), nil)
 	typs[128] = types.Types[types.TUINT16]
-	typs[129] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[128]), types.NewField(src.NoXPos, nil, typs[128])}, nil)
+	typs[129] = newSig(params(typs[128], typs[128]), nil)
-	typs[130] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[65]), types.NewField(src.NoXPos, nil, typs[65])}, nil)
+	typs[130] = newSig(params(typs[65], typs[65]), nil)
-	typs[131] = types.NewSignature(types.NoPkg, nil, []*types.Field{types.NewField(src.NoXPos, nil, typs[24]), types.NewField(src.NoXPos, nil, typs[24])}, nil)
+	typs[131] = newSig(params(typs[24], typs[24]), nil)
 	return typs[:]
 }

src/cmd/compile/internal/typecheck/const.go

@@ -449,7 +449,7 @@ func EvalConst(n ir.Node) ir.Node {
 		n := n.(*ir.BinaryExpr)
 		nl, nr := n.X, n.Y
 		if nl.Op() == ir.OLITERAL && nr.Op() == ir.OLITERAL {
-			// shiftBound from go/types; "so we can express smallestFloat64"
+			// shiftBound from go/types; "so we can express smallestFloat64" (see issue #44057)
 			const shiftBound = 1023 - 1 + 52
 			s, ok := constant.Uint64Val(nr.Val())
 			if !ok || s > shiftBound {

src/cmd/compile/internal/typecheck/dcl.go

@@ -470,5 +470,5 @@ func NewMethodType(sig *types.Type, recv *types.Type) *types.Type {
 		results[i] = types.NewField(base.Pos, nil, t.Type)
 	}
 
-	return types.NewSignature(types.LocalPkg, nil, params, results)
+	return types.NewSignature(types.LocalPkg, nil, nil, params, results)
 }

src/cmd/compile/internal/typecheck/iimport.go

@@ -607,7 +607,7 @@ func (r *importReader) signature(recv *types.Field) *types.Type {
 	if n := len(params); n > 0 {
 		params[n-1].SetIsDDD(r.bool())
 	}
-	return types.NewSignature(r.currPkg, recv, params, results)
+	return types.NewSignature(r.currPkg, recv, nil, params, results)
 }
 
 func (r *importReader) paramList() []*types.Field {

src/cmd/compile/internal/typecheck/mkbuiltin.go

@@ -102,6 +102,21 @@ func mkbuiltin(w io.Writer, name string) {
 	}
 	fmt.Fprintln(w, "}")
 
+	fmt.Fprintln(w, `
+// Not inlining this function removes a significant chunk of init code.
+//go:noinline
+func newSig(params, results []*types.Field) *types.Type {
+	return types.NewSignature(types.NoPkg, nil, nil, params, results)
+}
+
+func params(tlist ...*types.Type) []*types.Field {
+	flist := make([]*types.Field, len(tlist))
+	for i, typ := range tlist {
+		flist[i] = types.NewField(src.NoXPos, nil, typ)
+	}
+	return flist
+}`)
+
 	fmt.Fprintln(w)
 	fmt.Fprintf(w, "func %sTypes() []*types.Type {\n", name)
 	fmt.Fprintf(w, "var typs [%d]*types.Type\n", len(interner.typs))
@@ -169,7 +184,7 @@ func (i *typeInterner) mktype(t ast.Expr) string {
 		}
 		return fmt.Sprintf("types.NewChan(%s, %s)", i.subtype(t.Value), dir)
 	case *ast.FuncType:
-		return fmt.Sprintf("types.NewSignature(types.NoPkg, nil, %s, %s)", i.fields(t.Params, false), i.fields(t.Results, false))
+		return fmt.Sprintf("newSig(%s, %s)", i.fields(t.Params, false), i.fields(t.Results, false))
 	case *ast.InterfaceType:
 		if len(t.Methods.List) != 0 {
 			log.Fatal("non-empty interfaces unsupported")
@@ -192,23 +207,28 @@ func (i *typeInterner) fields(fl *ast.FieldList, keepNames bool) string {
 	if fl == nil || len(fl.List) == 0 {
 		return "nil"
 	}
+
 	var res []string
 	for _, f := range fl.List {
 		typ := i.subtype(f.Type)
 		if len(f.Names) == 0 {
-			res = append(res, fmt.Sprintf("types.NewField(src.NoXPos, nil, %s)", typ))
+			res = append(res, typ)
 		} else {
 			for _, name := range f.Names {
 				if keepNames {
 					res = append(res, fmt.Sprintf("types.NewField(src.NoXPos, Lookup(%q), %s)", name.Name, typ))
 				} else {
-					res = append(res, fmt.Sprintf("types.NewField(src.NoXPos, nil, %s)", typ))
+					res = append(res, typ)
 				}
 			}
 		}
 	}
+
+	if keepNames {
 		return fmt.Sprintf("[]*types.Field{%s}", strings.Join(res, ", "))
 	}
+	return fmt.Sprintf("params(%s)", strings.Join(res, ", "))
+}
 
 func intconst(e ast.Expr) int64 {
 	switch e := e.(type) {

src/cmd/compile/internal/typecheck/type.go

@@ -88,7 +88,7 @@ func tcFuncType(n *ir.FuncType) ir.Node {
 		recv = tcField(n.Recv, misc)
 	}
 
-	t := types.NewSignature(types.LocalPkg, recv, tcFields(n.Params, misc), tcFields(n.Results, misc))
+	t := types.NewSignature(types.LocalPkg, recv, nil, tcFields(n.Params, misc), tcFields(n.Results, misc))
 	checkdupfields("argument", t.Recvs().FieldSlice(), t.Params().FieldSlice(), t.Results().FieldSlice())
 
 	base.Pos = lno

src/cmd/compile/internal/typecheck/universe.go

@@ -329,7 +329,7 @@ func InitUniverse() {
 }
 
 func makeErrorInterface() *types.Type {
-	sig := types.NewSignature(types.NoPkg, fakeRecvField(), nil, []*types.Field{
+	sig := types.NewSignature(types.NoPkg, fakeRecvField(), nil, nil, []*types.Field{
 		types.NewField(src.NoXPos, nil, types.Types[types.TSTRING]),
 	})
 	method := types.NewField(src.NoXPos, Lookup("Error"), sig)

src/cmd/compile/internal/types/fmt.go

@@ -318,7 +318,7 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 	}
 
 	// Unless the 'L' flag was specified, if the type has a name, just print that name.
-	if verb != 'L' && t.Sym() != nil && t != Types[t.Kind()] {
+	if verb != 'L' && t.Sym() != nil && t != Types[t.Kind()] && t.Kind() != TTYPEPARAM {
 		switch mode {
 		case fmtTypeID, fmtTypeIDName:
 			if verb == 'S' {
@@ -478,6 +478,9 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 			}
 			b.WriteString("func")
 		}
+		if t.NumTParams() > 0 {
+			tconv2(b, t.TParams(), 0, mode, visited)
+		}
 		tconv2(b, t.Params(), 0, mode, visited)
 
 		switch t.NumResults() {
@@ -515,7 +518,11 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 		}
 
 		if funarg := t.StructType().Funarg; funarg != FunargNone {
-			b.WriteByte('(')
+			open, close := '(', ')'
+			if funarg == FunargTparams {
+				open, close = '[', ']'
+			}
+			b.WriteByte(byte(open))
 			fieldVerb := 'v'
 			switch mode {
 			case fmtTypeID, fmtTypeIDName, fmtGo:
@@ -528,7 +535,7 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 				}
 				fldconv(b, f, fieldVerb, mode, visited, funarg)
 			}
-			b.WriteByte(')')
+			b.WriteByte(byte(close))
 		} else {
 			b.WriteString("struct {")
 			for i, f := range t.Fields().Slice() {
@@ -554,6 +561,15 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 	case TUNSAFEPTR:
 		b.WriteString("unsafe.Pointer")
 
+	case TTYPEPARAM:
+		if t.Sym() != nil {
+			sconv2(b, t.Sym(), 'v', mode)
+		} else {
+			b.WriteString("tp")
+			// Print out the pointer value for now to disambiguate type params
+			b.WriteString(fmt.Sprintf("%p", t))
+		}
+
 	case Txxx:
 		b.WriteString("Txxx")
 

src/cmd/compile/internal/types/kind_string.go

@@ -37,20 +37,21 @@ func _() {
 	_ = x[TANY-26]
 	_ = x[TSTRING-27]
 	_ = x[TUNSAFEPTR-28]
-	_ = x[TIDEAL-29]
+	_ = x[TTYPEPARAM-29]
-	_ = x[TNIL-30]
+	_ = x[TIDEAL-30]
-	_ = x[TBLANK-31]
+	_ = x[TNIL-31]
-	_ = x[TFUNCARGS-32]
+	_ = x[TBLANK-32]
-	_ = x[TCHANARGS-33]
+	_ = x[TFUNCARGS-33]
-	_ = x[TSSA-34]
+	_ = x[TCHANARGS-34]
-	_ = x[TTUPLE-35]
+	_ = x[TSSA-35]
-	_ = x[TRESULTS-36]
+	_ = x[TTUPLE-36]
-	_ = x[NTYPE-37]
+	_ = x[TRESULTS-37]
+	_ = x[NTYPE-38]
 }
 
-const _Kind_name = "xxxINT8UINT8INT16UINT16INT32UINT32INT64UINT64INTUINTUINTPTRCOMPLEX64COMPLEX128FLOAT32FLOAT64BOOLPTRFUNCSLICEARRAYSTRUCTCHANMAPINTERFORWANYSTRINGUNSAFEPTRIDEALNILBLANKFUNCARGSCHANARGSSSATUPLERESULTSNTYPE"
+const _Kind_name = "xxxINT8UINT8INT16UINT16INT32UINT32INT64UINT64INTUINTUINTPTRCOMPLEX64COMPLEX128FLOAT32FLOAT64BOOLPTRFUNCSLICEARRAYSTRUCTCHANMAPINTERFORWANYSTRINGUNSAFEPTRTYPEPARAMIDEALNILBLANKFUNCARGSCHANARGSSSATUPLERESULTSNTYPE"
 
-var _Kind_index = [...]uint8{0, 3, 7, 12, 17, 23, 28, 34, 39, 45, 48, 52, 59, 68, 78, 85, 92, 96, 99, 103, 108, 113, 119, 123, 126, 131, 135, 138, 144, 153, 158, 161, 166, 174, 182, 185, 190, 197, 202}
+var _Kind_index = [...]uint8{0, 3, 7, 12, 17, 23, 28, 34, 39, 45, 48, 52, 59, 68, 78, 85, 92, 96, 99, 103, 108, 113, 119, 123, 126, 131, 135, 138, 144, 153, 162, 167, 170, 175, 183, 191, 194, 199, 206, 211}
 
 func (i Kind) String() string {
 	if i >= Kind(len(_Kind_index)-1) {

src/cmd/compile/internal/types/size.go

@@ -499,6 +499,11 @@ func CalcSize(t *Type) {
 			base.Warn("bad type %v %d\n", t1, w)
 		}
 		t.Align = 1
+
+	case TTYPEPARAM:
+		// TODO(danscales) - remove when we eliminate the need
+		// to do CalcSize in noder2 (which shouldn't be needed in the noder)
+		w = int64(PtrSize)
 	}
 
 	if PtrSize == 4 && w != int64(int32(w)) {

src/cmd/compile/internal/types/sizeof_test.go

@@ -21,10 +21,10 @@ func TestSizeof(t *testing.T) {
 		_64bit uintptr     // size on 64bit platforms
 	}{
 		{Sym{}, 44, 72},
-		{Type{}, 56, 96},
+		{Type{}, 68, 120},
 		{Map{}, 20, 40},
 		{Forward{}, 20, 32},
-		{Func{}, 24, 40},
+		{Func{}, 28, 48},
 		{Struct{}, 16, 32},
 		{Interface{}, 8, 16},
 		{Chan{}, 8, 16},

src/cmd/compile/internal/types/type.go

@@ -72,6 +72,7 @@ const (
 	TANY
 	TSTRING
 	TUNSAFEPTR
+	TTYPEPARAM
 
 	// pseudo-types for literals
 	TIDEAL // untyped numeric constants
@@ -150,6 +151,7 @@ type Type struct {
 	// TARRAY: *Array
 	// TSLICE: Slice
 	// TSSA: string
+	// TTYPEPARAM:  *Interface (though we may not need to store/use the Interface info)
 	Extra interface{}
 
 	// Width is the width of this Type in bytes.
@@ -174,6 +176,11 @@ type Type struct {
 	Align uint8 // the required alignment of this type, in bytes (0 means Width and Align have not yet been computed)
 
 	flags bitset8
+
+	// Type params (in order) of this named type that need to be instantiated.
+	// TODO(danscales): for space reasons, should probably be a pointer to a
+	// slice, possibly change the name of this field.
+	RParams []*Type
 }
 
 func (*Type) CanBeAnSSAAux() {}
@@ -184,6 +191,7 @@ const (
 	typeNoalg                  // suppress hash and eq algorithm generation
 	typeDeferwidth             // width computation has been deferred and type is on deferredTypeStack
 	typeRecur
+	typeHasTParam // there is a typeparam somewhere in the type (generic function or type)
 )
 
 func (t *Type) NotInHeap() bool  { return t.flags&typeNotInHeap != 0 }
@@ -191,18 +199,21 @@ func (t *Type) Broke() bool      { return t.flags&typeBroke != 0 }
 func (t *Type) Noalg() bool      { return t.flags&typeNoalg != 0 }
 func (t *Type) Deferwidth() bool { return t.flags&typeDeferwidth != 0 }
 func (t *Type) Recur() bool      { return t.flags&typeRecur != 0 }
+func (t *Type) HasTParam() bool  { return t.flags&typeHasTParam != 0 }
 
 func (t *Type) SetNotInHeap(b bool)  { t.flags.set(typeNotInHeap, b) }
 func (t *Type) SetBroke(b bool)      { t.flags.set(typeBroke, b) }
 func (t *Type) SetNoalg(b bool)      { t.flags.set(typeNoalg, b) }
 func (t *Type) SetDeferwidth(b bool) { t.flags.set(typeDeferwidth, b) }
 func (t *Type) SetRecur(b bool)      { t.flags.set(typeRecur, b) }
+func (t *Type) SetHasTParam(b bool)  { t.flags.set(typeHasTParam, b) }
 
 // Kind returns the kind of type t.
 func (t *Type) Kind() Kind { return t.kind }
 
 // Sym returns the name of type t.
 func (t *Type) Sym() *Sym       { return t.sym }
+func (t *Type) SetSym(sym *Sym) { t.sym = sym }
 
 // Underlying returns the underlying type of type t.
 func (t *Type) Underlying() *Type { return t.underlying }
@@ -283,6 +294,7 @@ type Func struct {
 	Receiver *Type // function receiver
 	Results  *Type // function results
 	Params   *Type // function params
+	TParams  *Type // type params of receiver (if method) or function
 
 	pkg *Pkg
 
@@ -318,6 +330,7 @@ const (
 	FunargRcvr           // receiver
 	FunargParams         // input parameters
 	FunargResults        // output results
+	FunargTparams        // type params
 )
 
 // StructType returns t's extra struct-specific fields.
@@ -508,6 +521,8 @@ func New(et Kind) *Type {
 		t.Extra = new(Tuple)
 	case TRESULTS:
 		t.Extra = new(Results)
+	case TTYPEPARAM:
+		t.Extra = new(Interface)
 	}
 	return t
 }
@@ -520,6 +535,9 @@ func NewArray(elem *Type, bound int64) *Type {
 	t := New(TARRAY)
 	t.Extra = &Array{Elem: elem, Bound: bound}
 	t.SetNotInHeap(elem.NotInHeap())
+	if elem.HasTParam() {
+		t.SetHasTParam(true)
+	}
 	return t
 }
 
@@ -535,6 +553,9 @@ func NewSlice(elem *Type) *Type {
 	t := New(TSLICE)
 	t.Extra = Slice{Elem: elem}
 	elem.cache.slice = t
+	if elem.HasTParam() {
+		t.SetHasTParam(true)
+	}
 	return t
 }
 
@@ -544,6 +565,9 @@ func NewChan(elem *Type, dir ChanDir) *Type {
 	ct := t.ChanType()
 	ct.Elem = elem
 	ct.Dir = dir
+	if elem.HasTParam() {
+		t.SetHasTParam(true)
+	}
 	return t
 }
 
@@ -551,6 +575,9 @@ func NewTuple(t1, t2 *Type) *Type {
 	t := New(TTUPLE)
 	t.Extra.(*Tuple).first = t1
 	t.Extra.(*Tuple).second = t2
+	if t1.HasTParam() || t2.HasTParam() {
+		t.SetHasTParam(true)
+	}
 	return t
 }
 
@@ -572,6 +599,9 @@ func NewMap(k, v *Type) *Type {
 	mt := t.MapType()
 	mt.Key = k
 	mt.Elem = v
+	if k.HasTParam() || v.HasTParam() {
+		t.SetHasTParam(true)
+	}
 	return t
 }
 
@@ -590,6 +620,12 @@ func NewPtr(elem *Type) *Type {
 		if t.Elem() != elem {
 			base.Fatalf("NewPtr: elem mismatch")
 		}
+		if elem.HasTParam() {
+			// Extra check when reusing the cache, since the elem
+			// might have still been undetermined (i.e. a TFORW type)
+			// when this entry was cached.
+			t.SetHasTParam(true)
+		}
 		return t
 	}
 
@@ -600,6 +636,9 @@ func NewPtr(elem *Type) *Type {
 	if NewPtrCacheEnabled {
 		elem.cache.ptr = t
 	}
+	if elem.HasTParam() {
+		t.SetHasTParam(true)
+	}
 	return t
 }
 
@@ -765,10 +804,12 @@ func (t *Type) wantEtype(et Kind) {
 }
 
 func (t *Type) Recvs() *Type   { return t.FuncType().Receiver }
+func (t *Type) TParams() *Type { return t.FuncType().TParams }
 func (t *Type) Params() *Type  { return t.FuncType().Params }
 func (t *Type) Results() *Type { return t.FuncType().Results }
 
 func (t *Type) NumRecvs() int   { return t.FuncType().Receiver.NumFields() }
+func (t *Type) NumTParams() int { return t.FuncType().TParams.NumFields() }
 func (t *Type) NumParams() int  { return t.FuncType().Params.NumFields() }
 func (t *Type) NumResults() int { return t.FuncType().Results.NumFields() }
 
@@ -1602,6 +1643,9 @@ func (t *Type) SetUnderlying(underlying *Type) {
 	if underlying.Broke() {
 		t.SetBroke(true)
 	}
+	if underlying.HasTParam() {
+		t.SetHasTParam(true)
+	}
 
 	// spec: "The declared type does not inherit any methods bound
 	// to the existing type, but the method set of an interface
@@ -1624,6 +1668,15 @@ func (t *Type) SetUnderlying(underlying *Type) {
 	}
 }
 
+func fieldsHasTParam(fields []*Field) bool {
+	for _, f := range fields {
+		if f.Type != nil && f.Type.HasTParam() {
+			return true
+		}
+	}
+	return false
+}
+
 // NewBasic returns a new basic type of the given kind.
 func NewBasic(kind Kind, obj Object) *Type {
 	t := New(kind)
@@ -1644,9 +1697,20 @@ func NewInterface(pkg *Pkg, methods []*Field) *Type {
 	return t
 }
 
+// NewTypeParam returns a new type param with the given constraint (which may
+// not really be needed except for the type checker).
+func NewTypeParam(pkg *Pkg, constraint *Type) *Type {
+	t := New(TTYPEPARAM)
+	constraint.wantEtype(TINTER)
+	t.methods = constraint.methods
+	t.Extra.(*Interface).pkg = pkg
+	t.SetHasTParam(true)
+	return t
+}
+
 // NewSignature returns a new function type for the given receiver,
-// parameters, and results, any of which may be nil.
+// parametes, results, and type parameters, any of which may be nil.
-func NewSignature(pkg *Pkg, recv *Field, params, results []*Field) *Type {
+func NewSignature(pkg *Pkg, recv *Field, tparams, params, results []*Field) *Type {
 	var recvs []*Field
 	if recv != nil {
 		recvs = []*Field{recv}
@@ -1665,9 +1729,14 @@ func NewSignature(pkg *Pkg, recv *Field, params, results []*Field) *Type {
 	}
 
 	ft.Receiver = funargs(recvs, FunargRcvr)
+	ft.TParams = funargs(tparams, FunargTparams)
 	ft.Params = funargs(params, FunargParams)
 	ft.Results = funargs(results, FunargResults)
 	ft.pkg = pkg
+	if len(tparams) > 0 || fieldsHasTParam(recvs) || fieldsHasTParam(params) ||
+		fieldsHasTParam(results) {
+		t.SetHasTParam(true)
+	}
 
 	return t
 }
@@ -1680,6 +1749,9 @@ func NewStruct(pkg *Pkg, fields []*Field) *Type {
 		t.SetBroke(true)
 	}
 	t.Extra.(*Struct).pkg = pkg
+	if fieldsHasTParam(fields) {
+		t.SetHasTParam(true)
+	}
 	return t
 }
 

src/cmd/compile/internal/types2/api.go

@@ -0,0 +1,443 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package types declares the data types and implements
+// the algorithms for type-checking of Go packages. Use
+// Config.Check to invoke the type checker for a package.
+// Alternatively, create a new type checker with NewChecker
+// and invoke it incrementally by calling Checker.Files.
+//
+// Type-checking consists of several interdependent phases:
+//
+// Name resolution maps each identifier (syntax.Name) in the program to the
+// language object (Object) it denotes.
+// Use Info.{Defs,Uses,Implicits} for the results of name resolution.
+//
+// Constant folding computes the exact constant value (constant.Value)
+// for every expression (syntax.Expr) that is a compile-time constant.
+// Use Info.Types[expr].Value for the results of constant folding.
+//
+// Type inference computes the type (Type) of every expression (syntax.Expr)
+// and checks for compliance with the language specification.
+// Use Info.Types[expr].Type for the results of type inference.
+//
+// For a tutorial, see https://golang.org/s/types-tutorial.
+//
+package types2
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/constant"
+)
+
+// An Error describes a type-checking error; it implements the error interface.
+// A "soft" error is an error that still permits a valid interpretation of a
+// package (such as "unused variable"); "hard" errors may lead to unpredictable
+// behavior if ignored.
+type Error struct {
+	Pos  syntax.Pos // error position
+	Msg  string     // default error message, user-friendly
+	Full string     // full error message, for debugging (may contain internal details)
+	Soft bool       // if set, error is "soft"
+}
+
+// Error returns an error string formatted as follows:
+// filename:line:column: message
+func (err Error) Error() string {
+	return fmt.Sprintf("%s: %s", err.Pos, err.Msg)
+}
+
+// FullError returns an error string like Error, buy it may contain
+// type-checker internal details such as subscript indices for type
+// parameters and more. Useful for debugging.
+func (err Error) FullError() string {
+	return fmt.Sprintf("%s: %s", err.Pos, err.Full)
+}
+
+// An Importer resolves import paths to Packages.
+//
+// CAUTION: This interface does not support the import of locally
+// vendored packages. See https://golang.org/s/go15vendor.
+// If possible, external implementations should implement ImporterFrom.
+type Importer interface {
+	// Import returns the imported package for the given import path.
+	// The semantics is like for ImporterFrom.ImportFrom except that
+	// dir and mode are ignored (since they are not present).
+	Import(path string) (*Package, error)
+}
+
+// ImportMode is reserved for future use.
+type ImportMode int
+
+// An ImporterFrom resolves import paths to packages; it
+// supports vendoring per https://golang.org/s/go15vendor.
+// Use go/importer to obtain an ImporterFrom implementation.
+type ImporterFrom interface {
+	// Importer is present for backward-compatibility. Calling
+	// Import(path) is the same as calling ImportFrom(path, "", 0);
+	// i.e., locally vendored packages may not be found.
+	// The types package does not call Import if an ImporterFrom
+	// is present.
+	Importer
+
+	// ImportFrom returns the imported package for the given import
+	// path when imported by a package file located in dir.
+	// If the import failed, besides returning an error, ImportFrom
+	// is encouraged to cache and return a package anyway, if one
+	// was created. This will reduce package inconsistencies and
+	// follow-on type checker errors due to the missing package.
+	// The mode value must be 0; it is reserved for future use.
+	// Two calls to ImportFrom with the same path and dir must
+	// return the same package.
+	ImportFrom(path, dir string, mode ImportMode) (*Package, error)
+}
+
+// A Config specifies the configuration for type checking.
+// The zero value for Config is a ready-to-use default configuration.
+type Config struct {
+	// GoVersion describes the accepted Go language version. The string
+	// must follow the format "go%d.%d" (e.g. "go1.12") or ist must be
+	// empty; an empty string indicates the latest language version.
+	// If the format is invalid, invoking the type checker will cause a
+	// panic.
+	GoVersion string
+
+	// If IgnoreFuncBodies is set, function bodies are not
+	// type-checked.
+	IgnoreFuncBodies bool
+
+	// If AcceptMethodTypeParams is set, methods may have type parameters.
+	AcceptMethodTypeParams bool
+
+	// If InferFromConstraints is set, constraint type inference is used
+	// if some function type arguments are missing.
+	InferFromConstraints bool
+
+	// If FakeImportC is set, `import "C"` (for packages requiring Cgo)
+	// declares an empty "C" package and errors are omitted for qualified
+	// identifiers referring to package C (which won't find an object).
+	// This feature is intended for the standard library cmd/api tool.
+	//
+	// Caution: Effects may be unpredictable due to follow-on errors.
+	//          Do not use casually!
+	FakeImportC bool
+
+	// If IgnoreLabels is set, correct label use is not checked.
+	// TODO(gri) Consolidate label checking and remove this flag.
+	IgnoreLabels bool
+
+	// If CompilerErrorMessages is set, errors are reported using
+	// cmd/compile error strings to match $GOROOT/test errors.
+	// TODO(gri) Consolidate error messages and remove this flag.
+	CompilerErrorMessages bool
+
+	// If go115UsesCgo is set, the type checker expects the
+	// _cgo_gotypes.go file generated by running cmd/cgo to be
+	// provided as a package source file. Qualified identifiers
+	// referring to package C will be resolved to cgo-provided
+	// declarations within _cgo_gotypes.go.
+	//
+	// It is an error to set both FakeImportC and go115UsesCgo.
+	go115UsesCgo bool
+
+	// If Trace is set, a debug trace is printed to stdout.
+	Trace bool
+
+	// If Error != nil, it is called with each error found
+	// during type checking; err has dynamic type Error.
+	// Secondary errors (for instance, to enumerate all types
+	// involved in an invalid recursive type declaration) have
+	// error strings that start with a '\t' character.
+	// If Error == nil, type-checking stops with the first
+	// error found.
+	Error func(err error)
+
+	// An importer is used to import packages referred to from
+	// import declarations.
+	// If the installed importer implements ImporterFrom, the type
+	// checker calls ImportFrom instead of Import.
+	// The type checker reports an error if an importer is needed
+	// but none was installed.
+	Importer Importer
+
+	// If Sizes != nil, it provides the sizing functions for package unsafe.
+	// Otherwise SizesFor("gc", "amd64") is used instead.
+	Sizes Sizes
+
+	// If DisableUnusedImportCheck is set, packages are not checked
+	// for unused imports.
+	DisableUnusedImportCheck bool
+}
+
+func srcimporter_setUsesCgo(conf *Config) {
+	conf.go115UsesCgo = true
+}
+
+// Info holds result type information for a type-checked package.
+// Only the information for which a map is provided is collected.
+// If the package has type errors, the collected information may
+// be incomplete.
+type Info struct {
+	// Types maps expressions to their types, and for constant
+	// expressions, also their values. Invalid expressions are
+	// omitted.
+	//
+	// For (possibly parenthesized) identifiers denoting built-in
+	// functions, the recorded signatures are call-site specific:
+	// if the call result is not a constant, the recorded type is
+	// an argument-specific signature. Otherwise, the recorded type
+	// is invalid.
+	//
+	// The Types map does not record the type of every identifier,
+	// only those that appear where an arbitrary expression is
+	// permitted. For instance, the identifier f in a selector
+	// expression x.f is found only in the Selections map, the
+	// identifier z in a variable declaration 'var z int' is found
+	// only in the Defs map, and identifiers denoting packages in
+	// qualified identifiers are collected in the Uses map.
+	Types map[syntax.Expr]TypeAndValue
+
+	// Inferred maps calls of parameterized functions that use
+	// type inference to the inferred type arguments and signature
+	// of the function called. The recorded "call" expression may be
+	// an *ast.CallExpr (as in f(x)), or an *ast.IndexExpr (s in f[T]).
+	Inferred map[syntax.Expr]Inferred
+
+	// Defs maps identifiers to the objects they define (including
+	// package names, dots "." of dot-imports, and blank "_" identifiers).
+	// For identifiers that do not denote objects (e.g., the package name
+	// in package clauses, or symbolic variables t in t := x.(type) of
+	// type switch headers), the corresponding objects are nil.
+	//
+	// For an embedded field, Defs returns the field *Var it defines.
+	//
+	// Invariant: Defs[id] == nil || Defs[id].Pos() == id.Pos()
+	Defs map[*syntax.Name]Object
+
+	// Uses maps identifiers to the objects they denote.
+	//
+	// For an embedded field, Uses returns the *TypeName it denotes.
+	//
+	// Invariant: Uses[id].Pos() != id.Pos()
+	Uses map[*syntax.Name]Object
+
+	// Implicits maps nodes to their implicitly declared objects, if any.
+	// The following node and object types may appear:
+	//
+	//     node               declared object
+	//
+	//     *syntax.ImportDecl    *PkgName for imports without renames
+	//     *syntax.CaseClause    type-specific *Var for each type switch case clause (incl. default)
+	//     *syntax.Field         anonymous parameter *Var (incl. unnamed results)
+	//
+	Implicits map[syntax.Node]Object
+
+	// Selections maps selector expressions (excluding qualified identifiers)
+	// to their corresponding selections.
+	Selections map[*syntax.SelectorExpr]*Selection
+
+	// Scopes maps syntax.Nodes to the scopes they define. Package scopes are not
+	// associated with a specific node but with all files belonging to a package.
+	// Thus, the package scope can be found in the type-checked Package object.
+	// Scopes nest, with the Universe scope being the outermost scope, enclosing
+	// the package scope, which contains (one or more) files scopes, which enclose
+	// function scopes which in turn enclose statement and function literal scopes.
+	// Note that even though package-level functions are declared in the package
+	// scope, the function scopes are embedded in the file scope of the file
+	// containing the function declaration.
+	//
+	// The following node types may appear in Scopes:
+	//
+	//     *syntax.File
+	//     *syntax.FuncType
+	//     *syntax.BlockStmt
+	//     *syntax.IfStmt
+	//     *syntax.SwitchStmt
+	//     *syntax.CaseClause
+	//     *syntax.CommClause
+	//     *syntax.ForStmt
+	//
+	Scopes map[syntax.Node]*Scope
+
+	// InitOrder is the list of package-level initializers in the order in which
+	// they must be executed. Initializers referring to variables related by an
+	// initialization dependency appear in topological order, the others appear
+	// in source order. Variables without an initialization expression do not
+	// appear in this list.
+	InitOrder []*Initializer
+}
+
+// TypeOf returns the type of expression e, or nil if not found.
+// Precondition: the Types, Uses and Defs maps are populated.
+//
+func (info *Info) TypeOf(e syntax.Expr) Type {
+	if t, ok := info.Types[e]; ok {
+		return t.Type
+	}
+	if id, _ := e.(*syntax.Name); id != nil {
+		if obj := info.ObjectOf(id); obj != nil {
+			return obj.Type()
+		}
+	}
+	return nil
+}
+
+// ObjectOf returns the object denoted by the specified id,
+// or nil if not found.
+//
+// If id is an embedded struct field, ObjectOf returns the field (*Var)
+// it defines, not the type (*TypeName) it uses.
+//
+// Precondition: the Uses and Defs maps are populated.
+//
+func (info *Info) ObjectOf(id *syntax.Name) Object {
+	if obj := info.Defs[id]; obj != nil {
+		return obj
+	}
+	return info.Uses[id]
+}
+
+// TypeAndValue reports the type and value (for constants)
+// of the corresponding expression.
+type TypeAndValue struct {
+	mode  operandMode
+	Type  Type
+	Value constant.Value
+}
+
+// IsVoid reports whether the corresponding expression
+// is a function call without results.
+func (tv TypeAndValue) IsVoid() bool {
+	return tv.mode == novalue
+}
+
+// IsType reports whether the corresponding expression specifies a type.
+func (tv TypeAndValue) IsType() bool {
+	return tv.mode == typexpr
+}
+
+// IsBuiltin reports whether the corresponding expression denotes
+// a (possibly parenthesized) built-in function.
+func (tv TypeAndValue) IsBuiltin() bool {
+	return tv.mode == builtin
+}
+
+// IsValue reports whether the corresponding expression is a value.
+// Builtins are not considered values. Constant values have a non-
+// nil Value.
+func (tv TypeAndValue) IsValue() bool {
+	switch tv.mode {
+	case constant_, variable, mapindex, value, nilvalue, commaok, commaerr:
+		return true
+	}
+	return false
+}
+
+// IsNil reports whether the corresponding expression denotes the
+// predeclared value nil. Depending on context, it may have been
+// given a type different from UntypedNil.
+func (tv TypeAndValue) IsNil() bool {
+	return tv.mode == nilvalue
+}
+
+// Addressable reports whether the corresponding expression
+// is addressable (https://golang.org/ref/spec#Address_operators).
+func (tv TypeAndValue) Addressable() bool {
+	return tv.mode == variable
+}
+
+// Assignable reports whether the corresponding expression
+// is assignable to (provided a value of the right type).
+func (tv TypeAndValue) Assignable() bool {
+	return tv.mode == variable || tv.mode == mapindex
+}
+
+// HasOk reports whether the corresponding expression may be
+// used on the rhs of a comma-ok assignment.
+func (tv TypeAndValue) HasOk() bool {
+	return tv.mode == commaok || tv.mode == mapindex
+}
+
+// Inferred reports the inferred type arguments and signature
+// for a parameterized function call that uses type inference.
+type Inferred struct {
+	Targs []Type
+	Sig   *Signature
+}
+
+// An Initializer describes a package-level variable, or a list of variables in case
+// of a multi-valued initialization expression, and the corresponding initialization
+// expression.
+type Initializer struct {
+	Lhs []*Var // var Lhs = Rhs
+	Rhs syntax.Expr
+}
+
+func (init *Initializer) String() string {
+	var buf bytes.Buffer
+	for i, lhs := range init.Lhs {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		buf.WriteString(lhs.Name())
+	}
+	buf.WriteString(" = ")
+	syntax.Fprint(&buf, init.Rhs, syntax.ShortForm)
+	return buf.String()
+}
+
+// Check type-checks a package and returns the resulting package object and
+// the first error if any. Additionally, if info != nil, Check populates each
+// of the non-nil maps in the Info struct.
+//
+// The package is marked as complete if no errors occurred, otherwise it is
+// incomplete. See Config.Error for controlling behavior in the presence of
+// errors.
+//
+// The package is specified by a list of *syntax.Files and corresponding
+// file set, and the package path the package is identified with.
+// The clean path must not be empty or dot (".").
+func (conf *Config) Check(path string, files []*syntax.File, info *Info) (*Package, error) {
+	pkg := NewPackage(path, "")
+	return pkg, NewChecker(conf, pkg, info).Files(files)
+}
+
+// AssertableTo reports whether a value of type V can be asserted to have type T.
+func AssertableTo(V *Interface, T Type) bool {
+	m, _ := (*Checker)(nil).assertableTo(V, T, false)
+	return m == nil
+}
+
+// AssignableTo reports whether a value of type V is assignable to a variable of type T.
+func AssignableTo(V, T Type) bool {
+	x := operand{mode: value, typ: V}
+	return x.assignableTo(nil, T, nil) // check not needed for non-constant x
+}
+
+// ConvertibleTo reports whether a value of type V is convertible to a value of type T.
+func ConvertibleTo(V, T Type) bool {
+	x := operand{mode: value, typ: V}
+	return x.convertibleTo(nil, T) // check not needed for non-constant x
+}
+
+// Implements reports whether type V implements interface T.
+func Implements(V Type, T *Interface) bool {
+	f, _ := MissingMethod(V, T, true)
+	return f == nil
+}
+
+// Identical reports whether x and y are identical types.
+// Receivers of Signature types are ignored.
+func Identical(x, y Type) bool {
+	return (*Checker)(nil).identical(x, y)
+}
+
+// IdenticalIgnoreTags reports whether x and y are identical types if tags are ignored.
+// Receivers of Signature types are ignored.
+func IdenticalIgnoreTags(x, y Type) bool {
+	return (*Checker)(nil).identicalIgnoreTags(x, y)
+}

src/cmd/compile/internal/types2/assignments.go

@@ -0,0 +1,368 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements initialization and assignment checks.
+
+package types2
+
+import "cmd/compile/internal/syntax"
+
+// assignment reports whether x can be assigned to a variable of type T,
+// if necessary by attempting to convert untyped values to the appropriate
+// type. context describes the context in which the assignment takes place.
+// Use T == nil to indicate assignment to an untyped blank identifier.
+// x.mode is set to invalid if the assignment failed.
+func (check *Checker) assignment(x *operand, T Type, context string) {
+	check.singleValue(x)
+
+	switch x.mode {
+	case invalid:
+		return // error reported before
+	case constant_, variable, mapindex, value, nilvalue, commaok, commaerr:
+		// ok
+	default:
+		// we may get here because of other problems (issue #39634, crash 12)
+		check.errorf(x, "cannot assign %s to %s in %s", x, T, context)
+		return
+	}
+
+	if isUntyped(x.typ) {
+		target := T
+		// spec: "If an untyped constant is assigned to a variable of interface
+		// type or the blank identifier, the constant is first converted to type
+		// bool, rune, int, float64, complex128 or string respectively, depending
+		// on whether the value is a boolean, rune, integer, floating-point, complex,
+		// or string constant."
+		if x.isNil() {
+			if T == nil {
+				check.errorf(x, "use of untyped nil in %s", context)
+				x.mode = invalid
+				return
+			}
+		} else if T == nil || IsInterface(T) {
+			target = Default(x.typ)
+		}
+		check.convertUntyped(x, target)
+		if x.mode == invalid {
+			return
+		}
+	}
+	// x.typ is typed
+
+	// A generic (non-instantiated) function value cannot be assigned to a variable.
+	if sig := asSignature(x.typ); sig != nil && len(sig.tparams) > 0 {
+		check.errorf(x, "cannot use generic function %s without instantiation in %s", x, context)
+	}
+
+	// spec: "If a left-hand side is the blank identifier, any typed or
+	// non-constant value except for the predeclared identifier nil may
+	// be assigned to it."
+	if T == nil {
+		return
+	}
+
+	if reason := ""; !x.assignableTo(check, T, &reason) {
+		if check.conf.CompilerErrorMessages {
+			check.errorf(x, "incompatible type: cannot use %s as %s value", x, T)
+		} else {
+			if reason != "" {
+				check.errorf(x, "cannot use %s as %s value in %s: %s", x, T, context, reason)
+			} else {
+				check.errorf(x, "cannot use %s as %s value in %s", x, T, context)
+			}
+		}
+		x.mode = invalid
+	}
+}
+
+func (check *Checker) initConst(lhs *Const, x *operand) {
+	if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		return
+	}
+
+	// rhs must be a constant
+	if x.mode != constant_ {
+		check.errorf(x, "%s is not constant", x)
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		return
+	}
+	assert(isConstType(x.typ))
+
+	// If the lhs doesn't have a type yet, use the type of x.
+	if lhs.typ == nil {
+		lhs.typ = x.typ
+	}
+
+	check.assignment(x, lhs.typ, "constant declaration")
+	if x.mode == invalid {
+		return
+	}
+
+	lhs.val = x.val
+}
+
+func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
+	if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		lhs.used = true // avoid follow-on "declared but not used" errors
+		return nil
+	}
+
+	// If the lhs doesn't have a type yet, use the type of x.
+	if lhs.typ == nil {
+		typ := x.typ
+		if isUntyped(typ) {
+			// convert untyped types to default types
+			if typ == Typ[UntypedNil] {
+				check.errorf(x, "use of untyped nil in %s", context)
+				lhs.typ = Typ[Invalid]
+				return nil
+			}
+			typ = Default(typ)
+		}
+		lhs.typ = typ
+	}
+
+	check.assignment(x, lhs.typ, context)
+	if x.mode == invalid {
+		return nil
+	}
+
+	return x.typ
+}
+
+func (check *Checker) assignVar(lhs syntax.Expr, x *operand) Type {
+	if x.mode == invalid || x.typ == Typ[Invalid] {
+		check.useLHS(lhs)
+		return nil
+	}
+
+	// Determine if the lhs is a (possibly parenthesized) identifier.
+	ident, _ := unparen(lhs).(*syntax.Name)
+
+	// Don't evaluate lhs if it is the blank identifier.
+	if ident != nil && ident.Value == "_" {
+		check.recordDef(ident, nil)
+		check.assignment(x, nil, "assignment to _ identifier")
+		if x.mode == invalid {
+			return nil
+		}
+		return x.typ
+	}
+
+	// If the lhs is an identifier denoting a variable v, this assignment
+	// is not a 'use' of v. Remember current value of v.used and restore
+	// after evaluating the lhs via check.expr.
+	var v *Var
+	var v_used bool
+	if ident != nil {
+		if obj := check.lookup(ident.Value); obj != nil {
+			// It's ok to mark non-local variables, but ignore variables
+			// from other packages to avoid potential race conditions with
+			// dot-imported variables.
+			if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
+				v = w
+				v_used = v.used
+			}
+		}
+	}
+
+	var z operand
+	check.expr(&z, lhs)
+	if v != nil {
+		v.used = v_used // restore v.used
+	}
+
+	if z.mode == invalid || z.typ == Typ[Invalid] {
+		return nil
+	}
+
+	// spec: "Each left-hand side operand must be addressable, a map index
+	// expression, or the blank identifier. Operands may be parenthesized."
+	switch z.mode {
+	case invalid:
+		return nil
+	case variable, mapindex:
+		// ok
+	case nilvalue:
+		check.errorf(&z, "cannot assign to nil") // default would print "untyped nil"
+		return nil
+	default:
+		if sel, ok := z.expr.(*syntax.SelectorExpr); ok {
+			var op operand
+			check.expr(&op, sel.X)
+			if op.mode == mapindex {
+				check.errorf(&z, "cannot assign to struct field %s in map", syntax.String(z.expr))
+				return nil
+			}
+		}
+		check.errorf(&z, "cannot assign to %s", &z)
+		return nil
+	}
+
+	check.assignment(x, z.typ, "assignment")
+	if x.mode == invalid {
+		return nil
+	}
+
+	return x.typ
+}
+
+// If returnPos is valid, initVars is called to type-check the assignment of
+// return expressions, and returnPos is the position of the return statement.
+func (check *Checker) initVars(lhs []*Var, orig_rhs []syntax.Expr, returnPos syntax.Pos) {
+	rhs, commaOk := check.exprList(orig_rhs, len(lhs) == 2 && !returnPos.IsKnown())
+
+	if len(lhs) != len(rhs) {
+		// invalidate lhs
+		for _, obj := range lhs {
+			if obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+		}
+		// don't report an error if we already reported one
+		for _, x := range rhs {
+			if x.mode == invalid {
+				return
+			}
+		}
+		if returnPos.IsKnown() {
+			check.errorf(returnPos, "wrong number of return values (want %d, got %d)", len(lhs), len(rhs))
+			return
+		}
+		check.errorf(rhs[0], "cannot initialize %d variables with %d values", len(lhs), len(rhs))
+		return
+	}
+
+	context := "assignment"
+	if returnPos.IsKnown() {
+		context = "return statement"
+	}
+
+	if commaOk {
+		var a [2]Type
+		for i := range a {
+			a[i] = check.initVar(lhs[i], rhs[i], context)
+		}
+		check.recordCommaOkTypes(orig_rhs[0], a)
+		return
+	}
+
+	for i, lhs := range lhs {
+		check.initVar(lhs, rhs[i], context)
+	}
+}
+
+func (check *Checker) assignVars(lhs, orig_rhs []syntax.Expr) {
+	rhs, commaOk := check.exprList(orig_rhs, len(lhs) == 2)
+
+	if len(lhs) != len(rhs) {
+		check.useLHS(lhs...)
+		// don't report an error if we already reported one
+		for _, x := range rhs {
+			if x.mode == invalid {
+				return
+			}
+		}
+		check.errorf(rhs[0], "cannot assign %d values to %d variables", len(rhs), len(lhs))
+		return
+	}
+
+	if commaOk {
+		var a [2]Type
+		for i := range a {
+			a[i] = check.assignVar(lhs[i], rhs[i])
+		}
+		check.recordCommaOkTypes(orig_rhs[0], a)
+		return
+	}
+
+	for i, lhs := range lhs {
+		check.assignVar(lhs, rhs[i])
+	}
+}
+
+// unpack unpacks a *syntax.ListExpr into a list of syntax.Expr.
+// Helper introduced for the go/types -> types2 port.
+// TODO(gri) Should find a more efficient solution that doesn't
+//           require introduction of a new slice for simple
+//           expressions.
+func unpackExpr(x syntax.Expr) []syntax.Expr {
+	if x, _ := x.(*syntax.ListExpr); x != nil {
+		return x.ElemList
+	}
+	if x != nil {
+		return []syntax.Expr{x}
+	}
+	return nil
+}
+
+func (check *Checker) shortVarDecl(pos syntax.Pos, lhs, rhs []syntax.Expr) {
+	top := len(check.delayed)
+	scope := check.scope
+
+	// collect lhs variables
+	var newVars []*Var
+	var lhsVars = make([]*Var, len(lhs))
+	for i, lhs := range lhs {
+		var obj *Var
+		if ident, _ := lhs.(*syntax.Name); ident != nil {
+			// Use the correct obj if the ident is redeclared. The
+			// variable's scope starts after the declaration; so we
+			// must use Scope.Lookup here and call Scope.Insert
+			// (via check.declare) later.
+			name := ident.Value
+			if alt := scope.Lookup(name); alt != nil {
+				// redeclared object must be a variable
+				if alt, _ := alt.(*Var); alt != nil {
+					obj = alt
+				} else {
+					check.errorf(lhs, "cannot assign to %s", lhs)
+				}
+				check.recordUse(ident, alt)
+			} else {
+				// declare new variable, possibly a blank (_) variable
+				obj = NewVar(ident.Pos(), check.pkg, name, nil)
+				if name != "_" {
+					newVars = append(newVars, obj)
+				}
+				check.recordDef(ident, obj)
+			}
+		} else {
+			check.useLHS(lhs)
+			check.errorf(lhs, "cannot declare %s", lhs)
+		}
+		if obj == nil {
+			obj = NewVar(lhs.Pos(), check.pkg, "_", nil) // dummy variable
+		}
+		lhsVars[i] = obj
+	}
+
+	check.initVars(lhsVars, rhs, nopos)
+
+	// process function literals in rhs expressions before scope changes
+	check.processDelayed(top)
+
+	// declare new variables
+	if len(newVars) > 0 {
+		// spec: "The scope of a constant or variable identifier declared inside
+		// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
+		// for short variable declarations) and ends at the end of the innermost
+		// containing block."
+		scopePos := endPos(rhs[len(rhs)-1])
+		for _, obj := range newVars {
+			check.declare(scope, nil, obj, scopePos) // recordObject already called
+		}
+	} else {
+		check.softErrorf(pos, "no new variables on left side of :=")
+	}
+}

src/cmd/compile/internal/types2/builtins.go

@@ -0,0 +1,776 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of builtin function calls.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"go/constant"
+	"go/token"
+)
+
+// builtin type-checks a call to the built-in specified by id and
+// reports whether the call is valid, with *x holding the result;
+// but x.expr is not set. If the call is invalid, the result is
+// false, and *x is undefined.
+//
+func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (_ bool) {
+	// append is the only built-in that permits the use of ... for the last argument
+	bin := predeclaredFuncs[id]
+	if call.HasDots && id != _Append {
+		//check.invalidOpf(call.Ellipsis, "invalid use of ... with built-in %s", bin.name)
+		check.invalidOpf(call, "invalid use of ... with built-in %s", bin.name)
+		check.use(call.ArgList...)
+		return
+	}
+
+	// For len(x) and cap(x) we need to know if x contains any function calls or
+	// receive operations. Save/restore current setting and set hasCallOrRecv to
+	// false for the evaluation of x so that we can check it afterwards.
+	// Note: We must do this _before_ calling exprList because exprList evaluates
+	//       all arguments.
+	if id == _Len || id == _Cap {
+		defer func(b bool) {
+			check.hasCallOrRecv = b
+		}(check.hasCallOrRecv)
+		check.hasCallOrRecv = false
+	}
+
+	// determine actual arguments
+	var arg func(*operand, int) // TODO(gri) remove use of arg getter in favor of using xlist directly
+	nargs := len(call.ArgList)
+	switch id {
+	default:
+		// make argument getter
+		xlist, _ := check.exprList(call.ArgList, false)
+		arg = func(x *operand, i int) { *x = *xlist[i]; x.typ = expand(x.typ) }
+		nargs = len(xlist)
+		// evaluate first argument, if present
+		if nargs > 0 {
+			arg(x, 0)
+			if x.mode == invalid {
+				return
+			}
+		}
+	case _Make, _New, _Offsetof, _Trace:
+		// arguments require special handling
+	}
+
+	// check argument count
+	{
+		msg := ""
+		if nargs < bin.nargs {
+			msg = "not enough"
+		} else if !bin.variadic && nargs > bin.nargs {
+			msg = "too many"
+		}
+		if msg != "" {
+			check.invalidOpf(call, "%s arguments for %s (expected %d, found %d)", msg, call, bin.nargs, nargs)
+			return
+		}
+	}
+
+	switch id {
+	case _Append:
+		// append(s S, x ...T) S, where T is the element type of S
+		// spec: "The variadic function append appends zero or more values x to s of type
+		// S, which must be a slice type, and returns the resulting slice, also of type S.
+		// The values x are passed to a parameter of type ...T where T is the element type
+		// of S and the respective parameter passing rules apply."
+		S := x.typ
+		var T Type
+		if s := asSlice(S); s != nil {
+			T = s.elem
+		} else {
+			check.invalidArgf(x, "%s is not a slice", x)
+			return
+		}
+
+		// remember arguments that have been evaluated already
+		alist := []operand{*x}
+
+		// spec: "As a special case, append also accepts a first argument assignable
+		// to type []byte with a second argument of string type followed by ... .
+		// This form appends the bytes of the string.
+		if nargs == 2 && call.HasDots && x.assignableTo(check, NewSlice(universeByte), nil) {
+			arg(x, 1)
+			if x.mode == invalid {
+				return
+			}
+			if isString(x.typ) {
+				if check.Types != nil {
+					sig := makeSig(S, S, x.typ)
+					sig.variadic = true
+					check.recordBuiltinType(call.Fun, sig)
+				}
+				x.mode = value
+				x.typ = S
+				break
+			}
+			alist = append(alist, *x)
+			// fallthrough
+		}
+
+		// check general case by creating custom signature
+		sig := makeSig(S, S, NewSlice(T)) // []T required for variadic signature
+		sig.variadic = true
+		var xlist []*operand
+		// convert []operand to []*operand
+		for i := range alist {
+			xlist = append(xlist, &alist[i])
+		}
+		for i := len(alist); i < nargs; i++ {
+			var x operand
+			arg(&x, i)
+			xlist = append(xlist, &x)
+		}
+		check.arguments(call, sig, xlist) // discard result (we know the result type)
+		// ok to continue even if check.arguments reported errors
+
+		x.mode = value
+		x.typ = S
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, sig)
+		}
+
+	case _Cap, _Len:
+		// cap(x)
+		// len(x)
+		mode := invalid
+		var typ Type
+		var val constant.Value
+		switch typ = implicitArrayDeref(optype(x.typ)); t := typ.(type) {
+		case *Basic:
+			if isString(t) && id == _Len {
+				if x.mode == constant_ {
+					mode = constant_
+					val = constant.MakeInt64(int64(len(constant.StringVal(x.val))))
+				} else {
+					mode = value
+				}
+			}
+
+		case *Array:
+			mode = value
+			// spec: "The expressions len(s) and cap(s) are constants
+			// if the type of s is an array or pointer to an array and
+			// the expression s does not contain channel receives or
+			// function calls; in this case s is not evaluated."
+			if !check.hasCallOrRecv {
+				mode = constant_
+				if t.len >= 0 {
+					val = constant.MakeInt64(t.len)
+				} else {
+					val = constant.MakeUnknown()
+				}
+			}
+
+		case *Slice, *Chan:
+			mode = value
+
+		case *Map:
+			if id == _Len {
+				mode = value
+			}
+
+		case *Sum:
+			if t.is(func(t Type) bool {
+				switch t := under(t).(type) {
+				case *Basic:
+					if isString(t) && id == _Len {
+						return true
+					}
+				case *Array, *Slice, *Chan:
+					return true
+				case *Map:
+					if id == _Len {
+						return true
+					}
+				}
+				return false
+			}) {
+				mode = value
+			}
+		}
+
+		if mode == invalid && typ != Typ[Invalid] {
+			check.invalidArgf(x, "%s for %s", x, bin.name)
+			return
+		}
+
+		x.mode = mode
+		x.typ = Typ[Int]
+		x.val = val
+		if check.Types != nil && mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, typ))
+		}
+
+	case _Close:
+		// close(c)
+		c := asChan(x.typ)
+		if c == nil {
+			check.invalidArgf(x, "%s is not a channel", x)
+			return
+		}
+		if c.dir == RecvOnly {
+			check.invalidArgf(x, "%s must not be a receive-only channel", x)
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, c))
+		}
+
+	case _Complex:
+		// complex(x, y floatT) complexT
+		var y operand
+		arg(&y, 1)
+		if y.mode == invalid {
+			return
+		}
+
+		// convert or check untyped arguments
+		d := 0
+		if isUntyped(x.typ) {
+			d |= 1
+		}
+		if isUntyped(y.typ) {
+			d |= 2
+		}
+		switch d {
+		case 0:
+			// x and y are typed => nothing to do
+		case 1:
+			// only x is untyped => convert to type of y
+			check.convertUntyped(x, y.typ)
+		case 2:
+			// only y is untyped => convert to type of x
+			check.convertUntyped(&y, x.typ)
+		case 3:
+			// x and y are untyped =>
+			// 1) if both are constants, convert them to untyped
+			//    floating-point numbers if possible,
+			// 2) if one of them is not constant (possible because
+			//    it contains a shift that is yet untyped), convert
+			//    both of them to float64 since they must have the
+			//    same type to succeed (this will result in an error
+			//    because shifts of floats are not permitted)
+			if x.mode == constant_ && y.mode == constant_ {
+				toFloat := func(x *operand) {
+					if isNumeric(x.typ) && constant.Sign(constant.Imag(x.val)) == 0 {
+						x.typ = Typ[UntypedFloat]
+					}
+				}
+				toFloat(x)
+				toFloat(&y)
+			} else {
+				check.convertUntyped(x, Typ[Float64])
+				check.convertUntyped(&y, Typ[Float64])
+				// x and y should be invalid now, but be conservative
+				// and check below
+			}
+		}
+		if x.mode == invalid || y.mode == invalid {
+			return
+		}
+
+		// both argument types must be identical
+		if !check.identical(x.typ, y.typ) {
+			check.invalidOpf(x, "%s (mismatched types %s and %s)", call, x.typ, y.typ)
+			return
+		}
+
+		// the argument types must be of floating-point type
+		f := func(x Type) Type {
+			if t := asBasic(x); t != nil {
+				switch t.kind {
+				case Float32:
+					return Typ[Complex64]
+				case Float64:
+					return Typ[Complex128]
+				case UntypedFloat:
+					return Typ[UntypedComplex]
+				}
+			}
+			return nil
+		}
+		resTyp := check.applyTypeFunc(f, x.typ)
+		if resTyp == nil {
+			check.invalidArgf(x, "arguments have type %s, expected floating-point", x.typ)
+			return
+		}
+
+		// if both arguments are constants, the result is a constant
+		if x.mode == constant_ && y.mode == constant_ {
+			x.val = constant.BinaryOp(constant.ToFloat(x.val), token.ADD, constant.MakeImag(constant.ToFloat(y.val)))
+		} else {
+			x.mode = value
+		}
+
+		if check.Types != nil && x.mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(resTyp, x.typ, x.typ))
+		}
+
+		x.typ = resTyp
+
+	case _Copy:
+		// copy(x, y []T) int
+		var dst Type
+		if t := asSlice(x.typ); t != nil {
+			dst = t.elem
+		}
+
+		var y operand
+		arg(&y, 1)
+		if y.mode == invalid {
+			return
+		}
+		var src Type
+		switch t := optype(y.typ).(type) {
+		case *Basic:
+			if isString(y.typ) {
+				src = universeByte
+			}
+		case *Slice:
+			src = t.elem
+		}
+
+		if dst == nil || src == nil {
+			check.invalidArgf(x, "copy expects slice arguments; found %s and %s", x, &y)
+			return
+		}
+
+		if !check.identical(dst, src) {
+			check.invalidArgf(x, "arguments to copy %s and %s have different element types %s and %s", x, &y, dst, src)
+			return
+		}
+
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(Typ[Int], x.typ, y.typ))
+		}
+		x.mode = value
+		x.typ = Typ[Int]
+
+	case _Delete:
+		// delete(m, k)
+		m := asMap(x.typ)
+		if m == nil {
+			check.invalidArgf(x, "%s is not a map", x)
+			return
+		}
+		arg(x, 1) // k
+		if x.mode == invalid {
+			return
+		}
+
+		check.assignment(x, m.key, "argument to delete")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, m, m.key))
+		}
+
+	case _Imag, _Real:
+		// imag(complexT) floatT
+		// real(complexT) floatT
+
+		// convert or check untyped argument
+		if isUntyped(x.typ) {
+			if x.mode == constant_ {
+				// an untyped constant number can always be considered
+				// as a complex constant
+				if isNumeric(x.typ) {
+					x.typ = Typ[UntypedComplex]
+				}
+			} else {
+				// an untyped non-constant argument may appear if
+				// it contains a (yet untyped non-constant) shift
+				// expression: convert it to complex128 which will
+				// result in an error (shift of complex value)
+				check.convertUntyped(x, Typ[Complex128])
+				// x should be invalid now, but be conservative and check
+				if x.mode == invalid {
+					return
+				}
+			}
+		}
+
+		// the argument must be of complex type
+		f := func(x Type) Type {
+			if t := asBasic(x); t != nil {
+				switch t.kind {
+				case Complex64:
+					return Typ[Float32]
+				case Complex128:
+					return Typ[Float64]
+				case UntypedComplex:
+					return Typ[UntypedFloat]
+				}
+			}
+			return nil
+		}
+		resTyp := check.applyTypeFunc(f, x.typ)
+		if resTyp == nil {
+			check.invalidArgf(x, "argument has type %s, expected complex type", x.typ)
+			return
+		}
+
+		// if the argument is a constant, the result is a constant
+		if x.mode == constant_ {
+			if id == _Real {
+				x.val = constant.Real(x.val)
+			} else {
+				x.val = constant.Imag(x.val)
+			}
+		} else {
+			x.mode = value
+		}
+
+		if check.Types != nil && x.mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(resTyp, x.typ))
+		}
+
+		x.typ = resTyp
+
+	case _Make:
+		// make(T, n)
+		// make(T, n, m)
+		// (no argument evaluated yet)
+		arg0 := call.ArgList[0]
+		T := check.varType(arg0)
+		if T == Typ[Invalid] {
+			return
+		}
+
+		min, max := -1, 10
+		var valid func(t Type) bool
+		valid = func(t Type) bool {
+			var m int
+			switch t := optype(t).(type) {
+			case *Slice:
+				m = 2
+			case *Map, *Chan:
+				m = 1
+			case *Sum:
+				return t.is(valid)
+			default:
+				return false
+			}
+			if m > min {
+				min = m
+			}
+			if m+1 < max {
+				max = m + 1
+			}
+			return true
+		}
+
+		if !valid(T) {
+			check.invalidArgf(arg0, "cannot make %s; type must be slice, map, or channel", arg0)
+			return
+		}
+		if nargs < min || max < nargs {
+			if min == max {
+				check.errorf(call, "%v expects %d arguments; found %d", call, min, nargs)
+			} else {
+				check.errorf(call, "%v expects %d or %d arguments; found %d", call, min, max, nargs)
+			}
+			return
+		}
+
+		types := []Type{T}
+		var sizes []int64 // constant integer arguments, if any
+		for _, arg := range call.ArgList[1:] {
+			typ, size := check.index(arg, -1) // ok to continue with typ == Typ[Invalid]
+			types = append(types, typ)
+			if size >= 0 {
+				sizes = append(sizes, size)
+			}
+		}
+		if len(sizes) == 2 && sizes[0] > sizes[1] {
+			check.invalidArgf(call.ArgList[1], "length and capacity swapped")
+			// safe to continue
+		}
+		x.mode = value
+		x.typ = T
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, types...))
+		}
+
+	case _New:
+		// new(T)
+		// (no argument evaluated yet)
+		T := check.varType(call.ArgList[0])
+		if T == Typ[Invalid] {
+			return
+		}
+
+		x.mode = value
+		x.typ = &Pointer{base: T}
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, T))
+		}
+
+	case _Panic:
+		// panic(x)
+		// record panic call if inside a function with result parameters
+		// (for use in Checker.isTerminating)
+		if check.sig != nil && check.sig.results.Len() > 0 {
+			// function has result parameters
+			p := check.isPanic
+			if p == nil {
+				// allocate lazily
+				p = make(map[*syntax.CallExpr]bool)
+				check.isPanic = p
+			}
+			p[call] = true
+		}
+
+		check.assignment(x, &emptyInterface, "argument to panic")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, &emptyInterface))
+		}
+
+	case _Print, _Println:
+		// print(x, y, ...)
+		// println(x, y, ...)
+		var params []Type
+		if nargs > 0 {
+			params = make([]Type, nargs)
+			for i := 0; i < nargs; i++ {
+				if i > 0 {
+					arg(x, i) // first argument already evaluated
+				}
+				check.assignment(x, nil, "argument to "+predeclaredFuncs[id].name)
+				if x.mode == invalid {
+					// TODO(gri) "use" all arguments?
+					return
+				}
+				params[i] = x.typ
+			}
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, params...))
+		}
+
+	case _Recover:
+		// recover() interface{}
+		x.mode = value
+		x.typ = &emptyInterface
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ))
+		}
+
+	case _Alignof:
+		// unsafe.Alignof(x T) uintptr
+		if asTypeParam(x.typ) != nil {
+			check.invalidOpf(call, "unsafe.Alignof undefined for %s", x)
+			return
+		}
+		check.assignment(x, nil, "argument to unsafe.Alignof")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = constant_
+		x.val = constant.MakeInt64(check.conf.alignof(x.typ))
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Offsetof:
+		// unsafe.Offsetof(x T) uintptr, where x must be a selector
+		// (no argument evaluated yet)
+		arg0 := call.ArgList[0]
+		selx, _ := unparen(arg0).(*syntax.SelectorExpr)
+		if selx == nil {
+			check.invalidArgf(arg0, "%s is not a selector expression", arg0)
+			check.use(arg0)
+			return
+		}
+
+		check.expr(x, selx.X)
+		if x.mode == invalid {
+			return
+		}
+
+		base := derefStructPtr(x.typ)
+		sel := selx.Sel.Value
+		obj, index, indirect := check.lookupFieldOrMethod(base, false, check.pkg, sel)
+		switch obj.(type) {
+		case nil:
+			check.invalidArgf(x, "%s has no single field %s", base, sel)
+			return
+		case *Func:
+			// TODO(gri) Using derefStructPtr may result in methods being found
+			// that don't actually exist. An error either way, but the error
+			// message is confusing. See: https://play.golang.org/p/al75v23kUy ,
+			// but go/types reports: "invalid argument: x.m is a method value".
+			check.invalidArgf(arg0, "%s is a method value", arg0)
+			return
+		}
+		if indirect {
+			check.invalidArgf(x, "field %s is embedded via a pointer in %s", sel, base)
+			return
+		}
+
+		// TODO(gri) Should we pass x.typ instead of base (and indirect report if derefStructPtr indirected)?
+		check.recordSelection(selx, FieldVal, base, obj, index, false)
+
+		offs := check.conf.offsetof(base, index)
+		x.mode = constant_
+		x.val = constant.MakeInt64(offs)
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Sizeof:
+		// unsafe.Sizeof(x T) uintptr
+		if asTypeParam(x.typ) != nil {
+			check.invalidOpf(call, "unsafe.Sizeof undefined for %s", x)
+			return
+		}
+		check.assignment(x, nil, "argument to unsafe.Sizeof")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = constant_
+		x.val = constant.MakeInt64(check.conf.sizeof(x.typ))
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Assert:
+		// assert(pred) causes a typechecker error if pred is false.
+		// The result of assert is the value of pred if there is no error.
+		// Note: assert is only available in self-test mode.
+		if x.mode != constant_ || !isBoolean(x.typ) {
+			check.invalidArgf(x, "%s is not a boolean constant", x)
+			return
+		}
+		if x.val.Kind() != constant.Bool {
+			check.errorf(x, "internal error: value of %s should be a boolean constant", x)
+			return
+		}
+		if !constant.BoolVal(x.val) {
+			check.errorf(call, "%v failed", call)
+			// compile-time assertion failure - safe to continue
+		}
+		// result is constant - no need to record signature
+
+	case _Trace:
+		// trace(x, y, z, ...) dumps the positions, expressions, and
+		// values of its arguments. The result of trace is the value
+		// of the first argument.
+		// Note: trace is only available in self-test mode.
+		// (no argument evaluated yet)
+		if nargs == 0 {
+			check.dump("%v: trace() without arguments", posFor(call))
+			x.mode = novalue
+			break
+		}
+		var t operand
+		x1 := x
+		for _, arg := range call.ArgList {
+			check.rawExpr(x1, arg, nil) // permit trace for types, e.g.: new(trace(T))
+			check.dump("%v: %s", posFor(x1), x1)
+			x1 = &t // use incoming x only for first argument
+		}
+		// trace is only available in test mode - no need to record signature
+
+	default:
+		unreachable()
+	}
+
+	return true
+}
+
+// applyTypeFunc applies f to x. If x is a type parameter,
+// the result is a type parameter constrained by an new
+// interface bound. The type bounds for that interface
+// are computed by applying f to each of the type bounds
+// of x. If any of these applications of f return nil,
+// applyTypeFunc returns nil.
+// If x is not a type parameter, the result is f(x).
+func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
+	if tp := asTypeParam(x); tp != nil {
+		// Test if t satisfies the requirements for the argument
+		// type and collect possible result types at the same time.
+		var rtypes []Type
+		if !tp.Bound().is(func(x Type) bool {
+			if r := f(x); r != nil {
+				rtypes = append(rtypes, r)
+				return true
+			}
+			return false
+		}) {
+			return nil
+		}
+
+		// TODO(gri) Would it be ok to return just the one type
+		//           if len(rtypes) == 1? What about top-level
+		//           uses of real() where the result is used to
+		//           define type and initialize a variable?
+
+		// construct a suitable new type parameter
+		tpar := NewTypeName(nopos, nil /* = Universe pkg */, "<type parameter>", nil)
+		ptyp := check.NewTypeParam(tpar, 0, &emptyInterface) // assigns type to tpar as a side-effect
+		tsum := NewSum(rtypes)
+		ptyp.bound = &Interface{types: tsum, allMethods: markComplete, allTypes: tsum}
+
+		return ptyp
+	}
+
+	return f(x)
+}
+
+// makeSig makes a signature for the given argument and result types.
+// Default types are used for untyped arguments, and res may be nil.
+func makeSig(res Type, args ...Type) *Signature {
+	list := make([]*Var, len(args))
+	for i, param := range args {
+		list[i] = NewVar(nopos, nil, "", Default(param))
+	}
+	params := NewTuple(list...)
+	var result *Tuple
+	if res != nil {
+		assert(!isUntyped(res))
+		result = NewTuple(NewVar(nopos, nil, "", res))
+	}
+	return &Signature{params: params, results: result}
+}
+
+// implicitArrayDeref returns A if typ is of the form *A and A is an array;
+// otherwise it returns typ.
+//
+func implicitArrayDeref(typ Type) Type {
+	if p, ok := typ.(*Pointer); ok {
+		if a := asArray(p.base); a != nil {
+			return a
+		}
+	}
+	return typ
+}
+
+// unparen returns e with any enclosing parentheses stripped.
+func unparen(e syntax.Expr) syntax.Expr {
+	for {
+		p, ok := e.(*syntax.ParenExpr)
+		if !ok {
+			return e
+		}
+		e = p.X
+	}
+}

src/cmd/compile/internal/types2/builtins_test.go

@@ -0,0 +1,218 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2_test
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"testing"
+
+	. "cmd/compile/internal/types2"
+)
+
+var builtinCalls = []struct {
+	name, src, sig string
+}{
+	{"append", `var s []int; _ = append(s)`, `func([]int, ...int) []int`},
+	{"append", `var s []int; _ = append(s, 0)`, `func([]int, ...int) []int`},
+	{"append", `var s []int; _ = (append)(s, 0)`, `func([]int, ...int) []int`},
+	{"append", `var s []byte; _ = ((append))(s, 0)`, `func([]byte, ...byte) []byte`},
+	{"append", `var s []byte; _ = append(s, "foo"...)`, `func([]byte, string...) []byte`},
+	{"append", `type T []byte; var s T; var str string; _ = append(s, str...)`, `func(p.T, string...) p.T`},
+	{"append", `type T []byte; type U string; var s T; var str U; _ = append(s, str...)`, `func(p.T, p.U...) p.T`},
+
+	{"cap", `var s [10]int; _ = cap(s)`, `invalid type`},  // constant
+	{"cap", `var s [10]int; _ = cap(&s)`, `invalid type`}, // constant
+	{"cap", `var s []int64; _ = cap(s)`, `func([]int64) int`},
+	{"cap", `var c chan<-bool; _ = cap(c)`, `func(chan<- bool) int`},
+
+	{"len", `_ = len("foo")`, `invalid type`}, // constant
+	{"len", `var s string; _ = len(s)`, `func(string) int`},
+	{"len", `var s [10]int; _ = len(s)`, `invalid type`},  // constant
+	{"len", `var s [10]int; _ = len(&s)`, `invalid type`}, // constant
+	{"len", `var s []int64; _ = len(s)`, `func([]int64) int`},
+	{"len", `var c chan<-bool; _ = len(c)`, `func(chan<- bool) int`},
+	{"len", `var m map[string]float32; _ = len(m)`, `func(map[string]float32) int`},
+
+	{"close", `var c chan int; close(c)`, `func(chan int)`},
+	{"close", `var c chan<- chan string; close(c)`, `func(chan<- chan string)`},
+
+	{"complex", `_ = complex(1, 0)`, `invalid type`}, // constant
+	{"complex", `var re float32; _ = complex(re, 1.0)`, `func(float32, float32) complex64`},
+	{"complex", `var im float64; _ = complex(1, im)`, `func(float64, float64) complex128`},
+	{"complex", `type F32 float32; var re, im F32; _ = complex(re, im)`, `func(p.F32, p.F32) complex64`},
+	{"complex", `type F64 float64; var re, im F64; _ = complex(re, im)`, `func(p.F64, p.F64) complex128`},
+
+	{"copy", `var src, dst []byte; copy(dst, src)`, `func([]byte, []byte) int`},
+	{"copy", `type T [][]int; var src, dst T; _ = copy(dst, src)`, `func(p.T, p.T) int`},
+	{"copy", `var src string; var dst []byte; copy(dst, src)`, `func([]byte, string) int`},
+	{"copy", `type T string; type U []byte; var src T; var dst U; copy(dst, src)`, `func(p.U, p.T) int`},
+	{"copy", `var dst []byte; copy(dst, "hello")`, `func([]byte, string) int`},
+
+	{"delete", `var m map[string]bool; delete(m, "foo")`, `func(map[string]bool, string)`},
+	{"delete", `type (K string; V int); var m map[K]V; delete(m, "foo")`, `func(map[p.K]p.V, p.K)`},
+
+	{"imag", `_ = imag(1i)`, `invalid type`}, // constant
+	{"imag", `var c complex64; _ = imag(c)`, `func(complex64) float32`},
+	{"imag", `var c complex128; _ = imag(c)`, `func(complex128) float64`},
+	{"imag", `type C64 complex64; var c C64; _ = imag(c)`, `func(p.C64) float32`},
+	{"imag", `type C128 complex128; var c C128; _ = imag(c)`, `func(p.C128) float64`},
+
+	{"real", `_ = real(1i)`, `invalid type`}, // constant
+	{"real", `var c complex64; _ = real(c)`, `func(complex64) float32`},
+	{"real", `var c complex128; _ = real(c)`, `func(complex128) float64`},
+	{"real", `type C64 complex64; var c C64; _ = real(c)`, `func(p.C64) float32`},
+	{"real", `type C128 complex128; var c C128; _ = real(c)`, `func(p.C128) float64`},
+
+	{"make", `_ = make([]int, 10)`, `func([]int, int) []int`},
+	{"make", `type T []byte; _ = make(T, 10, 20)`, `func(p.T, int, int) p.T`},
+
+	// issue #37349
+	{"make", `              _ = make([]int, 0   )`, `func([]int, int) []int`},
+	{"make", `var l    int; _ = make([]int, l   )`, `func([]int, int) []int`},
+	{"make", `              _ = make([]int, 0, 0)`, `func([]int, int, int) []int`},
+	{"make", `var l    int; _ = make([]int, l, 0)`, `func([]int, int, int) []int`},
+	{"make", `var    c int; _ = make([]int, 0, c)`, `func([]int, int, int) []int`},
+	{"make", `var l, c int; _ = make([]int, l, c)`, `func([]int, int, int) []int`},
+
+	// issue #37393
+	{"make", `                _ = make([]int       , 0   )`, `func([]int, int) []int`},
+	{"make", `var l    byte ; _ = make([]int8      , l   )`, `func([]int8, byte) []int8`},
+	{"make", `                _ = make([]int16     , 0, 0)`, `func([]int16, int, int) []int16`},
+	{"make", `var l    int16; _ = make([]string    , l, 0)`, `func([]string, int16, int) []string`},
+	{"make", `var    c int32; _ = make([]float64   , 0, c)`, `func([]float64, int, int32) []float64`},
+	{"make", `var l, c uint ; _ = make([]complex128, l, c)`, `func([]complex128, uint, uint) []complex128`},
+
+	{"new", `_ = new(int)`, `func(int) *int`},
+	{"new", `type T struct{}; _ = new(T)`, `func(p.T) *p.T`},
+
+	{"panic", `panic(0)`, `func(interface{})`},
+	{"panic", `panic("foo")`, `func(interface{})`},
+
+	{"print", `print()`, `func()`},
+	{"print", `print(0)`, `func(int)`},
+	{"print", `print(1, 2.0, "foo", true)`, `func(int, float64, string, bool)`},
+
+	{"println", `println()`, `func()`},
+	{"println", `println(0)`, `func(int)`},
+	{"println", `println(1, 2.0, "foo", true)`, `func(int, float64, string, bool)`},
+
+	{"recover", `recover()`, `func() interface{}`},
+	{"recover", `_ = recover()`, `func() interface{}`},
+
+	{"Alignof", `_ = unsafe.Alignof(0)`, `invalid type`},                 // constant
+	{"Alignof", `var x struct{}; _ = unsafe.Alignof(x)`, `invalid type`}, // constant
+
+	{"Offsetof", `var x struct{f bool}; _ = unsafe.Offsetof(x.f)`, `invalid type`},           // constant
+	{"Offsetof", `var x struct{_ int; f bool}; _ = unsafe.Offsetof((&x).f)`, `invalid type`}, // constant
+
+	{"Sizeof", `_ = unsafe.Sizeof(0)`, `invalid type`},                 // constant
+	{"Sizeof", `var x struct{}; _ = unsafe.Sizeof(x)`, `invalid type`}, // constant
+
+	{"assert", `assert(true)`, `invalid type`},                                    // constant
+	{"assert", `type B bool; const pred B = 1 < 2; assert(pred)`, `invalid type`}, // constant
+
+	// no tests for trace since it produces output as a side-effect
+}
+
+func TestBuiltinSignatures(t *testing.T) {
+	DefPredeclaredTestFuncs()
+
+	seen := map[string]bool{"trace": true} // no test for trace built-in; add it manually
+	for _, call := range builtinCalls {
+		testBuiltinSignature(t, call.name, call.src, call.sig)
+		seen[call.name] = true
+	}
+
+	// make sure we didn't miss one
+	for _, name := range Universe.Names() {
+		if _, ok := Universe.Lookup(name).(*Builtin); ok && !seen[name] {
+			t.Errorf("missing test for %s", name)
+		}
+	}
+	for _, name := range Unsafe.Scope().Names() {
+		if _, ok := Unsafe.Scope().Lookup(name).(*Builtin); ok && !seen[name] {
+			t.Errorf("missing test for unsafe.%s", name)
+		}
+	}
+}
+
+func testBuiltinSignature(t *testing.T, name, src0, want string) {
+	src := fmt.Sprintf(`package p; import "unsafe"; type _ unsafe.Pointer /* use unsafe */; func _() { %s }`, src0)
+	f, err := parseSrc("", src)
+	if err != nil {
+		t.Errorf("%s: %s", src0, err)
+		return
+	}
+
+	conf := Config{Importer: defaultImporter()}
+	uses := make(map[*syntax.Name]Object)
+	types := make(map[syntax.Expr]TypeAndValue)
+	_, err = conf.Check(f.PkgName.Value, []*syntax.File{f}, &Info{Uses: uses, Types: types})
+	if err != nil {
+		t.Errorf("%s: %s", src0, err)
+		return
+	}
+
+	// find called function
+	n := 0
+	var fun syntax.Expr
+	for x := range types {
+		if call, _ := x.(*syntax.CallExpr); call != nil {
+			fun = call.Fun
+			n++
+		}
+	}
+	if n != 1 {
+		t.Errorf("%s: got %d CallExprs; want 1", src0, n)
+		return
+	}
+
+	// check recorded types for fun and descendents (may be parenthesized)
+	for {
+		// the recorded type for the built-in must match the wanted signature
+		typ := types[fun].Type
+		if typ == nil {
+			t.Errorf("%s: no type recorded for %s", src0, syntax.String(fun))
+			return
+		}
+		if got := typ.String(); got != want {
+			t.Errorf("%s: got type %s; want %s", src0, got, want)
+			return
+		}
+
+		// called function must be a (possibly parenthesized, qualified)
+		// identifier denoting the expected built-in
+		switch p := fun.(type) {
+		case *syntax.Name:
+			obj := uses[p]
+			if obj == nil {
+				t.Errorf("%s: no object found for %s", src0, p.Value)
+				return
+			}
+			bin, _ := obj.(*Builtin)
+			if bin == nil {
+				t.Errorf("%s: %s does not denote a built-in", src0, p.Value)
+				return
+			}
+			if bin.Name() != name {
+				t.Errorf("%s: got built-in %s; want %s", src0, bin.Name(), name)
+				return
+			}
+			return // we're done
+
+		case *syntax.ParenExpr:
+			fun = p.X // unpack
+
+		case *syntax.SelectorExpr:
+			// built-in from package unsafe - ignore details
+			return // we're done
+
+		default:
+			t.Errorf("%s: invalid function call", src0)
+			return
+		}
+	}
+}

src/cmd/compile/internal/types2/call.go

@@ -0,0 +1,770 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of call and selector expressions.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"strings"
+	"unicode"
+)
+
+// funcInst type-checks a function instantiaton inst and returns the result in x.
+// The operand x must be the evaluation of inst.X and its type must be a signature.
+func (check *Checker) funcInst(x *operand, inst *syntax.IndexExpr) {
+	args, ok := check.exprOrTypeList(unpackExpr(inst.Index))
+	if !ok {
+		x.mode = invalid
+		x.expr = inst
+		return
+	}
+	if len(args) > 0 && args[0].mode != typexpr {
+		check.errorf(args[0], "%s is not a type", args[0])
+		ok = false
+	}
+
+	// check number of type arguments
+	n := len(args)
+	sig := x.typ.(*Signature)
+	if !check.conf.InferFromConstraints && n != len(sig.tparams) || n > len(sig.tparams) {
+		check.errorf(args[n-1], "got %d type arguments but want %d", n, len(sig.tparams))
+		x.mode = invalid
+		x.expr = inst
+		return
+	}
+
+	// collect types
+	targs := make([]Type, n)
+	poslist := make([]syntax.Pos, n)
+	for i, a := range args {
+		if a.mode != typexpr {
+			// error was reported earlier
+			x.mode = invalid
+			x.expr = inst
+			return
+		}
+		targs[i] = a.typ
+		poslist[i] = a.Pos()
+	}
+
+	// if we don't have enough type arguments, use constraint type inference
+	var inferred bool
+	if n < len(sig.tparams) {
+		var failed int
+		targs, failed = check.inferB(sig.tparams, targs)
+		if targs == nil {
+			// error was already reported
+			x.mode = invalid
+			x.expr = inst
+			return
+		}
+		if failed >= 0 {
+			// at least one type argument couldn't be inferred
+			assert(targs[failed] == nil)
+			tpar := sig.tparams[failed]
+			check.errorf(inst, "cannot infer %s (%s) (%s)", tpar.name, tpar.pos, targs)
+			x.mode = invalid
+			x.expr = inst
+			return
+		}
+		// all type arguments were inferred sucessfully
+		if debug {
+			for _, targ := range targs {
+				assert(targ != nil)
+			}
+		}
+		n = len(targs)
+		inferred = true
+	}
+	assert(n == len(sig.tparams))
+
+	// instantiate function signature
+	for i, typ := range targs {
+		// some positions may be missing if types are inferred
+		var pos syntax.Pos
+		if i < len(poslist) {
+			pos = poslist[i]
+		}
+		check.ordinaryType(pos, typ)
+	}
+	res := check.instantiate(x.Pos(), sig, targs, poslist).(*Signature)
+	assert(res.tparams == nil) // signature is not generic anymore
+	if inferred {
+		check.recordInferred(inst, targs, res)
+	}
+	x.typ = res
+	x.mode = value
+	x.expr = inst
+}
+
+func (check *Checker) call(x *operand, call *syntax.CallExpr) exprKind {
+	check.exprOrType(x, call.Fun)
+
+	switch x.mode {
+	case invalid:
+		check.use(call.ArgList...)
+		x.expr = call
+		return statement
+
+	case typexpr:
+		// conversion
+		T := x.typ
+		x.mode = invalid
+		switch n := len(call.ArgList); n {
+		case 0:
+			check.errorf(call, "missing argument in conversion to %s", T)
+		case 1:
+			check.expr(x, call.ArgList[0])
+			if x.mode != invalid {
+				if t := asInterface(T); t != nil {
+					check.completeInterface(nopos, t)
+					if t.IsConstraint() {
+						check.errorf(call, "cannot use interface %s in conversion (contains type list or is comparable)", T)
+						break
+					}
+				}
+				if call.HasDots {
+					check.errorf(call.ArgList[0], "invalid use of ... in type conversion to %s", T)
+					break
+				}
+				check.conversion(x, T)
+			}
+		default:
+			check.use(call.ArgList...)
+			check.errorf(call.ArgList[n-1], "too many arguments in conversion to %s", T)
+		}
+		x.expr = call
+		return conversion
+
+	case builtin:
+		id := x.id
+		if !check.builtin(x, call, id) {
+			x.mode = invalid
+		}
+		x.expr = call
+		// a non-constant result implies a function call
+		if x.mode != invalid && x.mode != constant_ {
+			check.hasCallOrRecv = true
+		}
+		return predeclaredFuncs[id].kind
+
+	default:
+		// function/method call
+		cgocall := x.mode == cgofunc
+
+		sig := asSignature(x.typ)
+		if sig == nil {
+			check.invalidOpf(x, "cannot call non-function %s", x)
+			x.mode = invalid
+			x.expr = call
+			return statement
+		}
+
+		// evaluate arguments
+		args, ok := check.exprOrTypeList(call.ArgList)
+		if !ok {
+			x.mode = invalid
+			x.expr = call
+			return expression
+		}
+
+		sig = check.arguments(call, sig, args)
+
+		// determine result
+		switch sig.results.Len() {
+		case 0:
+			x.mode = novalue
+		case 1:
+			if cgocall {
+				x.mode = commaerr
+			} else {
+				x.mode = value
+			}
+			x.typ = sig.results.vars[0].typ // unpack tuple
+		default:
+			x.mode = value
+			x.typ = sig.results
+		}
+		x.expr = call
+		check.hasCallOrRecv = true
+
+		// if type inference failed, a parametrized result must be invalidated
+		// (operands cannot have a parametrized type)
+		if x.mode == value && len(sig.tparams) > 0 && isParameterized(sig.tparams, x.typ) {
+			x.mode = invalid
+		}
+
+		return statement
+	}
+}
+
+// exprOrTypeList returns a list of operands and reports an error if the
+// list contains a mix of values and types (ignoring invalid operands).
+// TODO(gri) Now we can split this into exprList and typeList.
+func (check *Checker) exprOrTypeList(elist []syntax.Expr) (xlist []*operand, ok bool) {
+	ok = true
+
+	switch len(elist) {
+	case 0:
+		// nothing to do
+
+	case 1:
+		// single (possibly comma-ok) value or type, or function returning multiple values
+		e := elist[0]
+		var x operand
+		check.multiExprOrType(&x, e)
+		if t, ok := x.typ.(*Tuple); ok && x.mode != invalid && x.mode != typexpr {
+			// multiple values
+			xlist = make([]*operand, t.Len())
+			for i, v := range t.vars {
+				xlist[i] = &operand{mode: value, expr: e, typ: v.typ}
+			}
+			break
+		}
+
+		check.instantiatedOperand(&x)
+
+		// exactly one (possibly invalid or comma-ok) value or type
+		xlist = []*operand{&x}
+
+	default:
+		// multiple (possibly invalid) values or types
+		xlist = make([]*operand, len(elist))
+		ntypes := 0
+		for i, e := range elist {
+			var x operand
+			check.exprOrType(&x, e)
+			xlist[i] = &x
+			switch x.mode {
+			case invalid:
+				ntypes = len(xlist) // make 'if' condition fail below (no additional error in this case)
+			case typexpr:
+				ntypes++
+				check.instantiatedOperand(&x)
+			}
+		}
+		if 0 < ntypes && ntypes < len(xlist) {
+			check.errorf(xlist[0], "mix of value and type expressions")
+			ok = false
+		}
+	}
+
+	return
+}
+
+func (check *Checker) exprList(elist []syntax.Expr, allowCommaOk bool) (xlist []*operand, commaOk bool) {
+	switch len(elist) {
+	case 0:
+		// nothing to do
+
+	case 1:
+		// single (possibly comma-ok) value, or function returning multiple values
+		e := elist[0]
+		var x operand
+		check.multiExpr(&x, e)
+		if t, ok := x.typ.(*Tuple); ok && x.mode != invalid {
+			// multiple values
+			xlist = make([]*operand, t.Len())
+			for i, v := range t.vars {
+				xlist[i] = &operand{mode: value, expr: e, typ: v.typ}
+			}
+			break
+		}
+
+		// exactly one (possibly invalid or comma-ok) value
+		xlist = []*operand{&x}
+		if allowCommaOk && (x.mode == mapindex || x.mode == commaok || x.mode == commaerr) {
+			x.mode = value
+			xlist = append(xlist, &operand{mode: value, expr: e, typ: Typ[UntypedBool]})
+			commaOk = true
+		}
+
+	default:
+		// multiple (possibly invalid) values
+		xlist = make([]*operand, len(elist))
+		for i, e := range elist {
+			var x operand
+			check.expr(&x, e)
+			xlist[i] = &x
+		}
+	}
+
+	return
+}
+
+func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, args []*operand) (rsig *Signature) {
+	rsig = sig
+
+	// TODO(gri) try to eliminate this extra verification loop
+	for _, a := range args {
+		switch a.mode {
+		case typexpr:
+			check.errorf(a, "%s used as value", a)
+			return
+		case invalid:
+			return
+		}
+	}
+
+	// Function call argument/parameter count requirements
+	//
+	//               | standard call    | dotdotdot call |
+	// --------------+------------------+----------------+
+	// standard func | nargs == npars   | invalid        |
+	// --------------+------------------+----------------+
+	// variadic func | nargs >= npars-1 | nargs == npars |
+	// --------------+------------------+----------------+
+
+	nargs := len(args)
+	npars := sig.params.Len()
+	ddd := call.HasDots
+
+	// set up parameters
+	sig_params := sig.params // adjusted for variadic functions (may be nil for empty parameter lists!)
+	adjusted := false        // indicates if sig_params is different from t.params
+	if sig.variadic {
+		if ddd {
+			// variadic_func(a, b, c...)
+			if len(call.ArgList) == 1 && nargs > 1 {
+				// f()... is not permitted if f() is multi-valued
+				//check.errorf(call.Ellipsis, "cannot use ... with %d-valued %s", nargs, call.ArgList[0])
+				check.errorf(call, "cannot use ... with %d-valued %s", nargs, call.ArgList[0])
+				return
+			}
+		} else {
+			// variadic_func(a, b, c)
+			if nargs >= npars-1 {
+				// Create custom parameters for arguments: keep
+				// the first npars-1 parameters and add one for
+				// each argument mapping to the ... parameter.
+				vars := make([]*Var, npars-1) // npars > 0 for variadic functions
+				copy(vars, sig.params.vars)
+				last := sig.params.vars[npars-1]
+				typ := last.typ.(*Slice).elem
+				for len(vars) < nargs {
+					vars = append(vars, NewParam(last.pos, last.pkg, last.name, typ))
+				}
+				sig_params = NewTuple(vars...) // possibly nil!
+				adjusted = true
+				npars = nargs
+			} else {
+				// nargs < npars-1
+				npars-- // for correct error message below
+			}
+		}
+	} else {
+		if ddd {
+			// standard_func(a, b, c...)
+			//check.errorf(call.Ellipsis, "cannot use ... in call to non-variadic %s", call.Fun)
+			check.errorf(call, "cannot use ... in call to non-variadic %s", call.Fun)
+			return
+		}
+		// standard_func(a, b, c)
+	}
+
+	// check argument count
+	switch {
+	case nargs < npars:
+		check.errorf(call, "not enough arguments in call to %s", call.Fun)
+		return
+	case nargs > npars:
+		check.errorf(args[npars], "too many arguments in call to %s", call.Fun) // report at first extra argument
+		return
+	}
+
+	// infer type arguments and instantiate signature if necessary
+	if len(sig.tparams) > 0 {
+		// TODO(gri) provide position information for targs so we can feed
+		//           it to the instantiate call for better error reporting
+		targs, failed := check.infer(sig.tparams, sig_params, args)
+		if targs == nil {
+			return // error already reported
+		}
+		if failed >= 0 {
+			// Some type arguments couldn't be inferred. Use
+			// bounds type inference to try to make progress.
+			if check.conf.InferFromConstraints {
+				targs, failed = check.inferB(sig.tparams, targs)
+				if targs == nil {
+					return // error already reported
+				}
+			}
+			if failed >= 0 {
+				// at least one type argument couldn't be inferred
+				assert(targs[failed] == nil)
+				tpar := sig.tparams[failed]
+				// TODO(gri) here we'd like to use the position of the call's ')'
+				check.errorf(call.Pos(), "cannot infer %s (%s) (%s)", tpar.name, tpar.pos, targs)
+				return
+			}
+		}
+		// all type arguments were inferred sucessfully
+		if debug {
+			for _, targ := range targs {
+				assert(targ != nil)
+			}
+		}
+		//check.dump("### inferred targs = %s", targs)
+
+		// compute result signature
+		rsig = check.instantiate(call.Pos(), sig, targs, nil).(*Signature)
+		assert(rsig.tparams == nil) // signature is not generic anymore
+		check.recordInferred(call, targs, rsig)
+
+		// Optimization: Only if the parameter list was adjusted do we
+		// need to compute it from the adjusted list; otherwise we can
+		// simply use the result signature's parameter list.
+		if adjusted {
+			sig_params = check.subst(call.Pos(), sig_params, makeSubstMap(sig.tparams, targs)).(*Tuple)
+		} else {
+			sig_params = rsig.params
+		}
+	}
+
+	// check arguments
+	for i, a := range args {
+		check.assignment(a, sig_params.vars[i].typ, "argument")
+	}
+
+	return
+}
+
+var cgoPrefixes = [...]string{
+	"_Ciconst_",
+	"_Cfconst_",
+	"_Csconst_",
+	"_Ctype_",
+	"_Cvar_", // actually a pointer to the var
+	"_Cfpvar_fp_",
+	"_Cfunc_",
+	"_Cmacro_", // function to evaluate the expanded expression
+}
+
+func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
+	// these must be declared before the "goto Error" statements
+	var (
+		obj      Object
+		index    []int
+		indirect bool
+	)
+
+	sel := e.Sel.Value
+	// If the identifier refers to a package, handle everything here
+	// so we don't need a "package" mode for operands: package names
+	// can only appear in qualified identifiers which are mapped to
+	// selector expressions.
+	if ident, ok := e.X.(*syntax.Name); ok {
+		obj := check.lookup(ident.Value)
+		if pname, _ := obj.(*PkgName); pname != nil {
+			assert(pname.pkg == check.pkg)
+			check.recordUse(ident, pname)
+			pname.used = true
+			pkg := pname.imported
+
+			var exp Object
+			funcMode := value
+			if pkg.cgo {
+				// cgo special cases C.malloc: it's
+				// rewritten to _CMalloc and does not
+				// support two-result calls.
+				if sel == "malloc" {
+					sel = "_CMalloc"
+				} else {
+					funcMode = cgofunc
+				}
+				for _, prefix := range cgoPrefixes {
+					// cgo objects are part of the current package (in file
+					// _cgo_gotypes.go). Use regular lookup.
+					_, exp = check.scope.LookupParent(prefix+sel, check.pos)
+					if exp != nil {
+						break
+					}
+				}
+				if exp == nil {
+					check.errorf(e.Sel, "%s not declared by package C", sel)
+					goto Error
+				}
+				check.objDecl(exp, nil)
+			} else {
+				exp = pkg.scope.Lookup(sel)
+				if exp == nil {
+					if !pkg.fake {
+						if check.conf.CompilerErrorMessages {
+							check.errorf(e.Sel, "undefined: %s.%s", pkg.name, sel)
+						} else {
+							check.errorf(e.Sel, "%s not declared by package %s", sel, pkg.name)
+						}
+					}
+					goto Error
+				}
+				if !exp.Exported() {
+					check.errorf(e.Sel, "%s not exported by package %s", sel, pkg.name)
+					// ok to continue
+				}
+			}
+			check.recordUse(e.Sel, exp)
+
+			// Simplified version of the code for *syntax.Names:
+			// - imported objects are always fully initialized
+			switch exp := exp.(type) {
+			case *Const:
+				assert(exp.Val() != nil)
+				x.mode = constant_
+				x.typ = exp.typ
+				x.val = exp.val
+			case *TypeName:
+				x.mode = typexpr
+				x.typ = exp.typ
+			case *Var:
+				x.mode = variable
+				x.typ = exp.typ
+				if pkg.cgo && strings.HasPrefix(exp.name, "_Cvar_") {
+					x.typ = x.typ.(*Pointer).base
+				}
+			case *Func:
+				x.mode = funcMode
+				x.typ = exp.typ
+				if pkg.cgo && strings.HasPrefix(exp.name, "_Cmacro_") {
+					x.mode = value
+					x.typ = x.typ.(*Signature).results.vars[0].typ
+				}
+			case *Builtin:
+				x.mode = builtin
+				x.typ = exp.typ
+				x.id = exp.id
+			default:
+				check.dump("%v: unexpected object %v", posFor(e.Sel), exp)
+				unreachable()
+			}
+			x.expr = e
+			return
+		}
+	}
+
+	check.exprOrType(x, e.X)
+	if x.mode == invalid {
+		goto Error
+	}
+
+	check.instantiatedOperand(x)
+
+	obj, index, indirect = check.lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
+	if obj == nil {
+		switch {
+		case index != nil:
+			// TODO(gri) should provide actual type where the conflict happens
+			check.errorf(e.Sel, "ambiguous selector %s.%s", x.expr, sel)
+		case indirect:
+			check.errorf(e.Sel, "cannot call pointer method %s on %s", sel, x.typ)
+		default:
+			var why string
+			if tpar := asTypeParam(x.typ); tpar != nil {
+				// Type parameter bounds don't specify fields, so don't mention "field".
+				switch obj := tpar.Bound().obj.(type) {
+				case nil:
+					why = check.sprintf("type bound for %s has no method %s", x.typ, sel)
+				case *TypeName:
+					why = check.sprintf("interface %s has no method %s", obj.name, sel)
+				}
+			} else {
+				why = check.sprintf("type %s has no field or method %s", x.typ, sel)
+			}
+
+			// Check if capitalization of sel matters and provide better error message in that case.
+			if len(sel) > 0 {
+				var changeCase string
+				if r := rune(sel[0]); unicode.IsUpper(r) {
+					changeCase = string(unicode.ToLower(r)) + sel[1:]
+				} else {
+					changeCase = string(unicode.ToUpper(r)) + sel[1:]
+				}
+				if obj, _, _ = check.lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, changeCase); obj != nil {
+					why += ", but does have " + changeCase
+				}
+			}
+
+			check.errorf(e.Sel, "%s.%s undefined (%s)", x.expr, sel, why)
+
+		}
+		goto Error
+	}
+
+	// methods may not have a fully set up signature yet
+	if m, _ := obj.(*Func); m != nil {
+		// check.dump("### found method %s", m)
+		check.objDecl(m, nil)
+		// If m has a parameterized receiver type, infer the type parameter
+		// values from the actual receiver provided and then substitute the
+		// type parameters in the signature accordingly.
+		// TODO(gri) factor this code out
+		sig := m.typ.(*Signature)
+		if len(sig.rparams) > 0 {
+			//check.dump("### recv typ = %s", x.typ)
+			//check.dump("### method = %s rparams = %s tparams = %s", m, sig.rparams, sig.tparams)
+			// The method may have a pointer receiver, but the actually provided receiver
+			// may be a (hopefully addressable) non-pointer value, or vice versa. Here we
+			// only care about inferring receiver type parameters; to make the inference
+			// work, match up pointer-ness of receiver and argument.
+			arg := x
+			if ptrRecv := isPointer(sig.recv.typ); ptrRecv != isPointer(arg.typ) {
+				copy := *arg
+				if ptrRecv {
+					copy.typ = NewPointer(arg.typ)
+				} else {
+					copy.typ = arg.typ.(*Pointer).base
+				}
+				arg = &copy
+			}
+			targs, failed := check.infer(sig.rparams, NewTuple(sig.recv), []*operand{arg})
+			//check.dump("### inferred targs = %s", targs)
+			if failed >= 0 {
+				// We may reach here if there were other errors (see issue #40056).
+				// check.infer will report a follow-up error.
+				// TODO(gri) avoid the follow-up error or provide better explanation.
+				goto Error
+			}
+			// Don't modify m. Instead - for now - make a copy of m and use that instead.
+			// (If we modify m, some tests will fail; possibly because the m is in use.)
+			// TODO(gri) investigate and provide a correct explanation here
+			copy := *m
+			copy.typ = check.subst(e.Pos(), m.typ, makeSubstMap(sig.rparams, targs))
+			obj = &copy
+		}
+		// TODO(gri) we also need to do substitution for parameterized interface methods
+		//           (this breaks code in testdata/linalg.go2 at the moment)
+		//           12/20/2019: Is this TODO still correct?
+	}
+
+	if x.mode == typexpr {
+		// method expression
+		m, _ := obj.(*Func)
+		if m == nil {
+			// TODO(gri) should check if capitalization of sel matters and provide better error message in that case
+			check.errorf(e.Sel, "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
+			goto Error
+		}
+
+		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)
+
+		// the receiver type becomes the type of the first function
+		// argument of the method expression's function type
+		var params []*Var
+		sig := m.typ.(*Signature)
+		if sig.params != nil {
+			params = sig.params.vars
+		}
+		x.mode = value
+		x.typ = &Signature{
+			tparams:  sig.tparams,
+			params:   NewTuple(append([]*Var{NewVar(nopos, check.pkg, "_", x.typ)}, params...)...),
+			results:  sig.results,
+			variadic: sig.variadic,
+		}
+
+		check.addDeclDep(m)
+
+	} else {
+		// regular selector
+		switch obj := obj.(type) {
+		case *Var:
+			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
+			if x.mode == variable || indirect {
+				x.mode = variable
+			} else {
+				x.mode = value
+			}
+			x.typ = obj.typ
+
+		case *Func:
+			// TODO(gri) If we needed to take into account the receiver's
+			// addressability, should we report the type &(x.typ) instead?
+			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)
+
+			x.mode = value
+
+			// remove receiver
+			sig := *obj.typ.(*Signature)
+			sig.recv = nil
+			x.typ = &sig
+
+			check.addDeclDep(obj)
+
+		default:
+			unreachable()
+		}
+	}
+
+	// everything went well
+	x.expr = e
+	return
+
+Error:
+	x.mode = invalid
+	x.expr = e
+}
+
+// use type-checks each argument.
+// Useful to make sure expressions are evaluated
+// (and variables are "used") in the presence of other errors.
+// The arguments may be nil.
+// TODO(gri) make this accept a []syntax.Expr and use an unpack function when we have a ListExpr?
+func (check *Checker) use(arg ...syntax.Expr) {
+	var x operand
+	for _, e := range arg {
+		// Certain AST fields may legally be nil (e.g., the ast.SliceExpr.High field).
+		if e == nil {
+			continue
+		}
+		if l, _ := e.(*syntax.ListExpr); l != nil {
+			check.use(l.ElemList...)
+			continue
+		}
+		check.rawExpr(&x, e, nil)
+	}
+}
+
+// useLHS is like use, but doesn't "use" top-level identifiers.
+// It should be called instead of use if the arguments are
+// expressions on the lhs of an assignment.
+// The arguments must not be nil.
+func (check *Checker) useLHS(arg ...syntax.Expr) {
+	var x operand
+	for _, e := range arg {
+		// If the lhs is an identifier denoting a variable v, this assignment
+		// is not a 'use' of v. Remember current value of v.used and restore
+		// after evaluating the lhs via check.rawExpr.
+		var v *Var
+		var v_used bool
+		if ident, _ := unparen(e).(*syntax.Name); ident != nil {
+			// never type-check the blank name on the lhs
+			if ident.Value == "_" {
+				continue
+			}
+			if _, obj := check.scope.LookupParent(ident.Value, nopos); obj != nil {
+				// It's ok to mark non-local variables, but ignore variables
+				// from other packages to avoid potential race conditions with
+				// dot-imported variables.
+				if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
+					v = w
+					v_used = v.used
+				}
+			}
+		}
+		check.rawExpr(&x, e, nil)
+		if v != nil {
+			v.used = v_used // restore v.used
+		}
+	}
+}
+
+// instantiatedOperand reports an error of x is an uninstantiated (generic) type and sets x.typ to Typ[Invalid].
+func (check *Checker) instantiatedOperand(x *operand) {
+	if x.mode == typexpr && isGeneric(x.typ) {
+		check.errorf(x, "cannot use generic type %s without instantiation", x.typ)
+		x.typ = Typ[Invalid]
+	}
+}

src/cmd/compile/internal/types2/check.go

@@ -0,0 +1,455 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements the Check function, which drives type-checking.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"errors"
+	"fmt"
+	"go/constant"
+)
+
+var nopos syntax.Pos
+
+// debugging/development support
+const debug = true // leave on during development
+
+// If forceStrict is set, the type-checker enforces additional
+// rules not specified by the Go 1 spec, but which will
+// catch guaranteed run-time errors if the respective
+// code is executed. In other words, programs passing in
+// strict mode are Go 1 compliant, but not all Go 1 programs
+// will pass in strict mode. The additional rules are:
+//
+// - A type assertion x.(T) where T is an interface type
+//   is invalid if any (statically known) method that exists
+//   for both x and T have different signatures.
+//
+const forceStrict = false
+
+// If methodTypeParamsOk is set, type parameters are
+// permitted in method declarations (in interfaces, too).
+// Generalization and experimental feature.
+const methodTypeParamsOk = true
+
+// exprInfo stores information about an untyped expression.
+type exprInfo struct {
+	isLhs bool // expression is lhs operand of a shift with delayed type-check
+	mode  operandMode
+	typ   *Basic
+	val   constant.Value // constant value; or nil (if not a constant)
+}
+
+// A context represents the context within which an object is type-checked.
+type context struct {
+	decl          *declInfo                 // package-level declaration whose init expression/function body is checked
+	scope         *Scope                    // top-most scope for lookups
+	pos           syntax.Pos                // if valid, identifiers are looked up as if at position pos (used by Eval)
+	iota          constant.Value            // value of iota in a constant declaration; nil otherwise
+	errpos        syntax.Pos                // if valid, identifier position of a constant with inherited initializer
+	sig           *Signature                // function signature if inside a function; nil otherwise
+	isPanic       map[*syntax.CallExpr]bool // set of panic call expressions (used for termination check)
+	hasLabel      bool                      // set if a function makes use of labels (only ~1% of functions); unused outside functions
+	hasCallOrRecv bool                      // set if an expression contains a function call or channel receive operation
+}
+
+// lookup looks up name in the current context and returns the matching object, or nil.
+func (ctxt *context) lookup(name string) Object {
+	_, obj := ctxt.scope.LookupParent(name, ctxt.pos)
+	return obj
+}
+
+// An importKey identifies an imported package by import path and source directory
+// (directory containing the file containing the import). In practice, the directory
+// may always be the same, or may not matter. Given an (import path, directory), an
+// importer must always return the same package (but given two different import paths,
+// an importer may still return the same package by mapping them to the same package
+// paths).
+type importKey struct {
+	path, dir string
+}
+
+// A dotImportKey describes a dot-imported object in the given scope.
+type dotImportKey struct {
+	scope *Scope
+	obj   Object
+}
+
+// A Checker maintains the state of the type checker.
+// It must be created with NewChecker.
+type Checker struct {
+	// package information
+	// (initialized by NewChecker, valid for the life-time of checker)
+	conf *Config
+	pkg  *Package
+	*Info
+	version version                     // accepted language version
+	nextId  uint64                      // unique Id for type parameters (first valid Id is 1)
+	objMap  map[Object]*declInfo        // maps package-level objects and (non-interface) methods to declaration info
+	impMap  map[importKey]*Package      // maps (import path, source directory) to (complete or fake) package
+	posMap  map[*Interface][]syntax.Pos // maps interface types to lists of embedded interface positions
+	typMap  map[string]*Named           // maps an instantiated named type hash to a *Named type
+	pkgCnt  map[string]int              // counts number of imported packages with a given name (for better error messages)
+
+	// information collected during type-checking of a set of package files
+	// (initialized by Files, valid only for the duration of check.Files;
+	// maps and lists are allocated on demand)
+	files        []*syntax.File            // list of package files
+	imports      []*PkgName                // list of imported packages
+	dotImportMap map[dotImportKey]*PkgName // maps dot-imported objects to the package they were dot-imported through
+
+	firstErr error                    // first error encountered
+	methods  map[*TypeName][]*Func    // maps package scope type names to associated non-blank (non-interface) methods
+	untyped  map[syntax.Expr]exprInfo // map of expressions without final type
+	delayed  []func()                 // stack of delayed action segments; segments are processed in FIFO order
+	finals   []func()                 // list of final actions; processed at the end of type-checking the current set of files
+	objPath  []Object                 // path of object dependencies during type inference (for cycle reporting)
+
+	// context within which the current object is type-checked
+	// (valid only for the duration of type-checking a specific object)
+	context
+
+	// debugging
+	indent int // indentation for tracing
+}
+
+// addDeclDep adds the dependency edge (check.decl -> to) if check.decl exists
+func (check *Checker) addDeclDep(to Object) {
+	from := check.decl
+	if from == nil {
+		return // not in a package-level init expression
+	}
+	if _, found := check.objMap[to]; !found {
+		return // to is not a package-level object
+	}
+	from.addDep(to)
+}
+
+func (check *Checker) rememberUntyped(e syntax.Expr, lhs bool, mode operandMode, typ *Basic, val constant.Value) {
+	m := check.untyped
+	if m == nil {
+		m = make(map[syntax.Expr]exprInfo)
+		check.untyped = m
+	}
+	m[e] = exprInfo{lhs, mode, typ, val}
+}
+
+// later pushes f on to the stack of actions that will be processed later;
+// either at the end of the current statement, or in case of a local constant
+// or variable declaration, before the constant or variable is in scope
+// (so that f still sees the scope before any new declarations).
+func (check *Checker) later(f func()) {
+	check.delayed = append(check.delayed, f)
+}
+
+// atEnd adds f to the list of actions processed at the end
+// of type-checking, before initialization order computation.
+// Actions added by atEnd are processed after any actions
+// added by later.
+func (check *Checker) atEnd(f func()) {
+	check.finals = append(check.finals, f)
+}
+
+// push pushes obj onto the object path and returns its index in the path.
+func (check *Checker) push(obj Object) int {
+	check.objPath = append(check.objPath, obj)
+	return len(check.objPath) - 1
+}
+
+// pop pops and returns the topmost object from the object path.
+func (check *Checker) pop() Object {
+	i := len(check.objPath) - 1
+	obj := check.objPath[i]
+	check.objPath[i] = nil
+	check.objPath = check.objPath[:i]
+	return obj
+}
+
+// NewChecker returns a new Checker instance for a given package.
+// Package files may be added incrementally via checker.Files.
+func NewChecker(conf *Config, pkg *Package, info *Info) *Checker {
+	// make sure we have a configuration
+	if conf == nil {
+		conf = new(Config)
+	}
+
+	// make sure we have an info struct
+	if info == nil {
+		info = new(Info)
+	}
+
+	version, err := parseGoVersion(conf.GoVersion)
+	if err != nil {
+		panic(fmt.Sprintf("invalid Go version %q (%v)", conf.GoVersion, err))
+	}
+
+	return &Checker{
+		conf:    conf,
+		pkg:     pkg,
+		Info:    info,
+		version: version,
+		nextId:  1,
+		objMap:  make(map[Object]*declInfo),
+		impMap:  make(map[importKey]*Package),
+		posMap:  make(map[*Interface][]syntax.Pos),
+		typMap:  make(map[string]*Named),
+		pkgCnt:  make(map[string]int),
+	}
+}
+
+// initFiles initializes the files-specific portion of checker.
+// The provided files must all belong to the same package.
+func (check *Checker) initFiles(files []*syntax.File) {
+	// start with a clean slate (check.Files may be called multiple times)
+	check.files = nil
+	check.imports = nil
+	check.dotImportMap = nil
+
+	check.firstErr = nil
+	check.methods = nil
+	check.untyped = nil
+	check.delayed = nil
+	check.finals = nil
+
+	// determine package name and collect valid files
+	pkg := check.pkg
+	for _, file := range files {
+		switch name := file.PkgName.Value; pkg.name {
+		case "":
+			if name != "_" {
+				pkg.name = name
+			} else {
+				check.errorf(file.PkgName, "invalid package name _")
+			}
+			fallthrough
+
+		case name:
+			check.files = append(check.files, file)
+
+		default:
+			check.errorf(file, "package %s; expected %s", name, pkg.name)
+			// ignore this file
+		}
+	}
+}
+
+// A bailout panic is used for early termination.
+type bailout struct{}
+
+func (check *Checker) handleBailout(err *error) {
+	switch p := recover().(type) {
+	case nil, bailout:
+		// normal return or early exit
+		*err = check.firstErr
+	default:
+		// re-panic
+		panic(p)
+	}
+}
+
+// Files checks the provided files as part of the checker's package.
+func (check *Checker) Files(files []*syntax.File) error { return check.checkFiles(files) }
+
+var errBadCgo = errors.New("cannot use FakeImportC and go115UsesCgo together")
+
+func (check *Checker) checkFiles(files []*syntax.File) (err error) {
+	if check.conf.FakeImportC && check.conf.go115UsesCgo {
+		return errBadCgo
+	}
+
+	defer check.handleBailout(&err)
+
+	print := func(msg string) {
+		if check.conf.Trace {
+			fmt.Println(msg)
+		}
+	}
+
+	print("== initFiles ==")
+	check.initFiles(files)
+
+	print("== collectObjects ==")
+	check.collectObjects()
+
+	print("== packageObjects ==")
+	check.packageObjects()
+
+	print("== processDelayed ==")
+	check.processDelayed(0) // incl. all functions
+	check.processFinals()
+
+	print("== initOrder ==")
+	check.initOrder()
+
+	if !check.conf.DisableUnusedImportCheck {
+		print("== unusedImports ==")
+		check.unusedImports()
+	}
+	// no longer needed - release memory
+	check.imports = nil
+	check.dotImportMap = nil
+
+	print("== recordUntyped ==")
+	check.recordUntyped()
+
+	if check.Info != nil {
+		print("== sanitizeInfo ==")
+		sanitizeInfo(check.Info)
+	}
+
+	check.pkg.complete = true
+
+	// TODO(gri) There's more memory we should release at this point.
+
+	return
+}
+
+// processDelayed processes all delayed actions pushed after top.
+func (check *Checker) processDelayed(top int) {
+	// If each delayed action pushes a new action, the
+	// stack will continue to grow during this loop.
+	// However, it is only processing functions (which
+	// are processed in a delayed fashion) that may
+	// add more actions (such as nested functions), so
+	// this is a sufficiently bounded process.
+	for i := top; i < len(check.delayed); i++ {
+		check.delayed[i]() // may append to check.delayed
+	}
+	assert(top <= len(check.delayed)) // stack must not have shrunk
+	check.delayed = check.delayed[:top]
+}
+
+func (check *Checker) processFinals() {
+	n := len(check.finals)
+	for _, f := range check.finals {
+		f() // must not append to check.finals
+	}
+	if len(check.finals) != n {
+		panic("internal error: final action list grew")
+	}
+}
+
+func (check *Checker) recordUntyped() {
+	if !debug && check.Types == nil {
+		return // nothing to do
+	}
+
+	for x, info := range check.untyped {
+		if debug && isTyped(info.typ) {
+			check.dump("%v: %s (type %s) is typed", posFor(x), x, info.typ)
+			unreachable()
+		}
+		check.recordTypeAndValue(x, info.mode, info.typ, info.val)
+	}
+}
+
+func (check *Checker) recordTypeAndValue(x syntax.Expr, mode operandMode, typ Type, val constant.Value) {
+	assert(x != nil)
+	assert(typ != nil)
+	if mode == invalid {
+		return // omit
+	}
+	if mode == constant_ {
+		assert(val != nil)
+		assert(typ == Typ[Invalid] || isConstType(typ))
+	}
+	if m := check.Types; m != nil {
+		m[x] = TypeAndValue{mode, typ, val}
+	}
+}
+
+func (check *Checker) recordBuiltinType(f syntax.Expr, sig *Signature) {
+	// f must be a (possibly parenthesized) identifier denoting a built-in
+	// (built-ins in package unsafe always produce a constant result and
+	// we don't record their signatures, so we don't see qualified idents
+	// here): record the signature for f and possible children.
+	for {
+		check.recordTypeAndValue(f, builtin, sig, nil)
+		switch p := f.(type) {
+		case *syntax.Name:
+			return // we're done
+		case *syntax.ParenExpr:
+			f = p.X
+		default:
+			unreachable()
+		}
+	}
+}
+
+func (check *Checker) recordCommaOkTypes(x syntax.Expr, a [2]Type) {
+	assert(x != nil)
+	if a[0] == nil || a[1] == nil {
+		return
+	}
+	assert(isTyped(a[0]) && isTyped(a[1]) && (isBoolean(a[1]) || a[1] == universeError))
+	if m := check.Types; m != nil {
+		for {
+			tv := m[x]
+			assert(tv.Type != nil) // should have been recorded already
+			pos := x.Pos()
+			tv.Type = NewTuple(
+				NewVar(pos, check.pkg, "", a[0]),
+				NewVar(pos, check.pkg, "", a[1]),
+			)
+			m[x] = tv
+			// if x is a parenthesized expression (p.X), update p.X
+			p, _ := x.(*syntax.ParenExpr)
+			if p == nil {
+				break
+			}
+			x = p.X
+		}
+	}
+}
+
+func (check *Checker) recordInferred(call syntax.Expr, targs []Type, sig *Signature) {
+	assert(call != nil)
+	assert(sig != nil)
+	if m := check.Inferred; m != nil {
+		m[call] = Inferred{targs, sig}
+	}
+}
+
+func (check *Checker) recordDef(id *syntax.Name, obj Object) {
+	assert(id != nil)
+	if m := check.Defs; m != nil {
+		m[id] = obj
+	}
+}
+
+func (check *Checker) recordUse(id *syntax.Name, obj Object) {
+	assert(id != nil)
+	assert(obj != nil)
+	if m := check.Uses; m != nil {
+		m[id] = obj
+	}
+}
+
+func (check *Checker) recordImplicit(node syntax.Node, obj Object) {
+	assert(node != nil)
+	assert(obj != nil)
+	if m := check.Implicits; m != nil {
+		m[node] = obj
+	}
+}
+
+func (check *Checker) recordSelection(x *syntax.SelectorExpr, kind SelectionKind, recv Type, obj Object, index []int, indirect bool) {
+	assert(obj != nil && (recv == nil || len(index) > 0))
+	check.recordUse(x.Sel, obj)
+	if m := check.Selections; m != nil {
+		m[x] = &Selection{kind, recv, obj, index, indirect}
+	}
+}
+
+func (check *Checker) recordScope(node syntax.Node, scope *Scope) {
+	assert(node != nil)
+	assert(scope != nil)
+	if m := check.Scopes; m != nil {
+		m[node] = scope
+	}
+}

src/cmd/compile/internal/types2/check_test.go

@@ -0,0 +1,298 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a typechecker test harness. The packages specified
+// in tests are typechecked. Error messages reported by the typechecker are
+// compared against the error messages expected in the test files.
+//
+// Expected errors are indicated in the test files by putting a comment
+// of the form /* ERROR "rx" */ immediately following an offending token.
+// The harness will verify that an error matching the regular expression
+// rx is reported at that source position. Consecutive comments may be
+// used to indicate multiple errors for the same token position.
+//
+// For instance, the following test file indicates that a "not declared"
+// error should be reported for the undeclared variable x:
+//
+//	package p
+//	func f() {
+//		_ = x /* ERROR "not declared" */ + 1
+//	}
+
+// TODO(gri) Also collect strict mode errors of the form /* STRICT ... */
+//           and test against strict mode.
+
+package types2_test
+
+import (
+	"cmd/compile/internal/syntax"
+	"flag"
+	"fmt"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+
+	. "cmd/compile/internal/types2"
+)
+
+var (
+	haltOnError = flag.Bool("halt", false, "halt on error")
+	listErrors  = flag.Bool("errlist", false, "list errors")
+	testFiles   = flag.String("files", "", "comma-separated list of test files")
+	goVersion   = flag.String("lang", "", "Go language version (e.g. \"go1.12\"")
+)
+
+func parseFiles(t *testing.T, filenames []string, mode syntax.Mode) ([]*syntax.File, []error) {
+	var files []*syntax.File
+	var errlist []error
+	errh := func(err error) { errlist = append(errlist, err) }
+	for _, filename := range filenames {
+		file, err := syntax.ParseFile(filename, errh, nil, mode)
+		if file == nil {
+			t.Fatalf("%s: %s", filename, err)
+		}
+		files = append(files, file)
+	}
+	return files, errlist
+}
+
+func unpackError(err error) syntax.Error {
+	switch err := err.(type) {
+	case syntax.Error:
+		return err
+	case Error:
+		return syntax.Error{Pos: err.Pos, Msg: err.Msg}
+	default:
+		return syntax.Error{Msg: err.Error()}
+	}
+}
+
+func delta(x, y uint) uint {
+	switch {
+	case x < y:
+		return y - x
+	case x > y:
+		return x - y
+	default:
+		return 0
+	}
+}
+
+// goVersionRx matches a Go version string using '_', e.g. "go1_12".
+var goVersionRx = regexp.MustCompile(`^go[1-9][0-9]*_(0|[1-9][0-9]*)$`)
+
+// asGoVersion returns a regular Go language version string
+// if s is a Go version string using '_' rather than '.' to
+// separate the major and minor version numbers (e.g. "go1_12").
+// Otherwise it returns the empty string.
+func asGoVersion(s string) string {
+	if goVersionRx.MatchString(s) {
+		return strings.Replace(s, "_", ".", 1)
+	}
+	return ""
+}
+
+func checkFiles(t *testing.T, sources []string, goVersion string, colDelta uint, trace bool) {
+	if len(sources) == 0 {
+		t.Fatal("no source files")
+	}
+
+	var mode syntax.Mode
+	if strings.HasSuffix(sources[0], ".go2") {
+		mode |= syntax.AllowGenerics
+	}
+	// parse files and collect parser errors
+	files, errlist := parseFiles(t, sources, mode)
+
+	pkgName := "<no package>"
+	if len(files) > 0 {
+		pkgName = files[0].PkgName.Value
+	}
+
+	// if no Go version is given, consider the package name
+	if goVersion == "" {
+		goVersion = asGoVersion(pkgName)
+	}
+
+	if *listErrors && len(errlist) > 0 {
+		t.Errorf("--- %s:", pkgName)
+		for _, err := range errlist {
+			t.Error(err)
+		}
+	}
+
+	// typecheck and collect typechecker errors
+	var conf Config
+	conf.GoVersion = goVersion
+	conf.AcceptMethodTypeParams = true
+	conf.InferFromConstraints = true
+	// special case for importC.src
+	if len(sources) == 1 && strings.HasSuffix(sources[0], "importC.src") {
+		conf.FakeImportC = true
+	}
+	conf.Trace = trace
+	conf.Importer = defaultImporter()
+	conf.Error = func(err error) {
+		if *haltOnError {
+			defer panic(err)
+		}
+		if *listErrors {
+			t.Error(err)
+			return
+		}
+		// Ignore secondary error messages starting with "\t";
+		// they are clarifying messages for a primary error.
+		if !strings.Contains(err.Error(), ": \t") {
+			errlist = append(errlist, err)
+		}
+	}
+	conf.Check(pkgName, files, nil)
+
+	if *listErrors {
+		return
+	}
+
+	// collect expected errors
+	errmap := make(map[string]map[uint][]syntax.Error)
+	for _, filename := range sources {
+		f, err := os.Open(filename)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		if m := syntax.ErrorMap(f); len(m) > 0 {
+			errmap[filename] = m
+		}
+		f.Close()
+	}
+
+	// match against found errors
+	for _, err := range errlist {
+		got := unpackError(err)
+
+		// find list of errors for the respective error line
+		filename := got.Pos.Base().Filename()
+		filemap := errmap[filename]
+		var line uint
+		var list []syntax.Error
+		if filemap != nil {
+			line = got.Pos.Line()
+			list = filemap[line]
+		}
+		// list may be nil
+
+		// one of errors in list should match the current error
+		index := -1 // list index of matching message, if any
+		for i, want := range list {
+			rx, err := regexp.Compile(want.Msg)
+			if err != nil {
+				t.Errorf("%s:%d:%d: %v", filename, line, want.Pos.Col(), err)
+				continue
+			}
+			if rx.MatchString(got.Msg) {
+				index = i
+				break
+			}
+		}
+		if index < 0 {
+			t.Errorf("%s: no error expected: %q", got.Pos, got.Msg)
+			continue
+		}
+
+		// column position must be within expected colDelta
+		want := list[index]
+		if delta(got.Pos.Col(), want.Pos.Col()) > colDelta {
+			t.Errorf("%s: got col = %d; want %d", got.Pos, got.Pos.Col(), want.Pos.Col())
+		}
+
+		// eliminate from list
+		if n := len(list) - 1; n > 0 {
+			// not the last entry - swap in last element and shorten list by 1
+			list[index] = list[n]
+			filemap[line] = list[:n]
+		} else {
+			// last entry - remove list from filemap
+			delete(filemap, line)
+		}
+
+		// if filemap is empty, eliminate from errmap
+		if len(filemap) == 0 {
+			delete(errmap, filename)
+		}
+	}
+
+	// there should be no expected errors left
+	if len(errmap) > 0 {
+		t.Errorf("--- %s: unreported errors:", pkgName)
+		for filename, filemap := range errmap {
+			for line, list := range filemap {
+				for _, err := range list {
+					t.Errorf("%s:%d:%d: %s", filename, line, err.Pos.Col(), err.Msg)
+				}
+			}
+		}
+	}
+}
+
+// TestCheck is for manual testing of selected input files, provided with -files.
+// The accepted Go language version can be controlled with the -lang flag.
+func TestCheck(t *testing.T) {
+	if *testFiles == "" {
+		return
+	}
+	testenv.MustHaveGoBuild(t)
+	DefPredeclaredTestFuncs()
+	checkFiles(t, strings.Split(*testFiles, ","), *goVersion, 0, testing.Verbose())
+}
+
+func TestTestdata(t *testing.T)  { DefPredeclaredTestFuncs(); testDir(t, 75, "testdata") } // TODO(gri) narrow column tolerance
+func TestExamples(t *testing.T)  { testDir(t, 0, "examples") }
+func TestFixedbugs(t *testing.T) { testDir(t, 0, "fixedbugs") }
+
+func testDir(t *testing.T, colDelta uint, dir string) {
+	testenv.MustHaveGoBuild(t)
+
+	fis, err := ioutil.ReadDir(dir)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	for count, fi := range fis {
+		path := filepath.Join(dir, fi.Name())
+
+		// if fi is a directory, its files make up a single package
+		if fi.IsDir() {
+			if testing.Verbose() {
+				fmt.Printf("%3d %s\n", count, path)
+			}
+			fis, err := ioutil.ReadDir(path)
+			if err != nil {
+				t.Error(err)
+				continue
+			}
+			files := make([]string, len(fis))
+			for i, fi := range fis {
+				// if fi is a directory, checkFiles below will complain
+				files[i] = filepath.Join(path, fi.Name())
+				if testing.Verbose() {
+					fmt.Printf("\t%s\n", files[i])
+				}
+			}
+			checkFiles(t, files, "", colDelta, false)
+			continue
+		}
+
+		// otherwise, fi is a stand-alone file
+		if testing.Verbose() {
+			fmt.Printf("%3d %s\n", count, path)
+		}
+		checkFiles(t, []string{path}, "", colDelta, false)
+	}
+}

src/cmd/compile/internal/types2/conversions.go

@@ -0,0 +1,166 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of conversions.
+
+package types2
+
+import (
+	"go/constant"
+	"unicode"
+)
+
+// Conversion type-checks the conversion T(x).
+// The result is in x.
+func (check *Checker) conversion(x *operand, T Type) {
+	constArg := x.mode == constant_
+
+	var ok bool
+	switch {
+	case constArg && isConstType(T):
+		// constant conversion
+		switch t := asBasic(T); {
+		case representableConst(x.val, check, t, &x.val):
+			ok = true
+		case isInteger(x.typ) && isString(t):
+			codepoint := unicode.ReplacementChar
+			if i, ok := constant.Uint64Val(x.val); ok && i <= unicode.MaxRune {
+				codepoint = rune(i)
+			}
+			x.val = constant.MakeString(string(codepoint))
+			ok = true
+		}
+	case x.convertibleTo(check, T):
+		// non-constant conversion
+		x.mode = value
+		ok = true
+	}
+
+	if !ok {
+		check.errorf(x, "cannot convert %s to %s", x, T)
+		x.mode = invalid
+		return
+	}
+
+	// The conversion argument types are final. For untyped values the
+	// conversion provides the type, per the spec: "A constant may be
+	// given a type explicitly by a constant declaration or conversion,...".
+	if isUntyped(x.typ) {
+		final := T
+		// - For conversions to interfaces, except for untyped nil arguments,
+		//   use the argument's default type.
+		// - For conversions of untyped constants to non-constant types, also
+		//   use the default type (e.g., []byte("foo") should report string
+		//   not []byte as type for the constant "foo").
+		// - For integer to string conversions, keep the argument type.
+		//   (See also the TODO below.)
+		if x.typ == Typ[UntypedNil] {
+			// ok
+		} else if IsInterface(T) || constArg && !isConstType(T) {
+			final = Default(x.typ)
+		} else if isInteger(x.typ) && isString(T) {
+			final = x.typ
+		}
+		check.updateExprType(x.expr, final, true)
+	}
+
+	x.typ = T
+}
+
+// TODO(gri) convertibleTo checks if T(x) is valid. It assumes that the type
+// of x is fully known, but that's not the case for say string(1<<s + 1.0):
+// Here, the type of 1<<s + 1.0 will be UntypedFloat which will lead to the
+// (correct!) refusal of the conversion. But the reported error is essentially
+// "cannot convert untyped float value to string", yet the correct error (per
+// the spec) is that we cannot shift a floating-point value: 1 in 1<<s should
+// be converted to UntypedFloat because of the addition of 1.0. Fixing this
+// is tricky because we'd have to run updateExprType on the argument first.
+// (Issue #21982.)
+
+// convertibleTo reports whether T(x) is valid.
+// The check parameter may be nil if convertibleTo is invoked through an
+// exported API call, i.e., when all methods have been type-checked.
+func (x *operand) convertibleTo(check *Checker, T Type) bool {
+	// "x is assignable to T"
+	if x.assignableTo(check, T, nil) {
+		return true
+	}
+
+	// "x's type and T have identical underlying types if tags are ignored"
+	V := x.typ
+	Vu := under(V)
+	Tu := under(T)
+	if check.identicalIgnoreTags(Vu, Tu) {
+		return true
+	}
+
+	// "x's type and T are unnamed pointer types and their pointer base types
+	// have identical underlying types if tags are ignored"
+	if V, ok := V.(*Pointer); ok {
+		if T, ok := T.(*Pointer); ok {
+			if check.identicalIgnoreTags(under(V.base), under(T.base)) {
+				return true
+			}
+		}
+	}
+
+	// "x's type and T are both integer or floating point types"
+	if isIntegerOrFloat(V) && isIntegerOrFloat(T) {
+		return true
+	}
+
+	// "x's type and T are both complex types"
+	if isComplex(V) && isComplex(T) {
+		return true
+	}
+
+	// "x is an integer or a slice of bytes or runes and T is a string type"
+	if (isInteger(V) || isBytesOrRunes(Vu)) && isString(T) {
+		return true
+	}
+
+	// "x is a string and T is a slice of bytes or runes"
+	if isString(V) && isBytesOrRunes(Tu) {
+		return true
+	}
+
+	// package unsafe:
+	// "any pointer or value of underlying type uintptr can be converted into a unsafe.Pointer"
+	if (isPointer(Vu) || isUintptr(Vu)) && isUnsafePointer(T) {
+		return true
+	}
+	// "and vice versa"
+	if isUnsafePointer(V) && (isPointer(Tu) || isUintptr(Tu)) {
+		return true
+	}
+
+	return false
+}
+
+func isUintptr(typ Type) bool {
+	t := asBasic(typ)
+	return t != nil && t.kind == Uintptr
+}
+
+func isUnsafePointer(typ Type) bool {
+	// TODO(gri): Is this asBasic(typ) instead of typ.(*Basic) correct?
+	//            (The former calls under(), while the latter doesn't.)
+	//            The spec does not say so, but gc claims it is. See also
+	//            issue 6326.
+	t := asBasic(typ)
+	return t != nil && t.kind == UnsafePointer
+}
+
+func isPointer(typ Type) bool {
+	return asPointer(typ) != nil
+}
+
+func isBytesOrRunes(typ Type) bool {
+	if s := asSlice(typ); s != nil {
+		t := asBasic(s.elem)
+		return t != nil && (t.kind == Byte || t.kind == Rune)
+	}
+	return false
+}

src/cmd/compile/internal/types2/decl.go

@@ -0,0 +1,978 @@
+// UNREVIEWED
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/constant"
+)
+
+func (check *Checker) reportAltDecl(obj Object) {
+	if pos := obj.Pos(); pos.IsKnown() {
+		// We use "other" rather than "previous" here because
+		// the first declaration seen may not be textually
+		// earlier in the source.
+		check.errorf(pos, "\tother declaration of %s", obj.Name()) // secondary error, \t indented
+	}
+}
+
+func (check *Checker) declare(scope *Scope, id *syntax.Name, obj Object, pos syntax.Pos) {
+	// spec: "The blank identifier, represented by the underscore
+	// character _, may be used in a declaration like any other
+	// identifier but the declaration does not introduce a new
+	// binding."
+	if obj.Name() != "_" {
+		if alt := scope.Insert(obj); alt != nil {
+			check.errorf(obj.Pos(), "%s redeclared in this block", obj.Name())
+			check.reportAltDecl(alt)
+			return
+		}
+		obj.setScopePos(pos)
+	}
+	if id != nil {
+		check.recordDef(id, obj)
+	}
+}
+
+// pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
+func pathString(path []Object) string {
+	var s string
+	for i, p := range path {
+		if i > 0 {
+			s += "->"
+		}
+		s += p.Name()
+	}
+	return s
+}
+
+// objDecl type-checks the declaration of obj in its respective (file) context.
+// For the meaning of def, see Checker.definedType, in typexpr.go.
+func (check *Checker) objDecl(obj Object, def *Named) {
+	if check.conf.Trace && obj.Type() == nil {
+		if check.indent == 0 {
+			fmt.Println() // empty line between top-level objects for readability
+		}
+		check.trace(obj.Pos(), "-- checking %s (%s, objPath = %s)", obj, obj.color(), pathString(check.objPath))
+		check.indent++
+		defer func() {
+			check.indent--
+			check.trace(obj.Pos(), "=> %s (%s)", obj, obj.color())
+		}()
+	}
+
+	// Checking the declaration of obj means inferring its type
+	// (and possibly its value, for constants).
+	// An object's type (and thus the object) may be in one of
+	// three states which are expressed by colors:
+	//
+	// - an object whose type is not yet known is painted white (initial color)
+	// - an object whose type is in the process of being inferred is painted grey
+	// - an object whose type is fully inferred is painted black
+	//
+	// During type inference, an object's color changes from white to grey
+	// to black (pre-declared objects are painted black from the start).
+	// A black object (i.e., its type) can only depend on (refer to) other black
+	// ones. White and grey objects may depend on white and black objects.
+	// A dependency on a grey object indicates a cycle which may or may not be
+	// valid.
+	//
+	// When objects turn grey, they are pushed on the object path (a stack);
+	// they are popped again when they turn black. Thus, if a grey object (a
+	// cycle) is encountered, it is on the object path, and all the objects
+	// it depends on are the remaining objects on that path. Color encoding
+	// is such that the color value of a grey object indicates the index of
+	// that object in the object path.
+
+	// During type-checking, white objects may be assigned a type without
+	// traversing through objDecl; e.g., when initializing constants and
+	// variables. Update the colors of those objects here (rather than
+	// everywhere where we set the type) to satisfy the color invariants.
+	if obj.color() == white && obj.Type() != nil {
+		obj.setColor(black)
+		return
+	}
+
+	switch obj.color() {
+	case white:
+		assert(obj.Type() == nil)
+		// All color values other than white and black are considered grey.
+		// Because black and white are < grey, all values >= grey are grey.
+		// Use those values to encode the object's index into the object path.
+		obj.setColor(grey + color(check.push(obj)))
+		defer func() {
+			check.pop().setColor(black)
+		}()
+
+	case black:
+		assert(obj.Type() != nil)
+		return
+
+	default:
+		// Color values other than white or black are considered grey.
+		fallthrough
+
+	case grey:
+		// We have a cycle.
+		// In the existing code, this is marked by a non-nil type
+		// for the object except for constants and variables whose
+		// type may be non-nil (known), or nil if it depends on the
+		// not-yet known initialization value.
+		// In the former case, set the type to Typ[Invalid] because
+		// we have an initialization cycle. The cycle error will be
+		// reported later, when determining initialization order.
+		// TODO(gri) Report cycle here and simplify initialization
+		// order code.
+		switch obj := obj.(type) {
+		case *Const:
+			if check.cycle(obj) || obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+
+		case *Var:
+			if check.cycle(obj) || obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+
+		case *TypeName:
+			if check.cycle(obj) {
+				// break cycle
+				// (without this, calling underlying()
+				// below may lead to an endless loop
+				// if we have a cycle for a defined
+				// (*Named) type)
+				obj.typ = Typ[Invalid]
+			}
+
+		case *Func:
+			if check.cycle(obj) {
+				// Don't set obj.typ to Typ[Invalid] here
+				// because plenty of code type-asserts that
+				// functions have a *Signature type. Grey
+				// functions have their type set to an empty
+				// signature which makes it impossible to
+				// initialize a variable with the function.
+			}
+
+		default:
+			unreachable()
+		}
+		assert(obj.Type() != nil)
+		return
+	}
+
+	d := check.objMap[obj]
+	if d == nil {
+		check.dump("%v: %s should have been declared", obj.Pos(), obj)
+		unreachable()
+	}
+
+	// save/restore current context and setup object context
+	defer func(ctxt context) {
+		check.context = ctxt
+	}(check.context)
+	check.context = context{
+		scope: d.file,
+	}
+
+	// Const and var declarations must not have initialization
+	// cycles. We track them by remembering the current declaration
+	// in check.decl. Initialization expressions depending on other
+	// consts, vars, or functions, add dependencies to the current
+	// check.decl.
+	switch obj := obj.(type) {
+	case *Const:
+		check.decl = d // new package-level const decl
+		check.constDecl(obj, d.vtyp, d.init, d.inherited)
+	case *Var:
+		check.decl = d // new package-level var decl
+		check.varDecl(obj, d.lhs, d.vtyp, d.init)
+	case *TypeName:
+		// invalid recursive types are detected via path
+		check.typeDecl(obj, d.tdecl, def)
+		check.collectMethods(obj) // methods can only be added to top-level types
+	case *Func:
+		// functions may be recursive - no need to track dependencies
+		check.funcDecl(obj, d)
+	default:
+		unreachable()
+	}
+}
+
+// cycle checks if the cycle starting with obj is valid and
+// reports an error if it is not.
+func (check *Checker) cycle(obj Object) (isCycle bool) {
+	// The object map contains the package scope objects and the non-interface methods.
+	if debug {
+		info := check.objMap[obj]
+		inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
+		isPkgObj := obj.Parent() == check.pkg.scope
+		if isPkgObj != inObjMap {
+			check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
+			unreachable()
+		}
+	}
+
+	// Count cycle objects.
+	assert(obj.color() >= grey)
+	start := obj.color() - grey // index of obj in objPath
+	cycle := check.objPath[start:]
+	nval := 0 // number of (constant or variable) values in the cycle
+	ndef := 0 // number of type definitions in the cycle
+	for _, obj := range cycle {
+		switch obj := obj.(type) {
+		case *Const, *Var:
+			nval++
+		case *TypeName:
+			// Determine if the type name is an alias or not. For
+			// package-level objects, use the object map which
+			// provides syntactic information (which doesn't rely
+			// on the order in which the objects are set up). For
+			// local objects, we can rely on the order, so use
+			// the object's predicate.
+			// TODO(gri) It would be less fragile to always access
+			// the syntactic information. We should consider storing
+			// this information explicitly in the object.
+			var alias bool
+			if d := check.objMap[obj]; d != nil {
+				alias = d.tdecl.Alias // package-level object
+			} else {
+				alias = obj.IsAlias() // function local object
+			}
+			if !alias {
+				ndef++
+			}
+		case *Func:
+			// ignored for now
+		default:
+			unreachable()
+		}
+	}
+
+	if check.conf.Trace {
+		check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
+		check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
+		defer func() {
+			if isCycle {
+				check.trace(obj.Pos(), "=> error: cycle is invalid")
+			}
+		}()
+	}
+
+	// A cycle involving only constants and variables is invalid but we
+	// ignore them here because they are reported via the initialization
+	// cycle check.
+	if nval == len(cycle) {
+		return false
+	}
+
+	// A cycle involving only types (and possibly functions) must have at least
+	// one type definition to be permitted: If there is no type definition, we
+	// have a sequence of alias type names which will expand ad infinitum.
+	if nval == 0 && ndef > 0 {
+		return false // cycle is permitted
+	}
+
+	check.cycleError(cycle)
+
+	return true
+}
+
+type typeInfo uint
+
+// validType verifies that the given type does not "expand" infinitely
+// producing a cycle in the type graph. Cycles are detected by marking
+// defined types.
+// (Cycles involving alias types, as in "type A = [10]A" are detected
+// earlier, via the objDecl cycle detection mechanism.)
+func (check *Checker) validType(typ Type, path []Object) typeInfo {
+	const (
+		unknown typeInfo = iota
+		marked
+		valid
+		invalid
+	)
+
+	switch t := typ.(type) {
+	case *Array:
+		return check.validType(t.elem, path)
+
+	case *Struct:
+		for _, f := range t.fields {
+			if check.validType(f.typ, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Interface:
+		for _, etyp := range t.embeddeds {
+			if check.validType(etyp, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Named:
+		// don't touch the type if it is from a different package or the Universe scope
+		// (doing so would lead to a race condition - was issue #35049)
+		if t.obj.pkg != check.pkg {
+			return valid
+		}
+
+		// don't report a 2nd error if we already know the type is invalid
+		// (e.g., if a cycle was detected earlier, via Checker.underlying).
+		if t.underlying == Typ[Invalid] {
+			t.info = invalid
+			return invalid
+		}
+
+		switch t.info {
+		case unknown:
+			t.info = marked
+			t.info = check.validType(t.orig, append(path, t.obj)) // only types of current package added to path
+		case marked:
+			// cycle detected
+			for i, tn := range path {
+				if t.obj.pkg != check.pkg {
+					panic("internal error: type cycle via package-external type")
+				}
+				if tn == t.obj {
+					check.cycleError(path[i:])
+					t.info = invalid
+					return t.info
+				}
+			}
+			panic("internal error: cycle start not found")
+		}
+		return t.info
+
+	case *instance:
+		return check.validType(t.expand(), path)
+	}
+
+	return valid
+}
+
+// cycleError reports a declaration cycle starting with
+// the object in cycle that is "first" in the source.
+func (check *Checker) cycleError(cycle []Object) {
+	// TODO(gri) Should we start with the last (rather than the first) object in the cycle
+	//           since that is the earliest point in the source where we start seeing the
+	//           cycle? That would be more consistent with other error messages.
+	i := firstInSrc(cycle)
+	obj := cycle[i]
+	if check.conf.CompilerErrorMessages {
+		check.errorf(obj.Pos(), "invalid recursive type %s", obj.Name())
+	} else {
+		check.errorf(obj.Pos(), "illegal cycle in declaration of %s", obj.Name())
+	}
+	for range cycle {
+		check.errorf(obj.Pos(), "\t%s refers to", obj.Name()) // secondary error, \t indented
+		i++
+		if i >= len(cycle) {
+			i = 0
+		}
+		obj = cycle[i]
+	}
+	check.errorf(obj.Pos(), "\t%s", obj.Name())
+}
+
+// TODO(gri) This functionality should probably be with the Pos implementation.
+func cmpPos(p, q syntax.Pos) int {
+	// TODO(gri) is RelFilename correct here?
+	pname := p.RelFilename()
+	qname := q.RelFilename()
+	switch {
+	case pname < qname:
+		return -1
+	case pname > qname:
+		return +1
+	}
+
+	pline := p.Line()
+	qline := q.Line()
+	switch {
+	case pline < qline:
+		return -1
+	case pline > qline:
+		return +1
+	}
+
+	pcol := p.Col()
+	qcol := q.Col()
+	switch {
+	case pcol < qcol:
+		return -1
+	case pcol > qcol:
+		return +1
+	}
+
+	return 0
+}
+
+// firstInSrc reports the index of the object with the "smallest"
+// source position in path. path must not be empty.
+func firstInSrc(path []Object) int {
+	fst, pos := 0, path[0].Pos()
+	for i, t := range path[1:] {
+		if cmpPos(t.Pos(), pos) < 0 {
+			fst, pos = i+1, t.Pos()
+		}
+	}
+	return fst
+}
+
+func (check *Checker) constDecl(obj *Const, typ, init syntax.Expr, inherited bool) {
+	assert(obj.typ == nil)
+
+	// use the correct value of iota and errpos
+	defer func(iota constant.Value, errpos syntax.Pos) {
+		check.iota = iota
+		check.errpos = errpos
+	}(check.iota, check.errpos)
+	check.iota = obj.val
+	check.errpos = nopos
+
+	// provide valid constant value under all circumstances
+	obj.val = constant.MakeUnknown()
+
+	// determine type, if any
+	if typ != nil {
+		t := check.typ(typ)
+		if !isConstType(t) {
+			// don't report an error if the type is an invalid C (defined) type
+			// (issue #22090)
+			if under(t) != Typ[Invalid] {
+				check.errorf(typ, "invalid constant type %s", t)
+			}
+			obj.typ = Typ[Invalid]
+			return
+		}
+		obj.typ = t
+	}
+
+	// check initialization
+	var x operand
+	if init != nil {
+		if inherited {
+			// The initialization expression is inherited from a previous
+			// constant declaration, and (error) positions refer to that
+			// expression and not the current constant declaration. Use
+			// the constant identifier position for any errors during
+			// init expression evaluation since that is all we have
+			// (see issues #42991, #42992).
+			check.errpos = obj.pos
+		}
+		check.expr(&x, init)
+	}
+	check.initConst(obj, &x)
+}
+
+func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
+	assert(obj.typ == nil)
+
+	// If we have undefined variable types due to errors,
+	// mark variables as used to avoid follow-on errors.
+	// Matches compiler behavior.
+	defer func() {
+		if obj.typ == Typ[Invalid] {
+			obj.used = true
+		}
+		for _, lhs := range lhs {
+			if lhs.typ == Typ[Invalid] {
+				lhs.used = true
+			}
+		}
+	}()
+
+	// determine type, if any
+	if typ != nil {
+		obj.typ = check.varType(typ)
+		// We cannot spread the type to all lhs variables if there
+		// are more than one since that would mark them as checked
+		// (see Checker.objDecl) and the assignment of init exprs,
+		// if any, would not be checked.
+		//
+		// TODO(gri) If we have no init expr, we should distribute
+		// a given type otherwise we need to re-evalate the type
+		// expr for each lhs variable, leading to duplicate work.
+	}
+
+	// check initialization
+	if init == nil {
+		if typ == nil {
+			// error reported before by arityMatch
+			obj.typ = Typ[Invalid]
+		}
+		return
+	}
+
+	if lhs == nil || len(lhs) == 1 {
+		assert(lhs == nil || lhs[0] == obj)
+		var x operand
+		check.expr(&x, init)
+		check.initVar(obj, &x, "variable declaration")
+		return
+	}
+
+	if debug {
+		// obj must be one of lhs
+		found := false
+		for _, lhs := range lhs {
+			if obj == lhs {
+				found = true
+				break
+			}
+		}
+		if !found {
+			panic("inconsistent lhs")
+		}
+	}
+
+	// We have multiple variables on the lhs and one init expr.
+	// Make sure all variables have been given the same type if
+	// one was specified, otherwise they assume the type of the
+	// init expression values (was issue #15755).
+	if typ != nil {
+		for _, lhs := range lhs {
+			lhs.typ = obj.typ
+		}
+	}
+
+	check.initVars(lhs, []syntax.Expr{init}, nopos)
+}
+
+// under returns the expanded underlying type of n0; possibly by following
+// forward chains of named types. If an underlying type is found, resolve
+// the chain by setting the underlying type for each defined type in the
+// chain before returning it. If no underlying type is found or a cycle
+// is detected, the result is Typ[Invalid]. If a cycle is detected and
+// n0.check != nil, the cycle is reported.
+func (n0 *Named) under() Type {
+	u := n0.underlying
+	if u == nil {
+		return Typ[Invalid]
+	}
+
+	// If the underlying type of a defined type is not a defined
+	// type, then that is the desired underlying type.
+	n := asNamed(u)
+	if n == nil {
+		return u // common case
+	}
+
+	// Otherwise, follow the forward chain.
+	seen := map[*Named]int{n0: 0}
+	path := []Object{n0.obj}
+	for {
+		u = n.underlying
+		if u == nil {
+			u = Typ[Invalid]
+			break
+		}
+		n1 := asNamed(u)
+		if n1 == nil {
+			break // end of chain
+		}
+
+		seen[n] = len(seen)
+		path = append(path, n.obj)
+		n = n1
+
+		if i, ok := seen[n]; ok {
+			// cycle
+			// TODO(gri) revert this to a method on Checker. Having a possibly
+			// nil Checker on Named and TypeParam is too subtle.
+			if n0.check != nil {
+				n0.check.cycleError(path[i:])
+			}
+			u = Typ[Invalid]
+			break
+		}
+	}
+
+	for n := range seen {
+		// We should never have to update the underlying type of an imported type;
+		// those underlying types should have been resolved during the import.
+		// Also, doing so would lead to a race condition (was issue #31749).
+		// Do this check always, not just in debug more (it's cheap).
+		if n0.check != nil && n.obj.pkg != n0.check.pkg {
+			panic("internal error: imported type with unresolved underlying type")
+		}
+		n.underlying = u
+	}
+
+	return u
+}
+
+func (n *Named) setUnderlying(typ Type) {
+	if n != nil {
+		n.underlying = typ
+	}
+}
+
+func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named) {
+	assert(obj.typ == nil)
+
+	check.later(func() {
+		check.validType(obj.typ, nil)
+	})
+
+	alias := tdecl.Alias
+	if alias && tdecl.TParamList != nil {
+		// The parser will ensure this but we may still get an invalid AST.
+		// Complain and continue as regular type definition.
+		check.errorf(tdecl, "generic type cannot be alias")
+		alias = false
+	}
+
+	if alias {
+		// type alias declaration
+		if !check.allowVersion(obj.pkg, 1, 9) {
+			check.errorf(tdecl, "type aliases requires go1.9 or later")
+		}
+
+		obj.typ = Typ[Invalid]
+		obj.typ = check.anyType(tdecl.Type)
+
+	} else {
+		// defined type declaration
+
+		named := &Named{check: check, obj: obj}
+		def.setUnderlying(named)
+		obj.typ = named // make sure recursive type declarations terminate
+
+		if tdecl.TParamList != nil {
+			check.openScope(tdecl, "type parameters")
+			defer check.closeScope()
+			named.tparams = check.collectTypeParams(tdecl.TParamList)
+		}
+
+		// determine underlying type of named
+		named.orig = check.definedType(tdecl.Type, named)
+
+		// The underlying type of named may be itself a named type that is
+		// incomplete:
+		//
+		//	type (
+		//		A B
+		//		B *C
+		//		C A
+		//	)
+		//
+		// The type of C is the (named) type of A which is incomplete,
+		// and which has as its underlying type the named type B.
+		// Determine the (final, unnamed) underlying type by resolving
+		// any forward chain.
+		// TODO(gri) Investigate if we can just use named.origin here
+		//           and rely on lazy computation of the underlying type.
+		named.underlying = under(named)
+	}
+
+}
+
+func (check *Checker) collectTypeParams(list []*syntax.Field) (tparams []*TypeName) {
+	// Type parameter lists should not be empty. The parser will
+	// complain but we still may get an incorrect AST: ignore it.
+	if len(list) == 0 {
+		return
+	}
+
+	// Declare type parameters up-front, with empty interface as type bound.
+	// The scope of type parameters starts at the beginning of the type parameter
+	// list (so we can have mutually recursive parameterized interfaces).
+	for _, f := range list {
+		tparams = check.declareTypeParam(tparams, f.Name)
+	}
+
+	var bound Type
+	for i, j := 0, 0; i < len(list); i = j {
+		f := list[i]
+
+		// determine the range of type parameters list[i:j] with identical type bound
+		// (declared as in (type a, b, c B))
+		j = i + 1
+		for j < len(list) && list[j].Type == f.Type {
+			j++
+		}
+
+		// this should never be the case, but be careful
+		if f.Type == nil {
+			continue
+		}
+
+		// The predeclared identifier "any" is visible only as a constraint
+		// in a type parameter list. Look for it before general constraint
+		// resolution.
+		if tident, _ := f.Type.(*syntax.Name); tident != nil && tident.Value == "any" && check.lookup("any") == nil {
+			bound = universeAny
+		} else {
+			bound = check.typ(f.Type)
+		}
+
+		// type bound must be an interface
+		// TODO(gri) We should delay the interface check because
+		//           we may not have a complete interface yet:
+		//           type C(type T C) interface {}
+		//           (issue #39724).
+		if _, ok := under(bound).(*Interface); ok {
+			// set the type bounds
+			for i < j {
+				tparams[i].typ.(*TypeParam).bound = bound
+				i++
+			}
+		} else if bound != Typ[Invalid] {
+			check.errorf(f.Type, "%s is not an interface", bound)
+		}
+	}
+
+	return
+}
+
+func (check *Checker) declareTypeParam(tparams []*TypeName, name *syntax.Name) []*TypeName {
+	tpar := NewTypeName(name.Pos(), check.pkg, name.Value, nil)
+	check.NewTypeParam(tpar, len(tparams), &emptyInterface) // assigns type to tpar as a side-effect
+	check.declare(check.scope, name, tpar, check.scope.pos) // TODO(gri) check scope position
+	tparams = append(tparams, tpar)
+
+	if check.conf.Trace {
+		check.trace(name.Pos(), "type param = %v", tparams[len(tparams)-1])
+	}
+
+	return tparams
+}
+
+func (check *Checker) collectMethods(obj *TypeName) {
+	// get associated methods
+	// (Checker.collectObjects only collects methods with non-blank names;
+	// Checker.resolveBaseTypeName ensures that obj is not an alias name
+	// if it has attached methods.)
+	methods := check.methods[obj]
+	if methods == nil {
+		return
+	}
+	delete(check.methods, obj)
+	assert(!check.objMap[obj].tdecl.Alias) // don't use TypeName.IsAlias (requires fully set up object)
+
+	// use an objset to check for name conflicts
+	var mset objset
+
+	// spec: "If the base type is a struct type, the non-blank method
+	// and field names must be distinct."
+	base := asNamed(obj.typ) // shouldn't fail but be conservative
+	if base != nil {
+		if t, _ := base.underlying.(*Struct); t != nil {
+			for _, fld := range t.fields {
+				if fld.name != "_" {
+					assert(mset.insert(fld) == nil)
+				}
+			}
+		}
+
+		// Checker.Files may be called multiple times; additional package files
+		// may add methods to already type-checked types. Add pre-existing methods
+		// so that we can detect redeclarations.
+		for _, m := range base.methods {
+			assert(m.name != "_")
+			assert(mset.insert(m) == nil)
+		}
+	}
+
+	// add valid methods
+	for _, m := range methods {
+		// spec: "For a base type, the non-blank names of methods bound
+		// to it must be unique."
+		assert(m.name != "_")
+		if alt := mset.insert(m); alt != nil {
+			switch alt.(type) {
+			case *Var:
+				check.errorf(m.pos, "field and method with the same name %s", m.name)
+			case *Func:
+				if check.conf.CompilerErrorMessages {
+					check.errorf(m.pos, "%s.%s redeclared in this block", obj.Name(), m.name)
+				} else {
+					check.errorf(m.pos, "method %s already declared for %s", m.name, obj)
+				}
+			default:
+				unreachable()
+			}
+			check.reportAltDecl(alt)
+			continue
+		}
+
+		if base != nil {
+			base.methods = append(base.methods, m)
+		}
+	}
+}
+
+func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
+	assert(obj.typ == nil)
+
+	// func declarations cannot use iota
+	assert(check.iota == nil)
+
+	sig := new(Signature)
+	obj.typ = sig // guard against cycles
+
+	// Avoid cycle error when referring to method while type-checking the signature.
+	// This avoids a nuisance in the best case (non-parameterized receiver type) and
+	// since the method is not a type, we get an error. If we have a parameterized
+	// receiver type, instantiating the receiver type leads to the instantiation of
+	// its methods, and we don't want a cycle error in that case.
+	// TODO(gri) review if this is correct and/or whether we still need this?
+	saved := obj.color_
+	obj.color_ = black
+	fdecl := decl.fdecl
+	check.funcType(sig, fdecl.Recv, fdecl.TParamList, fdecl.Type)
+	obj.color_ = saved
+
+	// function body must be type-checked after global declarations
+	// (functions implemented elsewhere have no body)
+	if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
+		check.later(func() {
+			check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
+		})
+	}
+}
+
+func (check *Checker) declStmt(list []syntax.Decl) {
+	pkg := check.pkg
+
+	first := -1                // index of first ConstDecl in the current group, or -1
+	var last *syntax.ConstDecl // last ConstDecl with init expressions, or nil
+	for index, decl := range list {
+		if _, ok := decl.(*syntax.ConstDecl); !ok {
+			first = -1 // we're not in a constant declaration
+		}
+
+		switch s := decl.(type) {
+		case *syntax.ConstDecl:
+			top := len(check.delayed)
+
+			// iota is the index of the current constDecl within the group
+			if first < 0 || list[index-1].(*syntax.ConstDecl).Group != s.Group {
+				first = index
+				last = nil
+			}
+			iota := constant.MakeInt64(int64(index - first))
+
+			// determine which initialization expressions to use
+			inherited := true
+			switch {
+			case s.Type != nil || s.Values != nil:
+				last = s
+				inherited = false
+			case last == nil:
+				last = new(syntax.ConstDecl) // make sure last exists
+				inherited = false
+			}
+
+			// declare all constants
+			lhs := make([]*Const, len(s.NameList))
+			values := unpackExpr(last.Values)
+			for i, name := range s.NameList {
+				obj := NewConst(name.Pos(), pkg, name.Value, nil, iota)
+				lhs[i] = obj
+
+				var init syntax.Expr
+				if i < len(values) {
+					init = values[i]
+				}
+
+				check.constDecl(obj, last.Type, init, inherited)
+			}
+
+			// Constants must always have init values.
+			check.arity(s.Pos(), s.NameList, values, true, inherited)
+
+			// process function literals in init expressions before scope changes
+			check.processDelayed(top)
+
+			// spec: "The scope of a constant or variable identifier declared
+			// inside a function begins at the end of the ConstSpec or VarSpec
+			// (ShortVarDecl for short variable declarations) and ends at the
+			// end of the innermost containing block."
+			scopePos := endPos(s)
+			for i, name := range s.NameList {
+				check.declare(check.scope, name, lhs[i], scopePos)
+			}
+
+		case *syntax.VarDecl:
+			top := len(check.delayed)
+
+			lhs0 := make([]*Var, len(s.NameList))
+			for i, name := range s.NameList {
+				lhs0[i] = NewVar(name.Pos(), pkg, name.Value, nil)
+			}
+
+			// initialize all variables
+			values := unpackExpr(s.Values)
+			for i, obj := range lhs0 {
+				var lhs []*Var
+				var init syntax.Expr
+				switch len(values) {
+				case len(s.NameList):
+					// lhs and rhs match
+					init = values[i]
+				case 1:
+					// rhs is expected to be a multi-valued expression
+					lhs = lhs0
+					init = values[0]
+				default:
+					if i < len(values) {
+						init = values[i]
+					}
+				}
+				check.varDecl(obj, lhs, s.Type, init)
+				if len(values) == 1 {
+					// If we have a single lhs variable we are done either way.
+					// If we have a single rhs expression, it must be a multi-
+					// valued expression, in which case handling the first lhs
+					// variable will cause all lhs variables to have a type
+					// assigned, and we are done as well.
+					if debug {
+						for _, obj := range lhs0 {
+							assert(obj.typ != nil)
+						}
+					}
+					break
+				}
+			}
+
+			// If we have no type, we must have values.
+			if s.Type == nil || values != nil {
+				check.arity(s.Pos(), s.NameList, values, false, false)
+			}
+
+			// process function literals in init expressions before scope changes
+			check.processDelayed(top)
+
+			// declare all variables
+			// (only at this point are the variable scopes (parents) set)
+			scopePos := endPos(s) // see constant declarations
+			for i, name := range s.NameList {
+				// see constant declarations
+				check.declare(check.scope, name, lhs0[i], scopePos)
+			}
+
+		case *syntax.TypeDecl:
+			obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Value, nil)
+			// spec: "The scope of a type identifier declared inside a function
+			// begins at the identifier in the TypeSpec and ends at the end of
+			// the innermost containing block."
+			scopePos := s.Name.Pos()
+			check.declare(check.scope, s.Name, obj, scopePos)
+			// mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
+			obj.setColor(grey + color(check.push(obj)))
+			check.typeDecl(obj, s, nil)
+			check.pop().setColor(black)
+
+		default:
+			check.invalidASTf(s, "unknown syntax.Decl node %T", s)
+		}
+	}
+}

src/cmd/compile/internal/types2/errors.go

@@ -0,0 +1,172 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements various error reporters.
+
+package types2
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+func unimplemented() {
+	panic("unimplemented")
+}
+
+func assert(p bool) {
+	if !p {
+		panic("assertion failed")
+	}
+}
+
+func unreachable() {
+	panic("unreachable")
+}
+
+func (check *Checker) qualifier(pkg *Package) string {
+	// Qualify the package unless it's the package being type-checked.
+	if pkg != check.pkg {
+		// If the same package name was used by multiple packages, display the full path.
+		if check.pkgCnt[pkg.name] > 1 {
+			return strconv.Quote(pkg.path)
+		}
+		return pkg.name
+	}
+	return ""
+}
+
+func (check *Checker) sprintf(format string, args ...interface{}) string {
+	for i, arg := range args {
+		switch a := arg.(type) {
+		case nil:
+			arg = "<nil>"
+		case operand:
+			panic("internal error: should always pass *operand")
+		case *operand:
+			arg = operandString(a, check.qualifier)
+		case syntax.Pos:
+			arg = a.String()
+		case syntax.Expr:
+			arg = syntax.String(a)
+		case Object:
+			arg = ObjectString(a, check.qualifier)
+		case Type:
+			arg = TypeString(a, check.qualifier)
+		}
+		args[i] = arg
+	}
+	return fmt.Sprintf(format, args...)
+}
+
+func (check *Checker) trace(pos syntax.Pos, format string, args ...interface{}) {
+	fmt.Printf("%s:\t%s%s\n",
+		pos,
+		strings.Repeat(".  ", check.indent),
+		check.sprintf(format, args...),
+	)
+}
+
+// dump is only needed for debugging
+func (check *Checker) dump(format string, args ...interface{}) {
+	fmt.Println(check.sprintf(format, args...))
+}
+
+func (check *Checker) err(pos syntax.Pos, msg string, soft bool) {
+	// Cheap trick: Don't report errors with messages containing
+	// "invalid operand" or "invalid type" as those tend to be
+	// follow-on errors which don't add useful information. Only
+	// exclude them if these strings are not at the beginning,
+	// and only if we have at least one error already reported.
+	if check.firstErr != nil && (strings.Index(msg, "invalid operand") > 0 || strings.Index(msg, "invalid type") > 0) {
+		return
+	}
+
+	// If we are encountering an error while evaluating an inherited
+	// constant initialization expression, pos is the position of in
+	// the original expression, and not of the currently declared
+	// constant identifier. Use the provided errpos instead.
+	// TODO(gri) We may also want to augment the error message and
+	// refer to the position (pos) in the original expression.
+	if check.errpos.IsKnown() {
+		assert(check.iota != nil)
+		pos = check.errpos
+	}
+
+	err := Error{pos, stripAnnotations(msg), msg, soft}
+	if check.firstErr == nil {
+		check.firstErr = err
+	}
+
+	if check.conf.Trace {
+		check.trace(pos, "ERROR: %s", msg)
+	}
+
+	f := check.conf.Error
+	if f == nil {
+		panic(bailout{}) // report only first error
+	}
+	f(err)
+}
+
+type poser interface {
+	Pos() syntax.Pos
+}
+
+func (check *Checker) error(at poser, msg string) {
+	check.err(posFor(at), msg, false)
+}
+
+func (check *Checker) errorf(at poser, format string, args ...interface{}) {
+	check.err(posFor(at), check.sprintf(format, args...), false)
+}
+
+func (check *Checker) softErrorf(at poser, format string, args ...interface{}) {
+	check.err(posFor(at), check.sprintf(format, args...), true)
+}
+
+func (check *Checker) invalidASTf(at poser, format string, args ...interface{}) {
+	check.errorf(at, "invalid AST: "+format, args...)
+}
+
+func (check *Checker) invalidArgf(at poser, format string, args ...interface{}) {
+	check.errorf(at, "invalid argument: "+format, args...)
+}
+
+func (check *Checker) invalidOpf(at poser, format string, args ...interface{}) {
+	check.errorf(at, "invalid operation: "+format, args...)
+}
+
+// posFor reports the left (= start) position of at.
+func posFor(at poser) syntax.Pos {
+	switch x := at.(type) {
+	case *operand:
+		if x.expr != nil {
+			return startPos(x.expr)
+		}
+	case syntax.Node:
+		return startPos(x)
+	}
+	return at.Pos()
+}
+
+// stripAnnotations removes internal (type) annotations from s.
+func stripAnnotations(s string) string {
+	// Would like to use strings.Builder but it's not available in Go 1.4.
+	var b bytes.Buffer
+	for _, r := range s {
+		// strip #'s and subscript digits
+		if r != instanceMarker && !('₀' <= r && r < '₀'+10) { // '₀' == U+2080
+			b.WriteRune(r)
+		}
+	}
+	if b.Len() < len(s) {
+		return b.String()
+	}
+	return s
+}

src/cmd/compile/internal/types2/errors_test.go

@@ -0,0 +1,25 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import "testing"
+
+func TestStripAnnotations(t *testing.T) {
+	for _, test := range []struct {
+		in, want string
+	}{
+		{"", ""},
+		{"   ", "   "},
+		{"foo", "foo"},
+		{"foo₀", "foo"},
+		{"foo(T₀)", "foo(T)"},
+		{"#foo(T₀)", "foo(T)"},
+	} {
+		got := stripAnnotations(test.in)
+		if got != test.want {
+			t.Errorf("%q: got %q; want %q", test.in, got, test.want)
+		}
+	}
+}

src/cmd/compile/internal/types2/example_test.go

@@ -0,0 +1,269 @@
+// UNREVIEWED
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Only run where builders (build.golang.org) have
+// access to compiled packages for import.
+//
+// +build !arm,!arm64
+
+package types2_test
+
+// This file shows examples of basic usage of the go/types API.
+//
+// To locate a Go package, use (*go/build.Context).Import.
+// To load, parse, and type-check a complete Go program
+// from source, use golang.org/x/tools/go/loader.
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/types2"
+	"fmt"
+	"log"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+// ExampleScope prints the tree of Scopes of a package created from a
+// set of parsed files.
+func ExampleScope() {
+	// Parse the source files for a package.
+	var files []*syntax.File
+	for _, file := range []struct{ name, input string }{
+		{"main.go", `
+package main
+import "fmt"
+func main() {
+	freezing := FToC(-18)
+	fmt.Println(freezing, Boiling) }
+`},
+		{"celsius.go", `
+package main
+import "fmt"
+type Celsius float64
+func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }
+func FToC(f float64) Celsius { return Celsius(f - 32 / 9 * 5) }
+const Boiling Celsius = 100
+func Unused() { {}; {{ var x int; _ = x }} } // make sure empty block scopes get printed
+`},
+	} {
+		f, err := parseSrc(file.name, file.input)
+		if err != nil {
+			log.Fatal(err)
+		}
+		files = append(files, f)
+	}
+
+	// Type-check a package consisting of these files.
+	// Type information for the imported "fmt" package
+	// comes from $GOROOT/pkg/$GOOS_$GOOARCH/fmt.a.
+	conf := types2.Config{Importer: defaultImporter()}
+	pkg, err := conf.Check("temperature", files, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print the tree of scopes.
+	// For determinism, we redact addresses.
+	var buf bytes.Buffer
+	pkg.Scope().WriteTo(&buf, 0, true)
+	rx := regexp.MustCompile(` 0x[a-fA-F0-9]*`)
+	fmt.Println(rx.ReplaceAllString(buf.String(), ""))
+
+	// Output:
+	// package "temperature" scope {
+	// .  const temperature.Boiling temperature.Celsius
+	// .  type temperature.Celsius float64
+	// .  func temperature.FToC(f float64) temperature.Celsius
+	// .  func temperature.Unused()
+	// .  func temperature.main()
+	// .  main.go scope {
+	// .  .  package fmt
+	// .  .  function scope {
+	// .  .  .  var freezing temperature.Celsius
+	// .  .  }
+	// .  }
+	// .  celsius.go scope {
+	// .  .  package fmt
+	// .  .  function scope {
+	// .  .  .  var c temperature.Celsius
+	// .  .  }
+	// .  .  function scope {
+	// .  .  .  var f float64
+	// .  .  }
+	// .  .  function scope {
+	// .  .  .  block scope {
+	// .  .  .  }
+	// .  .  .  block scope {
+	// .  .  .  .  block scope {
+	// .  .  .  .  .  var x int
+	// .  .  .  .  }
+	// .  .  .  }
+	// .  .  }
+	// .  }
+	// }
+}
+
+// ExampleInfo prints various facts recorded by the type checker in a
+// types2.Info struct: definitions of and references to each named object,
+// and the type, value, and mode of every expression in the package.
+func ExampleInfo() {
+	// Parse a single source file.
+	const input = `
+package fib
+
+type S string
+
+var a, b, c = len(b), S(c), "hello"
+
+func fib(x int) int {
+	if x < 2 {
+		return x
+	}
+	return fib(x-1) - fib(x-2)
+}`
+	f, err := parseSrc("fib.go", input)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Type-check the package.
+	// We create an empty map for each kind of input
+	// we're interested in, and Check populates them.
+	info := types2.Info{
+		Types: make(map[syntax.Expr]types2.TypeAndValue),
+		Defs:  make(map[*syntax.Name]types2.Object),
+		Uses:  make(map[*syntax.Name]types2.Object),
+	}
+	var conf types2.Config
+	pkg, err := conf.Check("fib", []*syntax.File{f}, &info)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print package-level variables in initialization order.
+	fmt.Printf("InitOrder: %v\n\n", info.InitOrder)
+
+	// For each named object, print the line and
+	// column of its definition and each of its uses.
+	fmt.Println("Defs and Uses of each named object:")
+	usesByObj := make(map[types2.Object][]string)
+	for id, obj := range info.Uses {
+		posn := id.Pos()
+		lineCol := fmt.Sprintf("%d:%d", posn.Line(), posn.Col())
+		usesByObj[obj] = append(usesByObj[obj], lineCol)
+	}
+	var items []string
+	for obj, uses := range usesByObj {
+		sort.Strings(uses)
+		item := fmt.Sprintf("%s:\n  defined at %s\n  used at %s",
+			types2.ObjectString(obj, types2.RelativeTo(pkg)),
+			obj.Pos(),
+			strings.Join(uses, ", "))
+		items = append(items, item)
+	}
+	sort.Strings(items) // sort by line:col, in effect
+	fmt.Println(strings.Join(items, "\n"))
+	fmt.Println()
+
+	// TODO(gri) Enable once positions are updated/verified
+	// fmt.Println("Types and Values of each expression:")
+	// items = nil
+	// for expr, tv := range info.Types {
+	// 	var buf bytes.Buffer
+	// 	posn := expr.Pos()
+	// 	tvstr := tv.Type.String()
+	// 	if tv.Value != nil {
+	// 		tvstr += " = " + tv.Value.String()
+	// 	}
+	// 	// line:col | expr | mode : type = value
+	// 	fmt.Fprintf(&buf, "%2d:%2d | %-19s | %-7s : %s",
+	// 		posn.Line(), posn.Col(), types2.ExprString(expr),
+	// 		mode(tv), tvstr)
+	// 	items = append(items, buf.String())
+	// }
+	// sort.Strings(items)
+	// fmt.Println(strings.Join(items, "\n"))
+
+	// Output:
+	// InitOrder: [c = "hello" b = S(c) a = len(b)]
+	//
+	// Defs and Uses of each named object:
+	// builtin len:
+	//   defined at <unknown position>
+	//   used at 6:15
+	// func fib(x int) int:
+	//   defined at fib.go:8:6
+	//   used at 12:20, 12:9
+	// type S string:
+	//   defined at fib.go:4:6
+	//   used at 6:23
+	// type int:
+	//   defined at <unknown position>
+	//   used at 8:12, 8:17
+	// type string:
+	//   defined at <unknown position>
+	//   used at 4:8
+	// var b S:
+	//   defined at fib.go:6:8
+	//   used at 6:19
+	// var c string:
+	//   defined at fib.go:6:11
+	//   used at 6:25
+	// var x int:
+	//   defined at fib.go:8:10
+	//   used at 10:10, 12:13, 12:24, 9:5
+
+	// TODO(gri) Enable once positions are updated/verified
+	// Types and Values of each expression:
+	//  4: 8 | string              | type    : string
+	//  6:15 | len                 | builtin : func(string) int
+	//  6:15 | len(b)              | value   : int
+	//  6:19 | b                   | var     : fib.S
+	//  6:23 | S                   | type    : fib.S
+	//  6:23 | S(c)                | value   : fib.S
+	//  6:25 | c                   | var     : string
+	//  6:29 | "hello"             | value   : string = "hello"
+	//  8:12 | int                 | type    : int
+	//  8:17 | int                 | type    : int
+	//  9: 5 | x                   | var     : int
+	//  9: 5 | x < 2               | value   : untyped bool
+	//  9: 9 | 2                   | value   : int = 2
+	// 10:10 | x                   | var     : int
+	// 12: 9 | fib                 | value   : func(x int) int
+	// 12: 9 | fib(x - 1)          | value   : int
+	// 12: 9 | fib(x - 1) - fib(x - 2) | value   : int
+	// 12:13 | x                   | var     : int
+	// 12:13 | x - 1               | value   : int
+	// 12:15 | 1                   | value   : int = 1
+	// 12:20 | fib                 | value   : func(x int) int
+	// 12:20 | fib(x - 2)          | value   : int
+	// 12:24 | x                   | var     : int
+	// 12:24 | x - 2               | value   : int
+	// 12:26 | 2                   | value   : int = 2
+}
+
+func mode(tv types2.TypeAndValue) string {
+	switch {
+	case tv.IsVoid():
+		return "void"
+	case tv.IsType():
+		return "type"
+	case tv.IsBuiltin():
+		return "builtin"
+	case tv.IsNil():
+		return "nil"
+	case tv.Assignable():
+		if tv.Addressable() {
+			return "var"
+		}
+		return "mapindex"
+	case tv.IsValue():
+		return "value"
+	default:
+		return "unknown"
+	}
+}

src/cmd/compile/internal/types2/examples/functions.go2

@@ -0,0 +1,216 @@
+// UNREVIEWED
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of type-parameterized functions.
+
+package p
+
+// Reverse is a generic function that takes a []T argument and
+// reverses that slice in place.
+func Reverse[T any](list []T) {
+	i := 0
+	j := len(list)-1
+	for i < j {
+		list[i], list[j] = list[j], list[i]
+		i++
+		j--
+	}
+}
+
+func _() {
+	// Reverse can be called with an explicit type argument.
+	Reverse[int](nil)
+	Reverse[string]([]string{"foo", "bar"})
+	Reverse[struct{x, y int}]([]struct{x, y int}{{1, 2}, {2, 3}, {3, 4}})
+
+	// Since the type parameter is used for an incoming argument,
+	// it can be inferred from the provided argument's type.
+	Reverse([]string{"foo", "bar"})
+	Reverse([]struct{x, y int}{{1, 2}, {2, 3}, {3, 4}})
+
+	// But the incoming argument must have a type, even if it's a
+	// default type. An untyped nil won't work.
+	// Reverse(nil) // this won't type-check
+
+	// A typed nil will work, though.
+	Reverse([]int(nil))
+}
+
+// Certain functions, such as the built-in `new` could be written using
+// type parameters.
+func new[T any]() *T {
+	var x T
+	return &x
+}
+
+// When calling our own `new`, we need to pass the type parameter
+// explicitly since there is no (value) argument from which the
+// result type could be inferred. We don't try to infer the
+// result type from the assignment to keep things simple and
+// easy to understand.
+var _ = new[int]()
+var _ *float64 = new[float64]() // the result type is indeed *float64
+
+// A function may have multiple type parameters, of course.
+func foo[A, B, C any](a A, b []B, c *C) B {
+	// do something here
+	return b[0]
+}
+
+// As before, we can pass type parameters explicitly.
+var s = foo[int, string, float64](1, []string{"first"}, new[float64]())
+
+// Or we can use type inference.
+var _ float64 = foo(42, []float64{1.0}, &s)
+
+// Type inference works in a straight-forward manner even
+// for variadic functions.
+func variadic[A, B any](A, B, ...B) int
+
+// var _ = variadic(1) // ERROR not enough arguments
+var _ = variadic(1, 2.3)
+var _ = variadic(1, 2.3, 3.4, 4.5)
+var _ = variadic[int, float64](1, 2.3, 3.4, 4)
+
+// Type inference also works in recursive function calls where
+// the inferred type is the type parameter of the caller.
+func f1[T any](x T) {
+	f1(x)
+}
+
+func f2a[T any](x, y T) {
+	f2a(x, y)
+}
+
+func f2b[T any](x, y T) {
+	f2b(y, x)
+}
+
+func g2a[P, Q any](x P, y Q) {
+	g2a(x, y)
+}
+
+func g2b[P, Q any](x P, y Q) {
+	g2b(y, x)
+}
+
+// Here's an example of a recursive function call with variadic
+// arguments and type inference inferring the type parameter of
+// the caller (i.e., itself).
+func max[T interface{ type int }](x ...T) T {
+	var x0 T
+	if len(x) > 0 {
+		x0 = x[0]
+	}
+	if len(x) > 1 {
+		x1 := max(x[1:]...)
+		if x1 > x0 {
+			return x1
+		}
+	}
+	return x0
+}
+
+// When inferring channel types, the channel direction is ignored
+// for the purpose of type inference. Once the type has been in-
+// fered, the usual parameter passing rules are applied.
+// Thus even if a type can be inferred successfully, the function
+// call may not be valid.
+
+func fboth[T any](chan T)
+func frecv[T any](<-chan T)
+func fsend[T any](chan<- T)
+
+func _() {
+	var both chan int
+	var recv <-chan int
+	var send chan<-int
+
+	fboth(both)
+	fboth(recv /* ERROR cannot use */ )
+	fboth(send /* ERROR cannot use */ )
+
+	frecv(both)
+	frecv(recv)
+	frecv(send /* ERROR cannot use */ )
+
+	fsend(both)
+	fsend(recv /* ERROR cannot use */)
+	fsend(send)
+}
+
+func ffboth[T any](func(chan T))
+func ffrecv[T any](func(<-chan T))
+func ffsend[T any](func(chan<- T))
+
+func _() {
+	var both func(chan int)
+	var recv func(<-chan int)
+	var send func(chan<- int)
+
+	ffboth(both)
+	ffboth(recv /* ERROR cannot use */ )
+	ffboth(send /* ERROR cannot use */ )
+
+	ffrecv(both /* ERROR cannot use */ )
+	ffrecv(recv)
+	ffrecv(send /* ERROR cannot use */ )
+
+	ffsend(both /* ERROR cannot use */ )
+	ffsend(recv /* ERROR cannot use */ )
+	ffsend(send)
+}
+
+// When inferring elements of unnamed composite parameter types,
+// if the arguments are defined types, use their underlying types.
+// Even though the matching types are not exactly structurally the
+// same (one is a type literal, the other a named type), because
+// assignment is permitted, parameter passing is permitted as well,
+// so type inference should be able to handle these cases well.
+
+func g1[T any]([]T)
+func g2[T any]([]T, T)
+func g3[T any](*T, ...T)
+
+func _() {
+	type intSlize []int
+	g1([]int{})
+	g1(intSlize{})
+	g2(nil, 0)
+
+	type myString string
+	var s1 string
+	g3(nil, "1", myString("2"), "3")
+	g3(&s1, "1", myString /* ERROR does not match */ ("2"), "3")
+	_ = s1
+
+	type myStruct struct{x int}
+	var s2 myStruct
+	g3(nil, struct{x int}{}, myStruct{})
+	g3(&s2, struct{x int}{}, myStruct{})
+	g3(nil, myStruct{}, struct{x int}{})
+	g3(&s2, myStruct{}, struct{x int}{})
+}
+
+// Here's a realistic example.
+
+func append[T any](s []T, t ...T) []T
+
+func _() {
+	var f func()
+	type Funcs []func()
+	var funcs Funcs
+	_ = append(funcs, f)
+}
+
+// Generic type declarations cannot have empty type parameter lists
+// (that would indicate a slice type). Thus, generic functions cannot
+// have empty type parameter lists, either. This is a syntax error.
+
+func h[] /* ERROR empty type parameter list */ ()
+
+func _() {
+	h[] /* ERROR operand */ ()
+}

src/cmd/compile/internal/types2/examples/methods.go2

@@ -0,0 +1,97 @@
+// UNREVIEWED
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of methods on type-parameterized types.
+
+package p
+
+// Parameterized types may have methods.
+type T1[A any] struct{ a A }
+
+// When declaring a method for a parameterized type, the "instantiated"
+// receiver type acts as an implicit declaration of the type parameters
+// for the receiver type. In the example below, method m1 on type T1 has
+// the receiver type T1[A] which declares the type parameter A for use
+// with this method. That is, within the method m1, A stands for the
+// actual type argument provided to an instantiated T1.
+func (t T1[A]) m1() A { return t.a }
+
+// For instance, if T1 is instantiated with the type int, the type
+// parameter A in m1 assumes that type (int) as well and we can write
+// code like this:
+var x T1[int]
+var _ int = x.m1()
+
+// Because the type parameter provided to a parameterized receiver type
+// is declared through that receiver declaration, it must be an identifier.
+// It cannot possibly be some other type because the receiver type is not
+// instantiated with concrete types, it is standing for the parameterized
+// receiver type.
+func (t T1[[ /* ERROR must be an identifier */ ]int]) m2() {}
+
+// Note that using what looks like a predeclared identifier, say int,
+// as type parameter in this situation is deceptive and considered bad
+// style. In m3 below, int is the name of the local receiver type parameter
+// and it shadows the predeclared identifier int which then cannot be used
+// anymore as expected.
+// This is no different from locally redelaring a predeclared identifier
+// and usually should be avoided. There are some notable exceptions; e.g.,
+// sometimes it makes sense to use the identifier "copy" which happens to
+// also be the name of a predeclared built-in function.
+func (t T1[int]) m3() { var _ int = 42 /* ERROR cannot convert 42 .* to int */ }
+
+// The names of the type parameters used in a parameterized receiver
+// type don't have to match the type parameter names in the the declaration
+// of the type used for the receiver. In our example, even though T1 is
+// declared with type parameter named A, methods using that receiver type
+// are free to use their own name for that type parameter. That is, the
+// name of type parameters is always local to the declaration where they
+// are introduced. In our example we can write a method m2 and use the
+// name X instead of A for the type parameter w/o any difference.
+func (t T1[X]) m4() X { return t.a }
+
+// If the receiver type is parameterized, type parameters must always be
+// provided: this simply follows from the general rule that a parameterized
+// type must be instantiated before it can be used. A method receiver
+// declaration using a parameterized receiver type is no exception. It is
+// simply that such receiver type expressions perform two tasks simultaneously:
+// they declare the (local) type parameters and then use them to instantiate
+// the receiver type. Forgetting to provide a type parameter leads to an error.
+func (t T1 /* ERROR generic type .* without instantiation */ ) m5() {}
+
+// However, sometimes we don't need the type parameter, and thus it is
+// inconvenient to have to choose a name. Since the receiver type expression
+// serves as a declaration for its type parameters, we are free to choose the
+// blank identifier:
+func (t T1[_]) m6() {}
+
+// Naturally, these rules apply to any number of type parameters on the receiver
+// type. Here are some more complex examples.
+type T2[A, B, C any] struct {
+        a A
+        b B
+        c C
+}
+
+// Naming of the type parameters is local and has no semantic impact:
+func (t T2[A, B, C]) m1() (A, B, C) { return t.a, t.b, t.c }
+func (t T2[C, B, A]) m2() (C, B, A) { return t.a, t.b, t.c }
+func (t T2[X, Y, Z]) m3() (X, Y, Z) { return t.a, t.b, t.c }
+
+// Type parameters may be left blank if they are not needed:
+func (t T2[A, _, C]) m4() (A, C) { return t.a, t.c }
+func (t T2[_, _, X]) m5() X { return t.c }
+func (t T2[_, _, _]) m6() {}
+
+// As usual, blank names may be used for any object which we don't care about
+// using later. For instance, we may write an unnamed method with a receiver
+// that cannot be accessed:
+func (_ T2[_, _, _]) _() int { return 42 }
+
+// Because a receiver parameter list is simply a parameter list, we can
+// leave the receiver argument away for receiver types.
+type T0 struct{}
+func (T0) _() {}
+func (T1[A]) _() {}

src/cmd/compile/internal/types2/examples/types.go2

@@ -0,0 +1,280 @@
+// UNREVIEWED
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of generic types.
+
+package p
+
+// List is just what it says - a slice of E elements.
+type List[E any] []E
+
+// A generic (parameterized) type must always be instantiated
+// before it can be used to designate the type of a variable
+// (including a struct field, or function parameter); though
+// for the latter cases, the provided type may be another type
+// parameter. So:
+var _ List[byte] = []byte{}
+
+// A generic binary tree might be declared as follows.
+type Tree[E any] struct {
+	left, right *Tree[E]
+	payload E
+}
+
+// A simple instantiation of Tree:
+var root1 Tree[int]
+
+// The actual type parameter provided may be a generic type itself:
+var root2 Tree[List[int]]
+
+// A couple of more complex examples.
+// We don't need extra parentheses around the element type of the slices on
+// the right (unlike when we use ()'s rather than []'s for type parameters).
+var _ List[List[int]] = []List[int]{}
+var _ List[List[List[Tree[int]]]] = []List[List[Tree[int]]]{}
+
+// Type parameters act like type aliases when used in generic types
+// in the sense that we can "emulate" a specific type instantiation
+// with type aliases.
+type T1[P any] struct {
+	f P
+}
+
+type T2[P any] struct {
+	f struct {
+		g P
+	}
+}
+
+var x1 T1[struct{ g int }]
+var x2 T2[int]
+
+func _() {
+	// This assignment is invalid because the types of x1, x2 are T1(...)
+	// and T2(...) respectively, which are two different defined types.
+	x1 = x2 // ERROR assignment
+
+	// This assignment is valid because the types of x1.f and x2.f are
+	// both struct { g int }; the type parameters act like type aliases
+	// and their actual names don't come into play here.
+	x1.f = x2.f
+}
+
+// We can verify this behavior using type aliases instead:
+type T1a struct {
+	f A1
+}
+type A1 = struct { g int }
+
+type T2a struct {
+	f struct {
+		g A2
+	}
+}
+type A2 = int
+
+var x1a T1a
+var x2a T2a
+
+func _() {
+	x1a = x2a // ERROR assignment
+	x1a.f = x2a.f
+}
+
+// Another interesting corner case are generic types that don't use
+// their type arguments. For instance:
+type T[P any] struct{}
+
+var xint T[int]
+var xbool T[bool]
+
+// Are these two variables of the same type? After all, their underlying
+// types are identical. We consider them to be different because each type
+// instantiation creates a new named type, in this case T<int> and T<bool>
+// even if their underlying types are identical. This is sensible because
+// we might still have methods that have different signatures or behave
+// differently depending on the type arguments, and thus we can't possibly
+// consider such types identical. Consequently:
+func _() {
+	xint = xbool // ERROR assignment
+}
+
+// Generic types cannot be used without instantiation.
+var _ T // ERROR cannot use generic type T
+
+// In type context, generic (parameterized) types cannot be parenthesized before
+// being instantiated. See also NOTES entry from 12/4/2019.
+var _ (T /* ERROR cannot use generic type T */ )[ /* ERROR unexpected \[ */ int]
+
+// All types may be parameterized, including interfaces.
+type I1[T any] interface{
+	m1(T)
+}
+
+// There is no such thing as a variadic generic type.
+type _[T ... /* ERROR invalid use of ... */ interface{}] struct{}
+
+// Generic interfaces may be embedded as one would expect.
+type I2 interface {
+	I1(int)     // method!
+	I1[string]  // embedded I1
+}
+
+func _() {
+	var x I2
+	x.I1(0)
+	x.m1("foo")
+}
+
+type I0 interface {
+	m0()
+}
+
+type I3 interface {
+	I0
+	I1[bool]
+	m(string)
+}
+
+func _() {
+	var x I3
+	x.m0()
+	x.m1(true)
+	x.m("foo")
+}
+
+type _ struct {
+	( /* ERROR cannot parenthesize */ int8)
+	( /* ERROR cannot parenthesize */ *int16)
+	*( /* ERROR cannot parenthesize */ int32)
+	List[int]
+
+	int8 /* ERROR int8 redeclared */
+	* /* ERROR int16 redeclared */ int16
+	List /* ERROR List redeclared */ [int]
+}
+
+// It's possible to declare local types whose underlying types
+// are type parameters. As with ordinary type definitions, the
+// types underlying properties are "inherited" but the methods
+// are not.
+func _[T interface{ m(); type int }]() {
+	type L T
+	var x L
+
+	// m is not defined on L (it is not "inherited" from
+	// its underlying type).
+	x.m /* ERROR x.m undefined */ ()
+
+	// But the properties of T, such that as that it supports
+	// the operations of the types given by its type bound,
+	// are also the properties of L.
+	x++
+	_ = x - x
+
+	// On the other hand, if we define a local alias for T,
+	// that alias stands for T as expected.
+	type A = T
+	var y A
+	y.m()
+	_ = y < 0
+}
+
+// As a special case, an explicit type argument may be omitted
+// from a type parameter bound if the type bound expects exactly
+// one type argument. In that case, the type argument is the
+// respective type parameter to which the type bound applies.
+// Note: We may not permit this syntactic sugar at first.
+// Note: This is now disabled. All examples below are adjusted.
+type Adder[T any] interface {
+	Add(T) T
+}
+
+// We don't need to explicitly instantiate the Adder bound
+// if we have exactly one type parameter.
+func Sum[T Adder[T]](list []T) T {
+	var sum T
+	for _, x := range list {
+		sum = sum.Add(x)
+	}
+	return sum
+}
+
+// Valid and invalid variations.
+type B0 interface {}
+type B1[_ any] interface{}
+type B2[_, _ any] interface{}
+
+func _[T1 B0]()
+func _[T1 B1[T1]]()
+func _[T1 B2 /* ERROR cannot use generic type .* without instantiation */ ]()
+
+func _[T1, T2 B0]()
+func _[T1 B1[T1], T2 B1[T2]]()
+func _[T1, T2 B2 /* ERROR cannot use generic type .* without instantiation */ ]()
+
+func _[T1 B0, T2 B1[T2]]() // here B1 applies to T2
+
+// When the type argument is left away, the type bound is
+// instantiated for each type parameter with that type
+// parameter.
+// Note: We may not permit this syntactic sugar at first.
+func _[A Adder[A], B Adder[B], C Adder[A]]() {
+	var a A // A's type bound is Adder[A]
+	a = a.Add(a)
+	var b B // B's type bound is Adder[B]
+	b = b.Add(b)
+	var c C // C's type bound is Adder[A]
+	a = c.Add(a)
+}
+
+// The type of variables (incl. parameters and return values) cannot
+// be an interface with type constraints or be/embed comparable.
+type I interface {
+	type int
+}
+
+var (
+	_ interface /* ERROR contains type constraints */ {type int}
+	_ I /* ERROR contains type constraints */
+)
+
+func _(I /* ERROR contains type constraints */ )
+func _(x, y, z I /* ERROR contains type constraints */ )
+func _() I /* ERROR contains type constraints */
+
+func _() {
+	var _ I /* ERROR contains type constraints */
+}
+
+type C interface {
+	comparable
+}
+
+var _ comparable /* ERROR comparable */
+var _ C /* ERROR comparable */
+
+func _(_ comparable /* ERROR comparable */ , _ C /* ERROR comparable */ )
+
+func _() {
+	var _ comparable /* ERROR comparable */
+	var _ C /* ERROR comparable */
+}
+
+// Type parameters are never const types, i.e., it's
+// not possible to declare a constant of type parameter type.
+// (If a type list contains just a single const type, we could
+// allow it, but such type lists don't make much sense in the
+// first place.)
+func _[T interface { type int, float64 }]() {
+	// not valid
+	const _ = T /* ERROR not constant */ (0)
+	const _ T /* ERROR invalid constant type T */ = 1
+
+	// valid
+	var _ = T(0)
+	var _ T = 1
+	_ = T(0)
+}

src/cmd/compile/internal/types2/fixedbugs/issue20583.src

@@ -0,0 +1,12 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue20583
+const (
+	_ = 6e886451608 /* ERROR malformed constant */ /2
+	_ = 6e886451608i /* ERROR malformed constant */ /2
+	_ = 0 * 1e+1000000000 // ERROR malformed constant
+	x = 1e100000000
+	_ = x*x*x*x*x*x* /* ERROR not representable */ x
+)

src/cmd/compile/internal/types2/fixedbugs/issue39634.go2

@@ -0,0 +1,92 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Examples adjusted to match new [T any] syntax for type parameters.
+// Also, previously permitted empty type parameter lists and instantiations
+// are now syntax errors.
+
+package p
+
+// crash 1
+type nt1[_ any]interface{g /* ERROR undeclared name */ }
+type ph1[e nt1[e],g(d /* ERROR undeclared name */ )]s /* ERROR undeclared name */
+func(*ph1[e,e /* ERROR redeclared */ ])h(d /* ERROR undeclared name */ )
+
+// crash 2
+// Disabled: empty []'s are now syntax errors. This example leads to too many follow-on errors.
+// type Numeric2 interface{t2 /* ERROR not a type */ }
+// func t2[T Numeric2](s[]T){0 /* ERROR not a type */ []{s /* ERROR cannot index */ [0][0]}}
+
+// crash 3
+type t3 *interface{ t3.p /* ERROR no field or method p */ }
+
+// crash 4
+type Numeric4 interface{t4 /* ERROR not a type */ }
+func t4[T Numeric4](s[]T){if( /* ERROR non-boolean */ 0){*s /* ERROR cannot indirect */ [0]}}
+
+// crash 7
+type foo7 interface { bar() }
+type x7[A any] struct{ foo7 }
+func main7() { var _ foo7 = x7[int]{} }
+
+// crash 8
+type foo8[A any] interface { type A }
+func bar8[A foo8[A]](a A) {}
+func main8() {}
+
+// crash 9
+type foo9[A any] interface { type foo9 /* ERROR interface contains type constraints */ [A] }
+func _() { var _ = new(foo9 /* ERROR interface contains type constraints */ [int]) }
+
+// crash 12
+var u /* ERROR cycle */ , i [func /* ERROR used as value */ /* ERROR used as value */ (u, c /* ERROR undeclared */ /* ERROR undeclared */ ) {}(0, len)]c /* ERROR undeclared */ /* ERROR undeclared */
+
+// crash 15
+func y15() { var a /* ERROR declared but not used */ interface{ p() } = G15[string]{} }
+type G15[X any] s /* ERROR undeclared name */
+func (G15 /* ERROR generic type .* without instantiation */ ) p()
+
+// crash 16
+type Foo16[T any] r16 /* ERROR not a type */
+func r16[T any]() Foo16[Foo16[T]]
+
+// crash 17
+type Y17 interface{ c() }
+type Z17 interface {
+	c() Y17
+	Y17 /* ERROR duplicate method */
+}
+func F17[T Z17](T)
+
+// crash 18
+type o18[T any] []func(_ o18[[]_ /* ERROR cannot use _ */ ])
+
+// crash 19
+type Z19 [][[]Z19{}[0][0]]c19 /* ERROR undeclared */
+
+// crash 20
+type Z20 /* ERROR illegal cycle */ interface{ Z20 }
+func F20[t Z20]() { F20(t /* ERROR invalid composite literal type */ {}) }
+
+// crash 21
+type Z21 /* ERROR illegal cycle */ interface{ Z21 }
+func F21[T Z21]() { ( /* ERROR not used */ F21[Z21]) }
+
+// crash 24
+type T24[P any] P
+func (r T24[P]) m() { T24 /* ERROR without instantiation */ .m() }
+
+// crash 25
+type T25[A any] int
+func (t T25[A]) m1() {}
+var x T25 /* ERROR without instantiation */ .m1
+
+// crash 26
+type T26 = interface{ F26[ /* ERROR cannot have type parameters */ Z any]() }
+func F26[Z any]() T26 { return F26 /* ERROR without instantiation */ /* ERROR missing method */ [] /* ERROR operand */ }
+
+// crash 27
+func e27[T any]() interface{ x27 /* ERROR not a type */ }
+func x27() { e27( /* ERROR cannot infer T */ ) }

src/cmd/compile/internal/types2/fixedbugs/issue39664.go2

@@ -0,0 +1,16 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[_ any] struct {}
+
+func (T /* ERROR instantiation */ ) m()
+
+func _() {
+	var x interface { m() }
+	x = T[int]{}
+	_ = x
+}

src/cmd/compile/internal/types2/fixedbugs/issue39680.go2

@@ -0,0 +1,28 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "fmt"
+
+// Minimal test case.
+func _[T interface{type T}](x T) T{
+	return x
+}
+
+// Test case from issue.
+type constr[T any] interface {
+	type T
+}
+
+func Print[T constr[T]](s []T) {
+	for _, v := range s {
+		fmt.Print(v)
+	}
+}
+
+func f() {
+	Print([]string{"Hello, ", "playground\n"})
+}

src/cmd/compile/internal/types2/fixedbugs/issue39693.go2

@@ -0,0 +1,15 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Number interface {
+	int     /* ERROR int is not an interface */
+	float64 /* ERROR float64 is not an interface */
+}
+
+func Add[T Number](a, b T) T {
+	return a /* ERROR not defined */ + b
+}

src/cmd/compile/internal/types2/fixedbugs/issue39699.go2

@@ -0,0 +1,30 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T0 interface{
+}
+
+type T1 interface{
+	type int
+}
+
+type T2 interface{
+	comparable
+}
+
+type T3 interface {
+	T0
+	T1
+	T2
+}
+
+func _() {
+	_ = T0(0)
+	_ = T1 /* ERROR cannot use interface T1 in conversion */ (1)
+	_ = T2 /* ERROR cannot use interface T2 in conversion */ (2)
+	_ = T3 /* ERROR cannot use interface T3 in conversion */ (3)
+}

src/cmd/compile/internal/types2/fixedbugs/issue39711.go2

@@ -0,0 +1,12 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Do not report a duplicate type error for this type list.
+// (Check types after interfaces have been completed.)
+type _ interface {
+	type interface{ Error() string }, interface{ String() string }
+}