cmd/compile: better error message when referring to ambiguous method/field

Fixes #14321.

Change-Id: I9c92c767b01cf7938c4808a8fef9f2936fc667bc
Reviewed-on: https://go-review.googlesource.com/106119Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -2359,6 +2359,8 @@ func typecheckMethodExpr(n *Node) *Node {
 	if m == nil {
 		if lookdot1(n, s, t, ms, 1) != nil {
 			yyerror("%v undefined (cannot refer to unexported method %v)", n, s)
+		} else if _, ambig := dotpath(s, t, nil, false); ambig {
+			yyerror("%v undefined (ambiguous selector)", n) // method or field
 		} else {
 			yyerror("%v undefined (type %v has no method %v)", n, t, s)
 		}

test/fixedbugs/issue14321.go

+// errorcheck
+
+// Copyright 2018 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.
+
+// Verify that error message reports _ambiguous_ method.
+
+package p
+
+type A struct{
+	H int
+}
+
+func (A) F() {}
+func (A) G() {}
+
+type B struct{
+	G int
+	H int
+}
+
+func (B) F() {}
+
+type C struct {
+	A
+	B
+}
+
+var _ = C.F // ERROR "ambiguous selector"
+var _ = C.G // ERROR "ambiguous selector"
+var _ = C.H // ERROR "ambiguous selector"
+var _ = C.I // ERROR "no method I"