gc: require if condition

R=ken2, r2
CC=golang-dev
https://golang.org/cl/4195045

src/cmd/gc/go.y

@@ -640,10 +640,15 @@ if_stmt:
 	{
 		markdcl();
 	}
-	if_header loop_body
+	if_header
+	{
+		if($3->ntest == N)
+			yyerror("missing condition in if statement");
+	}
+	loop_body
 	{
 		$$ = $3;
-		$$->nbody = $4;
+		$$->nbody = $5;
 		// no popdcl; maybe there's an LELSE
 	}
 

test/syntax/if.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.
+
+package main
+
+func main() {
+	if {  // ERROR "missing condition"
+	}
+	
+	if x(); {  // ERROR "missing condition"
+	}
+}