cmd/compile: make case insensitive suggestions aware of package

Ensure that compiler error suggestions after case insensitive
field lookups don't mistakenly reported unexported fields if
those fields aren't in the local package being processed.

Fixes #25727

Change-Id: Icae84388c2a82c8cb539f3d43ad348f50a644caa
Reviewed-on: https://go-review.googlesource.com/117755
Run-TryBot: Emmanuel Odeke <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: odeke-em

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

@@ -919,7 +919,7 @@ func typecheck1(n *Node, top int) *Node {
 				yyerror("%v undefined (cannot refer to unexported field or method %v)", n, n.Sym)
 
 			default:
-				if mt := lookdot(n, t, 2); mt != nil { // Case-insensitive lookup.
+				if mt := lookdot(n, t, 2); mt != nil && visible(mt.Sym) { // Case-insensitive lookup.
 					yyerror("%v undefined (type %v has no field or method %v, but does have %v)", n, n.Left.Type, n.Sym, mt.Sym)
 				} else {
 					yyerror("%v undefined (type %v has no field or method %v)", n, n.Left.Type, n.Sym)
@@ -3132,7 +3132,11 @@ func typecheckcomplit(n *Node) *Node {
 				f := lookdot1(nil, l.Sym, t, t.Fields(), 0)
 				if f == nil {
 					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)
+						if visible(ci.Sym) {
+							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)
+						}
 						continue
 					}
 					p, _ := dotpath(l.Sym, t, nil, true)
@@ -3179,6 +3183,11 @@ func typecheckcomplit(n *Node) *Node {
 	return n
 }
 
+// visible reports whether sym is exported or locally defined.
+func visible(sym *types.Sym) bool {
+	return sym != nil && (types.IsExported(sym.Name) || sym.Pkg == localpkg)
+}
+
 // lvalue etc
 func islvalue(n *Node) bool {
 	switch n.Op {

test/fixedbugs/issue25727.go

@@ -0,0 +1,21 @@
+// 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
+
+import "net/http"
+
+var s = http.Server{}
+var _ = s.doneChan // ERROR "s.doneChan undefined .cannot refer to unexported field or method doneChan.$"
+var _ = s.DoneChan // ERROR "s.DoneChan undefined .type http.Server has no field or method DoneChan.$"
+var _ = http.Server{tlsConfig: nil} // ERROR "unknown field 'tlsConfig' in struct literal.+ .but does have TLSConfig.$"
+var _ = http.Server{DoneChan: nil} // ERROR "unknown field 'DoneChan' in struct literal of type http.Server$"
+
+type foo struct {
+    bar int
+}
+
+var _ = &foo{bAr: 10} // ERROR "unknown field 'bAr' in struct literal.+ .but does have bar.$"