go/types, types2: report error when using uninstantiated signature alias

For #67547.
Fixes #67683.

Change-Id: I9487820ab4e2bd257d253a7016df45729b29f836
Reviewed-on: https://go-review.googlesource.com/c/go/+/588855
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

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

@@ -1013,7 +1013,7 @@ func (check *Checker) nonGeneric(T *target, x *operand) {
 	}
 	var what string
 	switch t := x.typ.(type) {
-	case *Named:
+	case *Alias, *Named:
 		if isGeneric(t) {
 			what = "type"
 		}

src/go/types/expr.go

@@ -998,7 +998,7 @@ func (check *Checker) nonGeneric(T *target, x *operand) {
 	}
 	var what string
 	switch t := x.typ.(type) {
-	case *Named:
+	case *Alias, *Named:
 		if isGeneric(t) {
 			what = "type"
 		}

src/internal/types/testdata/fixedbugs/issue67683.go

@@ -0,0 +1,19 @@
+// -goexperiment=aliastypeparams -gotypesalias=1
+
+// Copyright 2024 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] func()
+
+// alias signature types
+type B[P any] = func()
+type C[P any] = B[P]
+
+var _ = A /* ERROR "cannot use generic type A without instantiation" */ (nil)
+
+// generic alias signature types must be instantiated before use
+var _ = B /* ERROR "cannot use generic type B without instantiation" */ (nil)
+var _ = C /* ERROR "cannot use generic type C without instantiation" */ (nil)