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spec: adjust rule for type parameter on RHS of alias declaration

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Per discussion on issue #75885, a type parameter on the RHS of an alias
declaration must not be declared in the same declaration (but it may be
declared by an enclosing function). This relaxes the spec slightly and
allows for (pre-existing) test cases.

Add a corresponding check to the type checker (there was no check for
type parameters on the RHS of alias declarations at all, before).

Fixes #75884.
Fixes #75885.

Change-Id: I1e5675978e6423d626c068829d4bf5e90035ea82
Reviewed-on: https://go-review.googlesource.com/c/go/+/721820
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Robert Griesemer 2025-11-18 15:47:44 -08:00 • committed by Gopher Robot
parent a49b0302d0
commit 790384c6c2
4 changed files with 38 additions and 9 deletions
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14
doc/go_spec.html
View file

@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Language version go1.26 (Nov 12, 2025)",
"Subtitle": "Language version go1.26 (Nov 18, 2025)",
"Path": "/ref/spec"
}-->
@ -2487,11 +2487,15 @@ type set[P comparable] = map[P]bool
</pre>
<p>
In an alias declaration the given type cannot be a type parameter.
In an alias declaration the given type cannot be a type parameter declared in the same declaration.
</p>
<pre>
type A[P any] = P // illegal: P is a type parameter
type A[P any] = P // illegal: P is a type parameter declared in the declaration of A
func f[P any]() {
type A = P // ok: T is a type parameter declared by the enclosing function
}
</pre>
<h4 id="Type_definitions">Type definitions</h4>
@ -2601,8 +2605,8 @@ In a type definition the given type cannot be a type parameter.
<pre>
type T[P any] P // illegal: P is a type parameter
func f[T any]() {
type L T // illegal: T is a type parameter declared by the enclosing function
func f[P any]() {
type L P // illegal: P is a type parameter declared by the enclosing function
}
</pre>

9
src/cmd/compile/internal/types2/decl.go
View file

@ -497,9 +497,14 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *TypeN
rhs = check.declaredType(tdecl.Type, obj)
assert(rhs != nil)
alias.fromRHS = rhs
unalias(alias) // populate alias.actual
// spec: In an alias declaration the given type cannot be a type parameter declared in the same declaration."
// (see also go.dev/issue/75884, go.dev/issue/#75885)
if tpar, ok := rhs.(*TypeParam); ok && alias.tparams != nil && slices.Index(alias.tparams.list(), tpar) >= 0 {
check.error(tdecl.Type, MisplacedTypeParam, "cannot use type parameter declared in alias declaration as RHS")
alias.fromRHS = Typ[Invalid]
}
} else {
if !versionErr && tparam0 != nil {
check.error(tdecl, UnsupportedFeature, "generic type alias requires GODEBUG=gotypesalias=1 or unset")

9
src/go/types/decl.go
View file

@ -572,9 +572,14 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *TypeName
rhs = check.declaredType(tdecl.Type, obj)
assert(rhs != nil)
alias.fromRHS = rhs
unalias(alias) // populate alias.actual
// spec: In an alias declaration the given type cannot be a type parameter declared in the same declaration."
// (see also go.dev/issue/75884, go.dev/issue/#75885)
if tpar, ok := rhs.(*TypeParam); ok && alias.tparams != nil && slices.Index(alias.tparams.list(), tpar) >= 0 {
check.error(tdecl.Type, MisplacedTypeParam, "cannot use type parameter declared in alias declaration as RHS")
alias.fromRHS = Typ[Invalid]
}
} else {
// With Go1.23, the default behavior is to use Alias nodes,
// reflected by check.enableAlias. Signal non-default behavior.

15
src/internal/types/testdata/fixedbugs/issue75885.go vendored Normal file
View file

@ -0,0 +1,15 @@
// -gotypesalias=1
// Copyright 2025 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 A[P any] = P // ERROR "cannot use type parameter declared in alias declaration as RHS"
func _[P any]() {
type A[P any] = P // ERROR "cannot use type parameter declared in alias declaration as RHS"
type B = P
type C[Q any] = P
}