go/types: use correct recv for parameterized embedded methods

This is a direct port of CL 298129 to go/types.

Fixes #44688

Change-Id: I950992ea7beea5b9c8bea0c296b5ce03b2aa9b12
Reviewed-on: https://go-review.googlesource.com/c/go/+/298349
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/call.go

@@ -592,31 +592,42 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 	// methods may not have a fully set up signature yet
 	if m, _ := obj.(*Func); m != nil {
 		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 in
+		// the signature accordingly.
 		// TODO(gri) factor this code out
 		sig := m.typ.(*Signature)
 		if len(sig.rparams) > 0 {
+			// 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 either 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
+			}
+
 			// 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})
 			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/go/types/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("")
+}