cmd/compile: lookup methods of base type for named pointer type

Passed toolstash-check.

Updates #21738
Fixes #21934

Change-Id: I59f0b2c9890146565ff913b04aeeeff7dc7a4499
Reviewed-on: https://go-review.googlesource.com/c/go/+/197561
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/subr.go

@@ -1189,7 +1189,12 @@ func lookdot0(s *types.Sym, t *types.Type, save **types.Field, ignorecase bool)
 		}
 	}
 
-	u = methtype(t)
+	u = t
+	if t.Sym != nil && t.IsPtr() && !t.Elem().IsPtr() {
+		// If t is a defined pointer type, then x.m is shorthand for (*x).m.
+		u = t.Elem()
+	}
+	u = methtype(u)
 	if u != nil {
 		for _, f := range u.Methods().Slice() {
 			if f.Embedded == 0 && (f.Sym == s || (ignorecase && strings.EqualFold(f.Sym.Name, s.Name))) {

test/fixedbugs/issue21934.go

@@ -0,0 +1,26 @@
+// errorcheck
+
+// Copyright 2019 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.
+
+// selector expression resolves incorrectly for defined
+// pointer types.
+
+package main
+
+type E struct{ f int }
+type T struct{ E }
+
+func (*T) f() int { return 0 }
+
+type P *T
+type PP **T
+
+func main() {
+	var x P
+	_ = x.f // ERROR "x\.f undefined \(type P has no field or method f\)"
+
+	var y PP
+	_ = y.f // ERROR "y\.f undefined \(type PP has no field or method f\)"
+}