go/*,cmd/gofmt: guard AST changes with the typeparams build tag

This CL changes our approach to guarding type parameter functionality
and API. Previously, we guarded type parameter functionality with the
parser.parseTypeParams parser mode, and were in the process of hiding
the type parameter API behind the go1.18 build constraint.

These mechanisms had several limitations:
 + Requiring the parser.parseTypeParams mode to be set meant that
   existing tooling would have to opt-in to type parameters in all
   places where it parses Go files.
 + The parseTypeParams mode value had to be copied in several places.
 + go1.18 is not specific to typeparams, making it difficult to set up
   the builders to run typeparams tests.

This CL addresses the above limitations, and completes the task of
hiding the AST API, by switching to a new 'typeparams' build constraint
and adding a new go/internal/typeparams helper package.

The typeparams build constraint is used to conditionally compile the new
AST changes. The typeparams package provides utilities for accessing and
writing the new AST data, so that we don't have to fragment our parser
or type checker logic across build constraints. The typeparams.Enabled
const is used to guard tests that require type parameter support.

The parseTypeParams parser mode is gone, replaced by a new
typeparams.DisableParsing mode with the opposite sense. Now, type
parameters are only parsed if go/parser is compiled with the typeparams
build constraint set AND typeparams.DisableParsing not set. This new
parser mode allows opting out of type parameter parsing for tests.

How exactly to run tests on builders is left to a follow-up CL.

Updates #44933

Change-Id: I3091e42a2e5e2f23e8b2ae584f415a784b9fbd65
Reviewed-on: https://go-review.googlesource.com/c/go/+/300649
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/gofmt/gofmt.go

@@ -33,10 +33,6 @@ var (
 	doDiff      = flag.Bool("d", false, "display diffs instead of rewriting files")
 	allErrors   = flag.Bool("e", false, "report all errors (not just the first 10 on different lines)")
 
-	// allowTypeParams controls whether type parameters are allowed in the code
-	// being formatted. It is enabled for go1.18 in gofmt_go1.18.go.
-	allowTypeParams = false
-
 	// debugging
 	cpuprofile = flag.String("cpuprofile", "", "write cpu profile to this file")
 )
@@ -53,10 +49,6 @@ const (
 	printerNormalizeNumbers = 1 << 30
 )
 
-// parseTypeParams tells go/parser to parse type parameters. Must be kept in
-// sync with go/parser/interface.go.
-const parseTypeParams parser.Mode = 1 << 30
-
 var (
 	fileSet    = token.NewFileSet() // per process FileSet
 	exitCode   = 0
@@ -79,9 +71,6 @@ func initParserMode() {
 	if *allErrors {
 		parserMode |= parser.AllErrors
 	}
-	if allowTypeParams {
-		parserMode |= parseTypeParams
-	}
 }
 
 func isGoFile(f fs.DirEntry) bool {

src/cmd/gofmt/gofmt_test.go

@@ -49,12 +49,13 @@ func gofmtFlags(filename string, maxLines int) string {
 		case scanner.EOF:
 			return ""
 		}
-
 	}
 
 	return ""
 }
 
+var typeParamsEnabled = false
+
 func runTest(t *testing.T, in, out string) {
 	// process flags
 	*simplifyAST = false
@@ -78,8 +79,10 @@ func runTest(t *testing.T, in, out string) {
 			// fake flag - pretend input is from stdin
 			stdin = true
 		case "-G":
-			// fake flag - allow parsing type parameters
+			// fake flag - test is for generic code
-			allowTypeParams = true
+			if !typeParamsEnabled {
+				return
+			}
 		default:
 			t.Errorf("unrecognized flag name: %s", name)
 		}

src/cmd/gofmt/gofmt_typeparams_test.go

@@ -2,11 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.18
+//go:build typeparams
-// +build go1.18
+// +build typeparams
 
 package main
 
 func init() {
-	allowTypeParams = true
+	typeParamsEnabled = true
 }

src/go/ast/ast.go

@@ -374,13 +374,6 @@ type (
 		Rparen   token.Pos // position of ")"
 	}
 
-	// A ListExpr node represents a list of expressions separated by commas.
-	// ListExpr nodes are used as index in IndexExpr nodes representing type
-	// or function instantiations with more than one type argument.
-	ListExpr struct {
-		ElemList []Expr
-	}
-
 	// A StarExpr node represents an expression of the form "*" Expression.
 	// Semantically it could be a unary "*" expression, or a pointer type.
 	//
@@ -447,14 +440,6 @@ type (
 
 	// Pointer types are represented via StarExpr nodes.
 
-	// A FuncType node represents a function type.
-	FuncType struct {
-		Func    token.Pos  // position of "func" keyword (token.NoPos if there is no "func")
-		TParams *FieldList // type parameters; or nil
-		Params  *FieldList // (incoming) parameters; non-nil
-		Results *FieldList // (outgoing) results; or nil
-	}
-
 	// An InterfaceType node represents an interface type.
 	InterfaceType struct {
 		Interface  token.Pos  // position of "interface" keyword
@@ -497,18 +482,12 @@ func (x *IndexExpr) Pos() token.Pos      { return x.X.Pos() }
 func (x *SliceExpr) Pos() token.Pos      { return x.X.Pos() }
 func (x *TypeAssertExpr) Pos() token.Pos { return x.X.Pos() }
 func (x *CallExpr) Pos() token.Pos       { return x.Fun.Pos() }
-func (x *ListExpr) Pos() token.Pos {
+func (x *StarExpr) Pos() token.Pos       { return x.Star }
-	if len(x.ElemList) > 0 {
+func (x *UnaryExpr) Pos() token.Pos      { return x.OpPos }
-		return x.ElemList[0].Pos()
+func (x *BinaryExpr) Pos() token.Pos     { return x.X.Pos() }
-	}
+func (x *KeyValueExpr) Pos() token.Pos   { return x.Key.Pos() }
-	return token.NoPos
+func (x *ArrayType) Pos() token.Pos      { return x.Lbrack }
-}
+func (x *StructType) Pos() token.Pos     { return x.Struct }
-func (x *StarExpr) Pos() token.Pos     { return x.Star }
-func (x *UnaryExpr) Pos() token.Pos    { return x.OpPos }
-func (x *BinaryExpr) Pos() token.Pos   { return x.X.Pos() }
-func (x *KeyValueExpr) Pos() token.Pos { return x.Key.Pos() }
-func (x *ArrayType) Pos() token.Pos    { return x.Lbrack }
-func (x *StructType) Pos() token.Pos   { return x.Struct }
 func (x *FuncType) Pos() token.Pos {
 	if x.Func.IsValid() || x.Params == nil { // see issue 3870
 		return x.Func
@@ -536,18 +515,12 @@ func (x *IndexExpr) End() token.Pos      { return x.Rbrack + 1 }
 func (x *SliceExpr) End() token.Pos      { return x.Rbrack + 1 }
 func (x *TypeAssertExpr) End() token.Pos { return x.Rparen + 1 }
 func (x *CallExpr) End() token.Pos       { return x.Rparen + 1 }
-func (x *ListExpr) End() token.Pos {
+func (x *StarExpr) End() token.Pos       { return x.X.End() }
-	if len(x.ElemList) > 0 {
+func (x *UnaryExpr) End() token.Pos      { return x.X.End() }
-		return x.ElemList[len(x.ElemList)-1].End()
+func (x *BinaryExpr) End() token.Pos     { return x.Y.End() }
-	}
+func (x *KeyValueExpr) End() token.Pos   { return x.Value.End() }
-	return token.NoPos
+func (x *ArrayType) End() token.Pos      { return x.Elt.End() }
-}
+func (x *StructType) End() token.Pos     { return x.Fields.End() }
-func (x *StarExpr) End() token.Pos     { return x.X.End() }
-func (x *UnaryExpr) End() token.Pos    { return x.X.End() }
-func (x *BinaryExpr) End() token.Pos   { return x.Y.End() }
-func (x *KeyValueExpr) End() token.Pos { return x.Value.End() }
-func (x *ArrayType) End() token.Pos    { return x.Elt.End() }
-func (x *StructType) End() token.Pos   { return x.Fields.End() }
 func (x *FuncType) End() token.Pos {
 	if x.Results != nil {
 		return x.Results.End()
@@ -573,7 +546,6 @@ func (*IndexExpr) exprNode()      {}
 func (*SliceExpr) exprNode()      {}
 func (*TypeAssertExpr) exprNode() {}
 func (*CallExpr) exprNode()       {}
-func (*ListExpr) exprNode()       {}
 func (*StarExpr) exprNode()       {}
 func (*UnaryExpr) exprNode()      {}
 func (*BinaryExpr) exprNode()     {}
@@ -920,16 +892,6 @@ type (
 		Values  []Expr        // initial values; or nil
 		Comment *CommentGroup // line comments; or nil
 	}
-
-	// A TypeSpec node represents a type declaration (TypeSpec production).
-	TypeSpec struct {
-		Doc     *CommentGroup // associated documentation; or nil
-		Name    *Ident        // type name
-		TParams *FieldList    // type parameters; or nil
-		Assign  token.Pos     // position of '=', if any
-		Type    Expr          // *Ident, *ParenExpr, *SelectorExpr, *StarExpr, or any of the *XxxTypes
-		Comment *CommentGroup // line comments; or nil
-	}
 )
 
 // Pos and End implementations for spec nodes.

src/go/ast/ast_notypeparams.go

@@ -0,0 +1,28 @@
+// 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.
+
+//go:build !typeparams
+// +build !typeparams
+
+package ast
+
+import "go/token"
+
+type (
+	// A FuncType node represents a function type.
+	FuncType struct {
+		Func    token.Pos  // position of "func" keyword (token.NoPos if there is no "func")
+		Params  *FieldList // (incoming) parameters; non-nil
+		Results *FieldList // (outgoing) results; or nil
+	}
+
+	// A TypeSpec node represents a type declaration (TypeSpec production).
+	TypeSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Name    *Ident        // type name
+		Assign  token.Pos     // position of '=', if any
+		Type    Expr          // *Ident, *ParenExpr, *SelectorExpr, *StarExpr, or any of the *XxxTypes
+		Comment *CommentGroup // line comments; or nil
+	}
+)

src/go/ast/ast_typeparams.go

@@ -0,0 +1,51 @@
+// 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.
+
+//go:build typeparams
+// +build typeparams
+
+package ast
+
+import "go/token"
+
+type (
+	// A FuncType node represents a function type.
+	FuncType struct {
+		Func    token.Pos  // position of "func" keyword (token.NoPos if there is no "func")
+		TParams *FieldList // type parameters; or nil
+		Params  *FieldList // (incoming) parameters; non-nil
+		Results *FieldList // (outgoing) results; or nil
+	}
+
+	// A TypeSpec node represents a type declaration (TypeSpec production).
+	TypeSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Name    *Ident        // type name
+		TParams *FieldList    // type parameters; or nil
+		Assign  token.Pos     // position of '=', if any
+		Type    Expr          // *Ident, *ParenExpr, *SelectorExpr, *StarExpr, or any of the *XxxTypes
+		Comment *CommentGroup // line comments; or nil
+	}
+
+	// A ListExpr node represents a list of expressions separated by commas.
+	// ListExpr nodes are used as index in IndexExpr nodes representing type
+	// or function instantiations with more than one type argument.
+	ListExpr struct {
+		ElemList []Expr
+	}
+)
+
+func (*ListExpr) exprNode() {}
+func (x *ListExpr) Pos() token.Pos {
+	if len(x.ElemList) > 0 {
+		return x.ElemList[0].Pos()
+	}
+	return token.NoPos
+}
+func (x *ListExpr) End() token.Pos {
+	if len(x.ElemList) > 0 {
+		return x.ElemList[len(x.ElemList)-1].End()
+	}
+	return token.NoPos
+}

src/go/ast/walk.go

@@ -4,8 +4,6 @@
 
 package ast
 
-import "fmt"
-
 // A Visitor's Visit method is invoked for each node encountered by Walk.
 // If the result visitor w is not nil, Walk visits each of the children
 // of node with the visitor w, followed by a call of w.Visit(nil).
@@ -116,9 +114,6 @@ func Walk(v Visitor, node Node) {
 		Walk(v, n.X)
 		Walk(v, n.Index)
 
-	case *ListExpr:
-		walkExprList(v, n.ElemList)
-
 	case *SliceExpr:
 		Walk(v, n.X)
 		if n.Low != nil {
@@ -166,9 +161,7 @@ func Walk(v Visitor, node Node) {
 		Walk(v, n.Fields)
 
 	case *FuncType:
-		if n.TParams != nil {
+		walkFuncTypeParams(v, n)
-			Walk(v, n.TParams)
-		}
 		if n.Params != nil {
 			Walk(v, n.Params)
 		}
@@ -323,9 +316,7 @@ func Walk(v Visitor, node Node) {
 			Walk(v, n.Doc)
 		}
 		Walk(v, n.Name)
-		if n.TParams != nil {
+		walkTypeSpecParams(v, n)
-			Walk(v, n.TParams)
-		}
 		Walk(v, n.Type)
 		if n.Comment != nil {
 			Walk(v, n.Comment)
@@ -372,7 +363,7 @@ func Walk(v Visitor, node Node) {
 		}
 
 	default:
-		panic(fmt.Sprintf("ast.Walk: unexpected node type %T", n))
+		walkOtherNodes(v, n)
 	}
 
 	v.Visit(nil)

src/go/ast/walk_notypeparams.go

@@ -0,0 +1,17 @@
+// 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.
+
+//go:build !typeparams
+// +build !typeparams
+
+package ast
+
+import "fmt"
+
+func walkFuncTypeParams(v Visitor, n *FuncType) {}
+func walkTypeSpecParams(v Visitor, n *TypeSpec) {}
+
+func walkOtherNodes(v Visitor, n Node) {
+	panic(fmt.Sprintf("ast.Walk: unexpected node type %T", n))
+}

src/go/ast/walk_typeparams.go

@@ -0,0 +1,30 @@
+// 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.
+
+//go:build typeparams
+// +build typeparams
+
+package ast
+
+func walkFuncTypeParams(v Visitor, n *FuncType) {
+	if n.TParams != nil {
+		Walk(v, n.TParams)
+	}
+}
+
+func walkTypeSpecParams(v Visitor, n *TypeSpec) {
+	if n.TParams != nil {
+		Walk(v, n.TParams)
+	}
+}
+
+func walkOtherNodes(v Visitor, n Node) {
+	if e, ok := n.(*ast.ListExpr); ok {
+		if e != nil {
+			Walk(v, e)
+		}
+	} else {
+		panic(fmt.Sprintf("ast.Walk: unexpected node type %T", n))
+	}
+}

src/go/build/deps_test.go

@@ -278,6 +278,7 @@ var depsRules = `
 	< go/token
 	< go/scanner
 	< go/ast
+	< go/internal/typeparams
 	< go/parser;
 
 	FMT

src/go/internal/typeparams/common.go

@@ -0,0 +1,13 @@
+// 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 typeparams provides functions to work with type parameter data
+// stored in the AST, while these AST changes are guarded by a build
+// constraint.
+package typeparams
+
+// DisallowParsing is the numeric value of a parsing mode that disallows type
+// parameters. This only matters if the typeparams experiment is active, and
+// may be used for running tests that disallow generics.
+const DisallowParsing = 1 << 30

src/go/internal/typeparams/notypeparams.go

@@ -0,0 +1,38 @@
+// 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.
+
+//go:build !typeparams
+// +build !typeparams
+
+package typeparams
+
+import (
+	"go/ast"
+)
+
+const Enabled = false
+
+func PackExpr(list []ast.Expr) ast.Expr {
+	switch len(list) {
+	case 0:
+		return nil
+	case 1:
+		return list[0]
+	default:
+		// The parser should not attempt to pack multiple expressions into an
+		// IndexExpr if type params are disabled.
+		panic("multiple index expressions are unsupported without type params")
+	}
+}
+
+func UnpackExpr(expr ast.Expr) []ast.Expr {
+	return []ast.Expr{expr}
+}
+
+func Get(ast.Node) *ast.FieldList {
+	return nil
+}
+
+func Set(node ast.Node, params *ast.FieldList) {
+}

src/go/internal/typeparams/typeparams.go

@@ -0,0 +1,61 @@
+// 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.
+
+//go:build typeparams
+// +build typeparams
+
+package typeparams
+
+import (
+	"fmt"
+	"go/ast"
+)
+
+const Enabled = true
+
+func PackExpr(list []ast.Expr) ast.Expr {
+	switch len(list) {
+	case 0:
+		return nil
+	case 1:
+		return list[0]
+	default:
+		return &ast.ListExpr{ElemList: list}
+	}
+}
+
+// TODO(gri) Should find a more efficient solution that doesn't
+//           require introduction of a new slice for simple
+//           expressions.
+func UnpackExpr(x ast.Expr) []ast.Expr {
+	if x, _ := x.(*ast.ListExpr); x != nil {
+		return x.ElemList
+	}
+	if x != nil {
+		return []ast.Expr{x}
+	}
+	return nil
+}
+
+func Get(n ast.Node) *ast.FieldList {
+	switch n := n.(type) {
+	case *ast.TypeSpec:
+		return n.TParams
+	case *ast.FuncType:
+		return n.TParams
+	default:
+		panic(fmt.Sprintf("node type %T has no type parameters", n))
+	}
+}
+
+func Set(n ast.Node, params *ast.FieldList) {
+	switch n := n.(type) {
+	case *ast.TypeSpec:
+		n.TParams = params
+	case *ast.FuncType:
+		n.TParams = params
+	default:
+		panic(fmt.Sprintf("node type %T has no type parameters", n))
+	}
+}

src/go/parser/error_test.go

@@ -23,6 +23,7 @@
 package parser
 
 import (
+	"go/internal/typeparams"
 	"go/scanner"
 	"go/token"
 	"os"
@@ -188,7 +189,11 @@ func TestErrors(t *testing.T) {
 		if !d.IsDir() && !strings.HasPrefix(name, ".") && (strings.HasSuffix(name, ".src") || strings.HasSuffix(name, ".go2")) {
 			mode := DeclarationErrors | AllErrors
 			if strings.HasSuffix(name, ".go2") {
-				mode |= parseTypeParams
+				if !typeparams.Enabled {
+					continue
+				}
+			} else {
+				mode |= typeparams.DisallowParsing
 			}
 			checkErrors(t, filepath.Join(testdata, name), nil, mode, true)
 		}

src/go/parser/interface.go

@@ -56,13 +56,6 @@ const (
 	DeclarationErrors                              // report declaration errors
 	SpuriousErrors                                 // same as AllErrors, for backward-compatibility
 	AllErrors         = SpuriousErrors             // report all errors (not just the first 10 on different lines)
-
-	// parseTypeParams controls the parsing of type parameters. Must be
-	// kept in sync with:
-	//  go/printer/printer_test.go
-	//  go/types/check_test.go
-	//  cmd/gofmt/gofmt.go
-	parseTypeParams = 1 << 30
 )
 
 // ParseFile parses the source code of a single Go source file and returns

src/go/parser/parser.go

@@ -19,6 +19,7 @@ package parser
 import (
 	"fmt"
 	"go/ast"
+	"go/internal/typeparams"
 	"go/scanner"
 	"go/token"
 	"strconv"
@@ -75,6 +76,10 @@ func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mod
 	p.next()
 }
 
+func (p *parser) parseTypeParams() bool {
+	return typeparams.Enabled && p.mode&typeparams.DisallowParsing == 0
+}
+
 // ----------------------------------------------------------------------------
 // Parsing support
 
@@ -494,7 +499,7 @@ func (p *parser) parseQualifiedIdent(ident *ast.Ident) ast.Expr {
 	}
 
 	typ := p.parseTypeName(ident)
-	if p.tok == token.LBRACK && p.mode&parseTypeParams != 0 {
+	if p.tok == token.LBRACK && p.parseTypeParams() {
 		typ = p.parseTypeInstance(typ)
 	}
 
@@ -553,7 +558,7 @@ func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Ex
 	// TODO(rfindley): consider changing parseRhsOrType so that this function variable
 	// is not needed.
 	argparser := p.parseRhsOrType
-	if p.mode&parseTypeParams == 0 {
+	if !p.parseTypeParams() {
 		argparser = p.parseRhs
 	}
 	if p.tok != token.RBRACK {
@@ -583,19 +588,19 @@ func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Ex
 			// x [P]E
 			return x, &ast.ArrayType{Lbrack: lbrack, Len: args[0], Elt: elt}
 		}
-		if p.mode&parseTypeParams == 0 {
+		if !p.parseTypeParams() {
 			p.error(rbrack, "missing element type in array type expression")
 			return nil, &ast.BadExpr{From: args[0].Pos(), To: args[0].End()}
 		}
 	}
 
-	if p.mode&parseTypeParams == 0 {
+	if !p.parseTypeParams() {
 		p.error(firstComma, "expected ']', found ','")
 		return x, &ast.BadExpr{From: args[0].Pos(), To: args[len(args)-1].End()}
 	}
 
 	// x[P], x[P1, P2], ...
-	return nil, &ast.IndexExpr{X: x, Lbrack: lbrack, Index: &ast.ListExpr{ElemList: args}, Rbrack: rbrack}
+	return nil, &ast.IndexExpr{X: x, Lbrack: lbrack, Index: typeparams.PackExpr(args), Rbrack: rbrack}
 }
 
 func (p *parser) parseFieldDecl() *ast.Field {
@@ -878,7 +883,7 @@ func (p *parser) parseParameters(acceptTParams bool) (tparams, params *ast.Field
 		defer un(trace(p, "Parameters"))
 	}
 
-	if p.mode&parseTypeParams != 0 && acceptTParams && p.tok == token.LBRACK {
+	if p.parseTypeParams() && acceptTParams && p.tok == token.LBRACK {
 		opening := p.pos
 		p.next()
 		// [T any](params) syntax
@@ -951,7 +956,7 @@ func (p *parser) parseMethodSpec() *ast.Field {
 	x := p.parseTypeName(nil)
 	if ident, _ := x.(*ast.Ident); ident != nil {
 		switch {
-		case p.tok == token.LBRACK && p.mode&parseTypeParams != 0:
+		case p.tok == token.LBRACK && p.parseTypeParams():
 			// generic method or embedded instantiated type
 			lbrack := p.pos
 			p.next()
@@ -967,7 +972,8 @@ func (p *parser) parseMethodSpec() *ast.Field {
 				_, params := p.parseParameters(false)
 				results := p.parseResult()
 				idents = []*ast.Ident{ident}
-				typ = &ast.FuncType{Func: token.NoPos, TParams: tparams, Params: params, Results: results}
+				typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
+				typeparams.Set(typ, tparams)
 			} else {
 				// embedded instantiated type
 				// TODO(rfindley) should resolve all identifiers in x.
@@ -984,7 +990,7 @@ func (p *parser) parseMethodSpec() *ast.Field {
 					p.exprLev--
 				}
 				rbrack := p.expectClosing(token.RBRACK, "type argument list")
-				typ = &ast.IndexExpr{X: ident, Lbrack: lbrack, Index: &ast.ListExpr{ElemList: list}, Rbrack: rbrack}
+				typ = &ast.IndexExpr{X: ident, Lbrack: lbrack, Index: typeparams.PackExpr(list), Rbrack: rbrack}
 			}
 		case p.tok == token.LPAREN:
 			// ordinary method
@@ -1000,7 +1006,7 @@ func (p *parser) parseMethodSpec() *ast.Field {
 	} else {
 		// embedded, possibly instantiated type
 		typ = x
-		if p.tok == token.LBRACK && p.mode&parseTypeParams != 0 {
+		if p.tok == token.LBRACK && p.parseTypeParams() {
 			// embedded instantiated interface
 			typ = p.parseTypeInstance(typ)
 		}
@@ -1020,7 +1026,7 @@ func (p *parser) parseInterfaceType() *ast.InterfaceType {
 	pos := p.expect(token.INTERFACE)
 	lbrace := p.expect(token.LBRACE)
 	var list []*ast.Field
-	for p.tok == token.IDENT || p.mode&parseTypeParams != 0 && p.tok == token.TYPE {
+	for p.tok == token.IDENT || p.parseTypeParams() && p.tok == token.TYPE {
 		if p.tok == token.IDENT {
 			list = append(list, p.parseMethodSpec())
 		} else {
@@ -1108,14 +1114,14 @@ func (p *parser) parseTypeInstance(typ ast.Expr) ast.Expr {
 
 	closing := p.expectClosing(token.RBRACK, "type argument list")
 
-	return &ast.IndexExpr{X: typ, Lbrack: opening, Index: &ast.ListExpr{ElemList: list}, Rbrack: closing}
+	return &ast.IndexExpr{X: typ, Lbrack: opening, Index: typeparams.PackExpr(list), Rbrack: closing}
 }
 
 func (p *parser) tryIdentOrType() ast.Expr {
 	switch p.tok {
 	case token.IDENT:
 		typ := p.parseTypeName(nil)
-		if p.tok == token.LBRACK && p.mode&parseTypeParams != 0 {
+		if p.tok == token.LBRACK && p.parseTypeParams() {
 			typ = p.parseTypeInstance(typ)
 		}
 		return typ
@@ -1360,13 +1366,13 @@ func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
 		return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
 	}
 
-	if p.mode&parseTypeParams == 0 {
+	if !p.parseTypeParams() {
 		p.error(firstComma, "expected ']' or ':', found ','")
 		return &ast.BadExpr{From: args[0].Pos(), To: args[len(args)-1].End()}
 	}
 
 	// instance expression
-	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: &ast.ListExpr{ElemList: args}, Rbrack: rbrack}
+	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: typeparams.PackExpr(args), Rbrack: rbrack}
 }
 
 func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
@@ -2406,7 +2412,7 @@ func (p *parser) parseValueSpec(doc *ast.CommentGroup, _ token.Pos, keyword toke
 func (p *parser) parseGenericType(spec *ast.TypeSpec, openPos token.Pos, name0 *ast.Ident, closeTok token.Token) {
 	list := p.parseParameterList(name0, closeTok, p.parseParamDecl, true)
 	closePos := p.expect(closeTok)
-	spec.TParams = &ast.FieldList{Opening: openPos, List: list, Closing: closePos}
+	typeparams.Set(spec, &ast.FieldList{Opening: openPos, List: list, Closing: closePos})
 	// Type alias cannot have type parameters. Accept them for robustness but complain.
 	if p.tok == token.ASSIGN {
 		p.error(p.pos, "generic type cannot be alias")
@@ -2432,7 +2438,7 @@ func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Pos, _ token.Token
 			p.exprLev++
 			x := p.parseExpr()
 			p.exprLev--
-			if name0, _ := x.(*ast.Ident); p.mode&parseTypeParams != 0 && name0 != nil && p.tok != token.RBRACK {
+			if name0, _ := x.(*ast.Ident); p.parseTypeParams() && name0 != nil && p.tok != token.RBRACK {
 				// generic type [T any];
 				p.parseGenericType(spec, lbrack, name0, token.RBRACK)
 			} else {
@@ -2537,12 +2543,12 @@ func (p *parser) parseFuncDecl() *ast.FuncDecl {
 		Name: ident,
 		Type: &ast.FuncType{
 			Func:    pos,
-			TParams: tparams,
 			Params:  params,
 			Results: results,
 		},
 		Body: body,
 	}
+	typeparams.Set(decl.Type, tparams)
 	return decl
 }
 

src/go/parser/resolver.go

@@ -7,6 +7,7 @@ package parser
 import (
 	"fmt"
 	"go/ast"
+	"go/internal/typeparams"
 	"go/token"
 )
 
@@ -450,10 +451,10 @@ func (r *resolver) Visit(node ast.Node) ast.Visitor {
 				// at the identifier in the TypeSpec and ends at the end of the innermost
 				// containing block.
 				r.declare(spec, nil, r.topScope, ast.Typ, spec.Name)
-				if spec.TParams != nil {
+				if tparams := typeparams.Get(spec); tparams != nil {
 					r.openScope(spec.Pos())
 					defer r.closeScope()
-					r.walkFieldList(r.topScope, spec.TParams, ast.Typ)
+					r.walkFieldList(r.topScope, tparams, ast.Typ)
 				}
 				ast.Walk(r, spec.Type)
 			}
@@ -476,7 +477,6 @@ func (r *resolver) Visit(node ast.Node) ast.Visitor {
 }
 
 func (r *resolver) walkFuncType(scope *ast.Scope, typ *ast.FuncType) {
-	r.walkFieldList(scope, typ.TParams, ast.Typ)
 	r.walkFieldList(scope, typ.Params, ast.Var)
 	r.walkFieldList(scope, typ.Results, ast.Var)
 }

src/go/parser/resolver_test.go

@@ -7,6 +7,7 @@ package parser
 import (
 	"fmt"
 	"go/ast"
+	"go/internal/typeparams"
 	"go/scanner"
 	"go/token"
 	"os"
@@ -41,7 +42,11 @@ func TestResolution(t *testing.T) {
 			src := readFile(path) // panics on failure
 			var mode Mode
 			if strings.HasSuffix(path, ".go2") {
-				mode = parseTypeParams
+				if !typeparams.Enabled {
+					t.Skip("type params are not enabled")
+				}
+			} else {
+				mode |= typeparams.DisallowParsing
 			}
 			file, err := ParseFile(fset, path, src, mode)
 			if err != nil {

src/go/parser/short_test.go

@@ -6,7 +6,10 @@
 
 package parser
 
-import "testing"
+import (
+	"go/internal/typeparams"
+	"testing"
+)
 
 var valids = []string{
 	"package p\n",
@@ -130,19 +133,22 @@ func TestValid(t *testing.T) {
 		}
 	})
 	t.Run("tparams", func(t *testing.T) {
+		if !typeparams.Enabled {
+			t.Skip("type params are not enabled")
+		}
 		for _, src := range valids {
-			checkErrors(t, src, src, DeclarationErrors|AllErrors|parseTypeParams, false)
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, false)
 		}
 		for _, src := range validWithTParamsOnly {
-			checkErrors(t, src, src, DeclarationErrors|AllErrors|parseTypeParams, false)
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, false)
 		}
 	})
 }
 
 // TestSingle is useful to track down a problem with a single short test program.
 func TestSingle(t *testing.T) {
-	const src = `package p; var _ = T[P]{}`
+	const src = `package p; var _ = T{}`
-	checkErrors(t, src, src, DeclarationErrors|AllErrors|parseTypeParams, true)
+	checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
 }
 
 var invalids = []string{
@@ -250,21 +256,24 @@ var invalidTParamErrs = []string{
 func TestInvalid(t *testing.T) {
 	t.Run("no tparams", func(t *testing.T) {
 		for _, src := range invalids {
-			checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
+			checkErrors(t, src, src, DeclarationErrors|AllErrors|typeparams.DisallowParsing, true)
 		}
 		for _, src := range validWithTParamsOnly {
-			checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
+			checkErrors(t, src, src, DeclarationErrors|AllErrors|typeparams.DisallowParsing, true)
 		}
 		for _, src := range invalidNoTParamErrs {
-			checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
+			checkErrors(t, src, src, DeclarationErrors|AllErrors|typeparams.DisallowParsing, true)
 		}
 	})
 	t.Run("tparams", func(t *testing.T) {
+		if !typeparams.Enabled {
+			t.Skip("type params are not enabled")
+		}
 		for _, src := range invalids {
-			checkErrors(t, src, src, DeclarationErrors|AllErrors|parseTypeParams, true)
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
 		}
 		for _, src := range invalidTParamErrs {
-			checkErrors(t, src, src, DeclarationErrors|AllErrors|parseTypeParams, true)
+			checkErrors(t, src, src, DeclarationErrors|AllErrors, true)
 		}
 	})
 }

src/go/printer/nodes.go

@@ -11,6 +11,7 @@ package printer
 import (
 	"bytes"
 	"go/ast"
+	"go/internal/typeparams"
 	"go/token"
 	"math"
 	"strconv"
@@ -382,8 +383,8 @@ func (p *printer) parameters(fields *ast.FieldList, isTypeParam bool) {
 }
 
 func (p *printer) signature(sig *ast.FuncType) {
-	if sig.TParams != nil {
+	if tparams := typeparams.Get(sig); tparams != nil {
-		p.parameters(sig.TParams, true)
+		p.parameters(tparams, true)
 	}
 	if sig.Params != nil {
 		p.parameters(sig.Params, false)
@@ -870,8 +871,14 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
 		// TODO(gri): should treat[] like parentheses and undo one level of depth
 		p.expr1(x.X, token.HighestPrec, 1)
 		p.print(x.Lbrack, token.LBRACK)
-		if e, _ := x.Index.(*ast.ListExpr); e != nil {
+		// Note: we're a bit defensive here to handle the case of a ListExpr of
-			p.exprList(x.Lbrack, e.ElemList, depth+1, commaTerm, x.Rbrack, false)
+		// length 1.
+		if list := typeparams.UnpackExpr(x.Index); len(list) > 0 {
+			if len(list) > 1 {
+				p.exprList(x.Lbrack, list, depth+1, commaTerm, x.Rbrack, false)
+			} else {
+				p.expr0(list[0], depth+1)
+			}
 		} else {
 			p.expr0(x.Index, depth+1)
 		}
@@ -1628,8 +1635,8 @@ func (p *printer) spec(spec ast.Spec, n int, doIndent bool) {
 	case *ast.TypeSpec:
 		p.setComment(s.Doc)
 		p.expr(s.Name)
-		if s.TParams != nil {
+		if tparams := typeparams.Get(s); tparams != nil {
-			p.parameters(s.TParams, true)
+			p.parameters(tparams, true)
 		}
 		if n == 1 {
 			p.print(blank)

src/go/printer/printer_test.go

@@ -10,6 +10,7 @@ import (
 	"flag"
 	"fmt"
 	"go/ast"
+	"go/internal/typeparams"
 	"go/parser"
 	"go/token"
 	"io"
@@ -19,10 +20,6 @@ import (
 	"time"
 )
 
-// parseTypeParams tells go/parser to parse type parameters. Must be kept in
-// sync with go/parser/interface.go.
-const parseTypeParams parser.Mode = 1 << 30
-
 const (
 	dataDir  = "testdata"
 	tabwidth = 8
@@ -47,11 +44,7 @@ const (
 // if any.
 func format(src []byte, mode checkMode) ([]byte, error) {
 	// parse src
-	parseMode := parser.ParseComments
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
-	if mode&allowTypeParams != 0 {
-		parseMode |= parseTypeParams
-	}
-	f, err := parser.ParseFile(fset, "", src, parseMode)
 	if err != nil {
 		return nil, fmt.Errorf("parse: %s\n%s", err, src)
 	}
@@ -79,7 +72,7 @@ func format(src []byte, mode checkMode) ([]byte, error) {
 
 	// make sure formatted output is syntactically correct
 	res := buf.Bytes()
-	if _, err := parser.ParseFile(fset, "", res, parseTypeParams); err != nil {
+	if _, err := parser.ParseFile(fset, "", res, parser.ParseComments); err != nil {
 		return nil, fmt.Errorf("re-parse: %s\n%s", err, buf.Bytes())
 	}
 
@@ -210,7 +203,7 @@ var data = []entry{
 	{"linebreaks.input", "linebreaks.golden", idempotent},
 	{"expressions.input", "expressions.golden", idempotent},
 	{"expressions.input", "expressions.raw", rawFormat | idempotent},
-	{"declarations.input", "declarations.golden", allowTypeParams},
+	{"declarations.input", "declarations.golden", 0},
 	{"statements.input", "statements.golden", 0},
 	{"slow.input", "slow.golden", idempotent},
 	{"complit.input", "complit.x", export},
@@ -229,6 +222,9 @@ var data = []entry{
 func TestFiles(t *testing.T) {
 	t.Parallel()
 	for _, e := range data {
+		if !typeparams.Enabled && e.mode&allowTypeParams != 0 {
+			continue
+		}
 		source := filepath.Join(dataDir, e.source)
 		golden := filepath.Join(dataDir, e.golden)
 		mode := e.mode

src/go/printer/testdata/declarations.golden

@@ -942,13 +942,6 @@ type _ interface {
 		x ...int)
 }
 
-// properly format one-line type lists
-type _ interface{ type a }
-
-type _ interface {
-	type a, b, c
-}
-
 // omit superfluous parentheses in parameter lists
 func _(int)
 func _(int)
@@ -999,10 +992,6 @@ func _(struct {
 	y	int
 })	// no extra comma between } and )
 
-// type parameters
-func _[A, B any](a A, b B) int	{}
-func _[T any](x, y T) T
-
 // alias declarations
 
 type c0 struct{}

src/go/printer/testdata/declarations.input

@@ -955,11 +955,6 @@ r string,
 		x ...int)
 }
 
-// properly format one-line type lists
-type _ interface { type a }
-
-type _ interface { type a,b,c }
-
 // omit superfluous parentheses in parameter lists
 func _((int))
 func _((((((int))))))
@@ -1010,10 +1005,6 @@ func _(struct {
 	y int
 }) // no extra comma between } and )
 
-// type parameters
-func _[A, B any](a A, b B) int {}
-func _[T any](x, y T) T
-
 // alias declarations
 
 type c0 struct{}

src/go/printer/testdata/generics.golden

@@ -4,6 +4,9 @@
 
 package generics
 
+func _[A, B any](a A, b B) int	{}
+func _[T any](x, y T) T
+
 type T[P any] struct{}
 type T[P1, P2, P3 any] struct{}
 
@@ -31,3 +34,10 @@ func _() {
 	var _ []T[P]
 	_ = []T[P]{}
 }
+
+// properly format one-line type lists
+type _ interface{ type a }
+
+type _ interface {
+	type a, b, c
+}

src/go/printer/testdata/generics.input

@@ -4,6 +4,9 @@
 
 package generics
 
+func _[A, B any](a A, b B) int {}
+func _[T any](x, y T) T
+
 type T[P any] struct{}
 type T[P1, P2, P3 any] struct{}
 
@@ -28,3 +31,8 @@ func _() {
 	var _ []T[P]
 	_ = []T[P]{}
 }
+
+// properly format one-line type lists
+type _ interface { type a }
+
+type _ interface { type a,b,c }

src/go/types/api_test.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/importer"
+	"go/internal/typeparams"
 	"go/parser"
 	"go/token"
 	"internal/testenv"
@@ -48,8 +49,8 @@ func mustTypecheck(t *testing.T, path, source string, info *Info) string {
 const genericPkg = "package generic_"
 
 func modeForSource(src string) parser.Mode {
-	if strings.HasPrefix(src, genericPkg) {
+	if !strings.HasPrefix(src, genericPkg) {
-		return parseTypeParams
+		return typeparams.DisallowParsing
 	}
 	return 0
 }
@@ -347,6 +348,9 @@ func TestTypesInfo(t *testing.T) {
 	}
 
 	for _, test := range tests {
+		if strings.HasPrefix(test.src, genericPkg) && !typeparams.Enabled {
+			continue
+		}
 		info := Info{Types: make(map[ast.Expr]TypeAndValue)}
 		var name string
 		if strings.HasPrefix(test.src, broken) {
@@ -401,6 +405,9 @@ func TestDefsInfo(t *testing.T) {
 	}
 
 	for _, test := range tests {
+		if strings.HasPrefix(test.src, genericPkg) && !typeparams.Enabled {
+			continue
+		}
 		info := Info{
 			Defs: make(map[*ast.Ident]Object),
 		}
@@ -446,6 +453,9 @@ func TestUsesInfo(t *testing.T) {
 	}
 
 	for _, test := range tests {
+		if strings.HasPrefix(test.src, genericPkg) && !typeparams.Enabled {
+			continue
+		}
 		info := Info{
 			Uses: make(map[*ast.Ident]Object),
 		}

src/go/types/api_typeparams.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.18
+//go:build typeparams
-// +build go1.18
+// +build typeparams
 
 package types
 

src/go/types/api_typeparams_test.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.18
+//go:build typeparams
-// +build go1.18
+// +build typeparams
 
 package types_test
 

src/go/types/assignments.go

@@ -303,20 +303,6 @@ func (check *Checker) assignVars(lhs, origRHS []ast.Expr) {
 	}
 }
 
-// unpack unpacks an *ast.ListExpr into a list of ast.Expr.
-// TODO(gri) Should find a more efficient solution that doesn't
-//           require introduction of a new slice for simple
-//           expressions.
-func unpackExpr(x ast.Expr) []ast.Expr {
-	if x, _ := x.(*ast.ListExpr); x != nil {
-		return x.ElemList
-	}
-	if x != nil {
-		return []ast.Expr{x}
-	}
-	return nil
-}
-
 func (check *Checker) shortVarDecl(pos positioner, lhs, rhs []ast.Expr) {
 	top := len(check.delayed)
 	scope := check.scope

src/go/types/call.go

@@ -8,6 +8,7 @@ package types
 
 import (
 	"go/ast"
+	"go/internal/typeparams"
 	"go/token"
 	"strings"
 	"unicode"
@@ -16,7 +17,8 @@ import (
 // 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 *ast.IndexExpr) {
-	args, ok := check.exprOrTypeList(unpackExpr(inst.Index))
+	exprs := typeparams.UnpackExpr(inst.Index)
+	args, ok := check.exprOrTypeList(exprs)
 	if !ok {
 		x.mode = invalid
 		x.expr = inst

src/go/types/check_test.go

@@ -30,6 +30,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/importer"
+	"go/internal/typeparams"
 	"go/parser"
 	"go/scanner"
 	"go/token"
@@ -43,10 +44,6 @@ import (
 	. "go/types"
 )
 
-// parseTypeParams tells go/parser to parse type parameters. Must be kept in
-// sync with go/parser/interface.go.
-const parseTypeParams parser.Mode = 1 << 30
-
 var (
 	haltOnError = flag.Bool("halt", false, "halt on error")
 	listErrors  = flag.Bool("errlist", false, "list errors")
@@ -213,7 +210,11 @@ func checkFiles(t *testing.T, goVersion string, filenames []string, srcs [][]byt
 
 	mode := parser.AllErrors
 	if strings.HasSuffix(filenames[0], ".go2") {
-		mode |= parseTypeParams
+		if !typeparams.Enabled {
+			t.Skip("type params are not enabled")
+		}
+	} else {
+		mode |= typeparams.DisallowParsing
 	}
 
 	// parse files and collect parser errors

src/go/types/decl.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/constant"
+	"go/internal/typeparams"
 	"go/token"
 )
 
@@ -645,7 +646,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 	})
 
 	alias := tdecl.Assign.IsValid()
-	if alias && tdecl.TParams != nil {
+	if alias && typeparams.Get(tdecl) != nil {
 		// The parser will ensure this but we may still get an invalid AST.
 		// Complain and continue as regular type definition.
 		check.error(atPos(tdecl.Assign), 0, "generic type cannot be alias")
@@ -668,10 +669,10 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 		def.setUnderlying(named)
 		obj.typ = named // make sure recursive type declarations terminate
 
-		if tdecl.TParams != nil {
+		if tparams := typeparams.Get(tdecl); tparams != nil {
 			check.openScope(tdecl, "type parameters")
 			defer check.closeScope()
-			named.tparams = check.collectTypeParams(tdecl.TParams)
+			named.tparams = check.collectTypeParams(tparams)
 		}
 
 		// determine underlying type of named

src/go/types/exprstring.go

@@ -10,6 +10,7 @@ import (
 	"bytes"
 	"fmt"
 	"go/ast"
+	"go/internal/typeparams"
 )
 
 // ExprString returns the (possibly shortened) string representation for x.
@@ -69,16 +70,14 @@ func WriteExpr(buf *bytes.Buffer, x ast.Expr) {
 	case *ast.IndexExpr:
 		WriteExpr(buf, x.X)
 		buf.WriteByte('[')
-		WriteExpr(buf, x.Index)
+		exprs := typeparams.UnpackExpr(x.Index)
-		buf.WriteByte(']')
+		for i, e := range exprs {
-
-	case *ast.ListExpr:
-		for i, e := range x.ElemList {
 			if i > 0 {
 				buf.WriteString(", ")
 			}
 			WriteExpr(buf, e)
 		}
+		buf.WriteByte(']')
 
 	case *ast.SliceExpr:
 		WriteExpr(buf, x.X)

src/go/types/resolver.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/constant"
+	"go/internal/typeparams"
 	"go/token"
 	"sort"
 	"strconv"
@@ -389,8 +390,8 @@ func (check *Checker) collectObjects() {
 						if name == "main" {
 							code = _InvalidMainDecl
 						}
-						if d.decl.Type.TParams != nil {
+						if tparams := typeparams.Get(d.decl.Type); tparams != nil {
-							check.softErrorf(d.decl.Type.TParams, code, "func %s must have no type parameters", name)
+							check.softErrorf(tparams, code, "func %s must have no type parameters", name)
 						}
 						if t := d.decl.Type; t.Params.NumFields() != 0 || t.Results != nil {
 							// TODO(rFindley) Should this be a hard error?
@@ -497,7 +498,7 @@ L: // unpack receiver type
 	if ptyp, _ := rtyp.(*ast.IndexExpr); ptyp != nil {
 		rtyp = ptyp.X
 		if unpackParams {
-			for _, arg := range unpackExpr(ptyp.Index) {
+			for _, arg := range typeparams.UnpackExpr(ptyp.Index) {
 				var par *ast.Ident
 				switch arg := arg.(type) {
 				case *ast.Ident:

src/go/types/typexpr.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/constant"
+	"go/internal/typeparams"
 	"go/token"
 	"sort"
 	"strconv"
@@ -209,27 +210,22 @@ func isubst(x ast.Expr, smap map[*ast.Ident]*ast.Ident) ast.Expr {
 			return &new
 		}
 	case *ast.IndexExpr:
-		index := isubst(n.Index, smap)
+		elems := typeparams.UnpackExpr(n.Index)
-		if index != n.Index {
+		var newElems []ast.Expr
-			new := *n
+		for i, elem := range elems {
-			new.Index = index
-			return &new
-		}
-	case *ast.ListExpr:
-		var elems []ast.Expr
-		for i, elem := range n.ElemList {
 			new := isubst(elem, smap)
 			if new != elem {
-				if elems == nil {
+				if newElems == nil {
-					elems = make([]ast.Expr, len(n.ElemList))
+					newElems = make([]ast.Expr, len(elems))
-					copy(elems, n.ElemList)
+					copy(newElems, elems)
 				}
-				elems[i] = new
+				newElems[i] = new
 			}
 		}
-		if elems != nil {
+		if newElems != nil {
+			index := typeparams.PackExpr(newElems)
 			new := *n
-			new.ElemList = elems
+			new.Index = index
 			return &new
 		}
 	case *ast.ParenExpr:
@@ -316,13 +312,13 @@ func (check *Checker) funcType(sig *Signature, recvPar *ast.FieldList, ftyp *ast
 		}
 	}
 
-	if ftyp.TParams != nil {
+	if tparams := typeparams.Get(ftyp); tparams != nil {
-		sig.tparams = check.collectTypeParams(ftyp.TParams)
+		sig.tparams = check.collectTypeParams(tparams)
 		// Always type-check method type parameters but complain that they are not allowed.
 		// (A separate check is needed when type-checking interface method signatures because
 		// they don't have a receiver specification.)
 		if recvPar != nil {
-			check.errorf(ftyp.TParams, _Todo, "methods cannot have type parameters")
+			check.errorf(tparams, _Todo, "methods cannot have type parameters")
 		}
 	}
 
@@ -467,7 +463,8 @@ func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
 		}
 
 	case *ast.IndexExpr:
-		return check.instantiatedType(e.X, unpackExpr(e.Index), def)
+		exprs := typeparams.UnpackExpr(e.Index)
+		return check.instantiatedType(e.X, exprs, def)
 
 	case *ast.ParenExpr:
 		// Generic types must be instantiated before they can be used in any form.
@@ -801,7 +798,11 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 			// (This extra check is needed here because interface method signatures don't have
 			// a receiver specification.)
 			if sig.tparams != nil {
-				check.errorf(f.Type.(*ast.FuncType).TParams, _Todo, "methods cannot have type parameters")
+				var at positioner = f.Type
+				if tparams := typeparams.Get(f.Type); tparams != nil {
+					at = tparams
+				}
+				check.errorf(at, _Todo, "methods cannot have type parameters")
 			}
 
 			// use named receiver type if available (for better error messages)