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

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>

3 files changed, 29 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index 60371651ab..5cb52fdbe4 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -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")
diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index 4b374fb66d..fffcb590e6 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -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.
diff --git a/src/internal/types/testdata/fixedbugs/issue75885.go b/src/internal/types/testdata/fixedbugs/issue75885.go
new file mode 100644
index 0000000000..f0cf4a65ed
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue75885.go
@@ -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
+}