go/types, types2: fix misleading internal comment

See also the discussion in #75885.

Change-Id: Ieb964ea6ee51600c0c08ecba0af50a1deb209a4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/712141
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

Author: Robert Griesemer

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

@@ -575,11 +575,8 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *TypeN
 		named.underlying = Typ[Invalid]
 	}
 
-	// Disallow a lone type parameter as the RHS of a type declaration (go.dev/issue/45639).
-	// We don't need this restriction anymore if we make the underlying type of a type
-	// parameter its constraint interface: if the RHS is a lone type parameter, we will
-	// use its underlying type (like we do for any RHS in a type declaration), and its
-	// underlying type is an interface and the type declaration is well defined.
+	// spec: "In a type definition the given type cannot be a type parameter."
+	// (See also go.dev/issue/45639.)
 	if isTypeParam(rhs) {
 		check.error(tdecl.Type, MisplacedTypeParam, "cannot use a type parameter as RHS in type declaration")
 		named.underlying = Typ[Invalid]

src/go/types/decl.go

@@ -657,11 +657,8 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *TypeName
 		named.underlying = Typ[Invalid]
 	}
 
-	// Disallow a lone type parameter as the RHS of a type declaration (go.dev/issue/45639).
-	// We don't need this restriction anymore if we make the underlying type of a type
-	// parameter its constraint interface: if the RHS is a lone type parameter, we will
-	// use its underlying type (like we do for any RHS in a type declaration), and its
-	// underlying type is an interface and the type declaration is well defined.
+	// spec: "In a type definition the given type cannot be a type parameter."
+	// (See also go.dev/issue/45639.)
 	if isTypeParam(rhs) {
 		check.error(tdecl.Type, MisplacedTypeParam, "cannot use a type parameter as RHS in type declaration")
 		named.underlying = Typ[Invalid]