gc: disallow declaration of variables outside package.

Fixes #2231.

Declaring main.i in package main in the same way already triggers syntax errors.

R=rsc
CC=golang-dev
https://golang.org/cl/5483078

src/cmd/gc/dcl.c

@@ -175,6 +175,11 @@ declare(Node *n, int ctxt)
 
 	n->lineno = parserline();
 	s = n->sym;
+
+	// kludgy: typecheckok means we're past parsing.  Eg genwrapper may declare out of package names later.
+	if(importpkg == nil && !typecheckok && s->pkg != localpkg)
+		yyerror("cannot declare name %S", s);
+
 	gen = 0;
 	if(ctxt == PEXTERN) {
 		externdcl = list(externdcl, n);

test/fixedbugs/bug388.go

+// errchk $G $D/$F.go
+
+// Copyright 2011 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.
+
+// Issue 2231
+
+package main
+import "runtime"
+
+func foo(runtime.UintType, i int) {  // ERROR "cannot declare name runtime.UintType"
+	println(i, runtime.UintType) 
+}
+
+func bar(i int) {
+	runtime.UintType := i       // ERROR "cannot declare name runtime.UintType"
+	println(runtime.UintType)
+}
+
+func baz() {
+	main.i := 1	// ERROR "non-name main.i"
+	println(main.i)
+}
+
+func qux() {
+	var main.i	// ERROR "unexpected [.]"
+	println(main.i)
+}
+
+func corge() {
+	var foo.i int  // ERROR "unexpected [.]"
+	println(foo.i)
+}
+
+func main() {
+	foo(42,43)
+	bar(1969)
+}