cmd/compile/internal/types2: use correct recv for parameterized embedded methods

Methods of generic types are instantiated lazily (upon use). Thus,
when we encounter a method of such a type, we need to instantiate
the method signature with the receiver type arguments. We infer
those type arguments from the method receiver. If the method is
embedded, we must use the actual embedded receiver type, otherwise
the receiver type declared with the method doesn't match up and
inference will fail.

(Note that there's no type inference in the source code here, it's
only the implementation which uses the existing inference mechanism
to easily identify the actual type arguments. If the implementation
is correct, the inference will always succeed.)

Updates #44688.

Change-Id: Ie35b62bebaeaf42037f2ca00cf8bd34fec2ddd9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/298129
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -597,34 +597,43 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 	if m, _ := obj.(*Func); m != nil {
 		// check.dump("### found method %s", m)
 		check.objDecl(m, nil)
-		// If m has a parameterized receiver type, infer the type parameter
-		// values from the actual receiver provided and then substitute the
-		// type parameters in the signature accordingly.
+		// If m has a parameterized receiver type, infer the type arguments
+		// from the actual receiver provided and then substitute the type
+		// parameters accordingly.
 		// TODO(gri) factor this code out
 		sig := m.typ.(*Signature)
 		if len(sig.rparams) > 0 {
-			//check.dump("### recv typ = %s", x.typ)
+			// For inference to work, we must use the receiver type
+			// matching the receiver in the actual method declaration.
+			// If the method is embedded, the matching receiver is the
+			// embedded struct or interface that declared the method.
+			// Traverse the embedding to find that type (issue #44688).
+			recv := x.typ
+			for i := 0; i < len(index)-1; i++ {
+				// The embedded type is always a struct or a pointer to
+				// a struct except for the last one (which we don't need).
+				recv = asStruct(derefStructPtr(recv)).Field(index[i]).typ
+			}
+			//check.dump("### recv = %s", recv)
 			//check.dump("### method = %s rparams = %s tparams = %s", m, sig.rparams, sig.tparams)
 			// The method may have a pointer receiver, but the actually provided receiver
 			// may be a (hopefully addressable) non-pointer value, or vice versa. Here we
 			// only care about inferring receiver type parameters; to make the inference
 			// work, match up pointer-ness of receiver and argument.
-			arg := x
-			if ptrRecv := isPointer(sig.recv.typ); ptrRecv != isPointer(arg.typ) {
-				copy := *arg
+			if ptrRecv := isPointer(sig.recv.typ); ptrRecv != isPointer(recv) {
 				if ptrRecv {
-					copy.typ = NewPointer(arg.typ)
+					recv = NewPointer(recv)
 				} else {
-					copy.typ = arg.typ.(*Pointer).base
+					recv = recv.(*Pointer).base
 				}
-				arg = &copy
 			}
-			targs, failed := check.infer(sig.rparams, NewTuple(sig.recv), []*operand{arg})
+			arg := operand{mode: variable, expr: x.expr, typ: recv}
+			targs, failed := check.infer(sig.rparams, NewTuple(sig.recv), []*operand{&arg})
 			//check.dump("### inferred targs = %s", targs)
 			if failed >= 0 {
 				// We may reach here if there were other errors (see issue #40056).
 				// check.infer will report a follow-up error.
-				// TODO(gri) avoid the follow-up error or provide better explanation.
+				// TODO(gri) avoid the follow-up error as it is confusing (there's no inference in the source code)
 				goto Error
 			}
 			// Don't modify m. Instead - for now - make a copy of m and use that instead.

src/cmd/compile/internal/types2/fixedbugs/issue44688.go2

@@ -0,0 +1,83 @@
+// 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 A1[T any] struct{}
+
+func (*A1[T]) m1(T) {}
+
+type A2[T any] interface {
+	m2(T)
+}
+
+type B1[T any] struct {
+	filler int
+	*A1[T]
+	A2[T]
+}
+
+type B2[T any] interface {
+	A2[T]
+}
+
+type C[T any] struct {
+	filler1 int
+	filler2 int
+	B1[T]
+}
+
+type D[T any] struct {
+	filler1 int
+	filler2 int
+	filler3 int
+	C[T]
+}
+
+func _() {
+	// calling embedded methods
+	var b1 B1[string]
+
+	b1.A1.m1("")
+	b1.m1("")
+
+	b1.A2.m2("")
+	b1.m2("")
+
+	var b2 B2[string]
+	b2.m2("")
+
+	// a deeper nesting
+	var d D[string]
+	d.m1("")
+	d.m2("")
+
+	// calling method expressions
+	m1x := B1[string].m1
+	m1x(b1, "")
+	m2x := B2[string].m2
+	m2x(b2, "")
+
+	// calling method values
+	m1v := b1.m1
+	m1v("")
+	m2v := b1.m2
+	m2v("")
+	b2v := b2.m2
+	b2v("")
+}
+
+// actual test case from issue
+
+type A[T any] struct{}
+
+func (*A[T]) f(T) {}
+
+type B[T any] struct{ A[T] }
+
+func _() {
+	var b B[string]
+	b.A.f("")
+	b.f("")
+}

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

@@ -51,8 +51,8 @@ func (s *Selection) Kind() SelectionKind { return s.kind }
 // Recv returns the type of x in x.f.
 func (s *Selection) Recv() Type { return s.recv }
 
-// Work-around for bug where a (*instance) shows up in a final type.
-// TODO(gri):  fix this bug.
+// Work-around for a compiler issue where an (*instance) escapes.
+// TODO(gri): Is this still needed?
 func (s *Selection) TArgs() []Type {
 	r := s.recv
 	if p := asPointer(r); p != nil {