cmd/compile/internal/types2: do not declare new methods on instantiated types

Report an error if an alias is used to declare a method on an
instantiated type.

Also, when resolving the receiver type, don't use asNamed to
avoid premature expansion of the type.

Fixes #47968.

Change-Id: Ie5acc4cfb1944deaaeeaee98707f31e256f8ef5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/345472
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -132,7 +132,7 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 				// Also: Don't report an error via genericType since it will be reported
 				//       again when we type-check the signature.
 				// TODO(gri) maybe the receiver should be marked as invalid instead?
-				if recv := asNamed(check.genericType(rname, false)); recv != nil {
+				if recv, _ := check.genericType(rname, false).(*Named); recv != nil {
 					recvTParams = recv.TParams().list()
 				}
 			}
@@ -211,6 +211,12 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 			switch T := rtyp.(type) {
 			case *Named:
 				T.expand(nil)
+				// The receiver type may be an instantiated type referred to
+				// by an alias (which cannot have receiver parameters for now).
+				if T.TArgs() != nil && sig.RParams() == nil {
+					check.errorf(recv.pos, "cannot define methods on instantiated type %s", recv.typ)
+					break
+				}
 				// spec: "The type denoted by T is called the receiver base type; it must not
 				// be a pointer or interface type and it must be declared in the same package
 				// as the method."

src/cmd/compile/internal/types2/testdata/fixedbugs/issue47968.go2

@@ -0,0 +1,21 @@
+// 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.
+
+package p
+
+type T[P any] struct{}
+
+func (T[P]) m1()
+
+type A1 = T
+
+func (A1[P]) m2() {}
+
+type A2 = T[int]
+
+func (A2 /* ERROR cannot define methods on instantiated type T\[int\] */) m3()   {}
+func (_ /* ERROR cannot define methods on instantiated type T\[int\] */ A2) m4() {}
+
+func (T[int]) m5()                                     {} // int is the type parameter name, not an instantiation
+func (T[* /* ERROR must be an identifier */ int]) m6() {} // syntax error