[dev.go2go] go/types: permit alias to generic type

Fixes #39768.

Change-Id: I68c8fbec5adc50e448dabe90b212ef147a384a06
Reviewed-on: https://go-review.googlesource.com/c/go/+/239387
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/check_test.go

@@ -131,6 +131,7 @@ var tests = [][]string{
 	{"fixedbugs/issue39711.go2"},
 	{"fixedbugs/issue39723.go2"},
 	{"fixedbugs/issue39755.go2"},
+	{"fixedbugs/issue39768.go2"},
 }
 
 var fset = token.NewFileSet()

src/go/types/decl.go

@@ -562,7 +562,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 		}
 
 		obj.typ = Typ[Invalid]
-		obj.typ = check.typ(tdecl.Type)
+		obj.typ = check.anyType(tdecl.Type)
 
 	} else {
 		// defined type declaration

src/go/types/fixedbugs/issue39768.go2

@@ -0,0 +1,20 @@
+// Copyright 2020 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 T(type P) P
+type A = T
+var x A(int)
+var _ A /* ERROR cannot use generic type */
+
+type B = T(int)
+var y B = x
+var _ B /* ERROR not a generic type */ (int)
+
+// test case from issue
+
+type Vector(type T) []T
+type VectorAlias = Vector
+var v Vector(int)