[dev.typeparams] cmd/compile/internal/types2: type alias decl requires go1.9

Add respective check to type checker.
Remove respective check from the compiler's new type2-based noder.

Updates #31793.

Change-Id: I907e3acab4c136027a8c3db1e9bac301d209c2e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/289570
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -102,10 +102,6 @@ func (g *irgen) funcDecl(out *ir.Nodes, decl *syntax.FuncDecl) {
 
 func (g *irgen) typeDecl(out *ir.Nodes, decl *syntax.TypeDecl) {
 	if decl.Alias {
-		if !types.AllowsGoVersion(types.LocalPkg, 1, 9) {
-			base.ErrorfAt(g.pos(decl), "type aliases only supported as of -lang=go1.9")
-		}
-
 		name, _ := g.def(decl.Name)
 		g.pragmaFlags(decl.Pragma, 0)
 

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

@@ -629,6 +629,9 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 
 	if alias {
 		// type alias declaration
+		if !check.allowVersion(obj.pkg, 1, 9) {
+			check.errorf(tdecl, "type aliases requires go1.9 or later")
+		}
 
 		obj.typ = Typ[Invalid]
 		obj.typ = check.anyType(tdecl.Type)

src/cmd/compile/internal/types2/testdata/go1_8.src

@@ -0,0 +1,10 @@
+// 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.
+
+// Check Go language version-specific errors.
+
+package go1_8 // go1.8
+
+// type alias declarations
+type any /* ERROR type aliases requires go1.9 or later */ = interface{}