gc: handle use of builtin function outside function call

tweaks & tests of last bug fix too.

R=ken2
CC=golang-dev
https://golang.org/cl/1207044

src/cmd/gc/typecheck.c

@@ -89,6 +89,10 @@ typecheck(Node **np, int top)
 redo:
 	lno = setlineno(n);
 	if(n->sym) {
+		if(n->op == ONAME && n->etype != 0) {
+			yyerror("use of builtin %S not in function call", n->sym);
+			goto error;
+		}
 		walkdef(n);
 		if(n->op == ONONAME)
 			goto error;

src/cmd/gc/walk.c

@@ -221,7 +221,9 @@ walkdef(Node *n)
 	if(n->op == ONONAME) {
 		if(!n->diag) {
 			n->diag = 1;
-			yyerrorl(n->lineno, "undefined: %S", n->sym);
+			if(n->lineno != 0)
+				lineno = n->lineno;
+			yyerror("undefined: %S", n->sym);
 		}
 		return;
 	}

test/fixedbugs/bug208.go

@@ -11,10 +11,10 @@ type	T struct
 	f int;
 }
 
-var _ = T{f: 1}
-
 // 6g used to get confused by the f:1 above
 // and allow uses of f that would be silently
 // dropped during the compilation.
 var _ = f;	// ERROR "undefined"
 
+var _ = T{f: 1}
+

test/varerr.go

+// errchk $G $D/$F.go
+
+// Copyright 2010 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
+
+func main() {
+	_ = asdf	// ERROR "undefined: asdf"
+
+	new = 1	// ERROR "use of builtin new not in function call"
+}
+