cmd/compile: fix wrong typeparams for selector expr with embedded generic type

For selector expression "x.M" where "M" is a promoted method, irgen is using
the type of receiver "x" for determining the typeparams for instantiation.
However, because M is a promoted method, so its associated receiver is
not "x", but "x.T" where "T" is the embedded field of "x". That casues a
mismatch when converting non-shape types arguments.

Fixing it by using the actual receiver which has the method, instead of
using the base receiver.

Fixes #53982

Change-Id: I1836fc422d734df14e9e6664d4bd014503960bfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/419294
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -334,10 +334,6 @@ func (g *genInst) buildClosure(outer *ir.Func, x ir.Node) ir.Node {
 	} else { // ir.OMETHEXPR or ir.METHVALUE
 		// Method expression T.M where T is a generic type.
 		se := x.(*ir.SelectorExpr)
-		targs := deref(se.X.Type()).RParams()
-		if len(targs) == 0 {
-			panic("bad")
-		}
 		if x.Op() == ir.OMETHVALUE {
 			rcvrValue = se.X
 		}
@@ -348,7 +344,8 @@ func (g *genInst) buildClosure(outer *ir.Func, x ir.Node) ir.Node {
 		// of se.Selection, since that will be the type that actually has
 		// the method.
 		recv := deref(se.Selection.Type.Recv().Type)
-		if len(recv.RParams()) == 0 {
+		targs := recv.RParams()
+		if len(targs) == 0 {
 			// The embedded type that actually has the method is not
 			// actually generic, so no need to build a closure.
 			return x

test/fixedbugs/issue53982.go

@@ -0,0 +1,25 @@
+// build
+
+// Copyright 2022 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 main
+
+type S[K, V any] struct {
+	E[V]
+}
+
+type E[K any] struct{}
+
+func (e E[K]) M() E[K] {
+	return e
+}
+
+func G[K, V any](V) {
+	_ = (*S[K, V]).M
+}
+
+func main() {
+	G[*int](new(int))
+}