gc: diagnose (x) := 0

Fixes #1756.

R=ken2
CC=golang-dev
https://golang.org/cl/4810057

src/cmd/gc/dcl.c

@@ -457,17 +457,19 @@ colasname(Node *n)
 void
 colasdefn(NodeList *left, Node *defn)
 {
-	int nnew;
+	int nnew, nerr;
 	NodeList *l;
 	Node *n;
 
 	nnew = 0;
+	nerr = 0;
 	for(l=left; l; l=l->next) {
 		n = l->n;
 		if(isblank(n))
 			continue;
 		if(!colasname(n)) {
 			yyerror("non-name %#N on left side of :=", n);
+			nerr++;
 			continue;
 		}
 		if(n->sym->block == block)
@@ -480,7 +482,7 @@ colasdefn(NodeList *left, Node *defn)
 		defn->ninit = list(defn->ninit, nod(ODCL, n, N));
 		l->n = n;
 	}
-	if(nnew == 0)
+	if(nnew == 0 && nerr == 0)
 		yyerror("no new variables on left side of :=");
 }
 

src/cmd/gc/go.h

@@ -393,6 +393,7 @@ enum
 	ONOT, OCOM, OPLUS, OMINUS,
 	OOROR,
 	OPANIC, OPRINT, OPRINTN,
+	OPAREN,
 	OSEND,
 	OSLICE, OSLICEARR, OSLICESTR,
 	ORECOVER,

src/cmd/gc/go.y

@@ -916,6 +916,18 @@ pexpr:
 |	'(' expr_or_type ')'
 	{
 		$$ = $2;
+		
+		// Need to know on lhs of := whether there are ( ).
+		// Don't bother with the OPAREN in other cases:
+		// it's just a waste of memory and time.
+		switch($$->op) {
+		case ONAME:
+		case ONONAME:
+		case OPACK:
+		case OTYPE:
+		case OLITERAL:
+			$$ = nod(OPAREN, $$, N);
+		}
 	}
 
 expr_or_type:

src/cmd/gc/print.c

@@ -78,6 +78,7 @@ exprfmt(Fmt *f, Node *n, int prec)
 	case OTPAREN:
 	case OINDEX:
 	case OINDEXMAP:
+	case OPAREN:
 		nprec = 7;
 		break;
 
@@ -134,6 +135,10 @@ exprfmt(Fmt *f, Node *n, int prec)
 		fmtprint(f, "(node %O)", n->op);
 		break;
 
+	case OPAREN:
+		fmtprint(f, "(%#N)", n->left);
+		break;
+
 	case OREGISTER:
 		fmtprint(f, "%R", n->val.u.reg);
 		break;

src/cmd/gc/typecheck.c

@@ -124,9 +124,16 @@ typecheck(Node **np, int top)
 	n = *np;
 	if(n == N)
 		return N;
+	
+	lno = setlineno(n);
+
+	// Skip over parens.
+	while(n->op == OPAREN)
+		n = n->left;
 
 	// Resolve definition of name and value of iota lazily.
 	n = resolve(n);
+
 	*np = n;
 
 	// Skip typecheck if already done.

test/fixedbugs/bug351.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() {
+	(x) := 0  // ERROR "non-name [(]x[)]"
+}