cmd/compile: improve compiler error on embedded structs

Fixes #23609

Change-Id: I751aae3d849de7fce1306324fcb1a4c3842d873e
Reviewed-on: https://go-review.googlesource.com/97076Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -3114,7 +3114,18 @@ func typecheckcomplit(n *Node) *Node {
 					if ci := lookdot1(nil, l.Sym, t, t.Fields(), 2); ci != nil { // Case-insensitive lookup.
 						yyerror("unknown field '%v' in struct literal of type %v (but does have %v)", l.Sym, t, ci.Sym)
 					} else {
-						yyerror("unknown field '%v' in struct literal of type %v", l.Sym, t)
+						p, _ := dotpath(l.Sym, t, nil, true)
+						if p == nil {
+							yyerror("unknown field '%v' in struct literal of type %v", l.Sym, t)
+							continue
+						}
+						// dotpath returns the parent embedded types in reverse order.
+						var ep []string
+						for ei := len(p) - 1; ei >= 0; ei-- {
+							ep = append(ep, p[ei].field.Type.Sym.Name)
+						}
+						ep = append(ep, l.Sym.Name)
+						yyerror("cannot use promoted field %v in struct literal of type %v", strings.Join(ep, "."), t)
 					}
 					continue
 				}

test/fixedbugs/issue23609.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.
+
+package main
+
+type t1 struct {
+	t1f1 int
+	t1f2 int
+}
+type t2 struct {
+	t2f1 int
+	t2f2 int
+	t1
+}
+type t3 struct {
+	t3f1 int
+	t2
+}
+
+var (
+	_ = t2{t1f1: 600} // ERROR "cannot use promoted field t1.t1f1 in struct literal of type t2"
+	_ = t3{t1f2: 800} // ERROR "cannot use promoted field t2.t1.t1f2 in struct literal of type t3"
+	_ = t3{t2f1: 900} // ERROR "cannot use promoted field t2.t2f1 in struct literal of type t3"
+)