go/types: handle import "C" more like cmd/compile

Fixes #12667.

Change-Id: I68e73e26da9938606304163ae2637e3c6bacd6f6
Reviewed-on: https://go-review.googlesource.com/14722Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/check_test.go

@@ -55,6 +55,7 @@ var tests = [][]string{
 	{"testdata/errors.src"},
 	{"testdata/importdecl0a.src", "testdata/importdecl0b.src"},
 	{"testdata/importdecl1a.src", "testdata/importdecl1b.src"},
+	{"testdata/importC.src"}, // special handling in checkFiles
 	{"testdata/cycles.src"},
 	{"testdata/cycles1.src"},
 	{"testdata/cycles2.src"},
@@ -245,6 +246,10 @@ func checkFiles(t *testing.T, testfiles []string) {
 
 	// typecheck and collect typechecker errors
 	var conf Config
+	// special case for importC.src
+	if len(testfiles) == 1 && testfiles[0] == "testdata/importC.src" {
+		conf.FakeImportC = true
+	}
 	conf.Importer = importer.Default()
 	conf.Error = func(err error) {
 		if *listErrors {

src/go/types/resolver.go

@@ -202,6 +202,11 @@ func (check *Checker) collectObjects() {
 						name := imp.name
 						if s.Name != nil {
 							name = s.Name.Name
+							if path == "C" {
+								// match cmd/compile (not prescribed by spec)
+								check.errorf(s.Name.Pos(), `cannot rename import "C"`)
+								continue
+							}
 							if name == "init" {
 								check.errorf(s.Name.Pos(), "cannot declare init - must be func")
 								continue
@@ -216,6 +221,11 @@ func (check *Checker) collectObjects() {
 							check.recordImplicit(s, obj)
 						}
 
+						if path == "C" {
+							// match cmd/compile (not prescribed by spec)
+							obj.used = true
+						}
+
 						// add import to file scope
 						if name == "." {
 							// merge imported scope with file scope

src/go/types/testdata/importC.src

+// Copyright 2015 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 importC
+
+import "C"
+import _ /* ERROR cannot rename import "C" */ "C"
+import foo /* ERROR cannot rename import "C" */ "C"
+import . /* ERROR cannot rename import "C" */ "C"