gc: export nil literals without inferred type.

Fixes #2678

R=rsc
CC=golang-dev
https://golang.org/cl/5529066

src/cmd/gc/fmt.c

@@ -1072,9 +1072,11 @@ exprfmt(Fmt *f, Node *n, int prec)
 	case OREGISTER:
 		return fmtprint(f, "%R", n->val.u.reg);
 
-	case OLITERAL:  // this is still a bit of a mess
+	case OLITERAL:  // this is a bit of a mess
 		if(fmtmode == FErr && n->sym != S)
 			return fmtprint(f, "%S", n->sym);
+		if(n->val.ctype == CTNIL)
+			n = n->orig; // if this node was a nil decorated with at type, print the original naked nil
 		if(n->type != types[n->type->etype] && n->type != idealbool && n->type != idealstring) {
 			if(isptr[n->type->etype])
 				return fmtprint(f, "(%T)(%V)", n->type, &n->val);

test/fixedbugs/bug392.dir/one.go

+package one
+
+type file int
+func (file *file) isnil() bool { return file == nil }
+func (fil *file)  isnil2() bool { return fil == nil }
+

test/fixedbugs/bug392.dir/two.go

+package two
+
+import _ "./one"
+

test/fixedbugs/bug392.go

+// $G $D/$F.dir/one.go && $G -ll $D/$F.dir/two.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 2678
+// -ll flag in command above is to force typecheck on import, needed to trigger the bug.
+// fixedbugs/bug392.dir/two.go:3: cannot call non-function *one.file (type one.file)
+
+package ignored