gc: fix return variable named _

Fixes #1712.

R=ken2
CC=golang-dev
https://golang.org/cl/4445055

src/cmd/gc/dcl.c

@@ -560,6 +560,7 @@ funcargs(Node *nt)
 {
 	Node *n;
 	NodeList *l;
+	int gen;
 
 	if(nt->op != OTFUNC)
 		fatal("funcargs %O", nt->op);
@@ -589,6 +590,7 @@ funcargs(Node *nt)
 	}
 
 	// declare the out arguments.
+	gen = 0;
 	for(l=nt->rlist; l; l=l->next) {
 		n = l->n;
 		if(n->op != ODCLFIELD)
@@ -596,6 +598,11 @@ funcargs(Node *nt)
 		if(n->left != N) {
 			n->left->op = ONAME;
 			n->left->ntype = n->right;
+			if(isblank(n->left)) {
+				// Give it a name so we can assign to it during return.
+				snprint(namebuf, sizeof(namebuf), ".anon%d", gen++);
+				n->left->sym = lookup(namebuf);
+			}
 			declare(n->left, PPARAMOUT);
 		}
 	}

test/fixedbugs/bug331.go

+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: bug331
+
+// 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
+
+import "os"
+
+func f() (_ string, x float64, err os.Error) {
+	return
+}
+
+func g() (_ string, x float64, err os.Error) {
+	return "hello", 3.14, os.EOF
+}
+
+var _ func() (string, float64, os.Error) = f
+var _ func() (string, float64, os.Error) = g
+
+func main() {
+	x, y, z := g()
+	if x != "hello" || y != 3.14 || z != os.EOF {
+		println("wrong", x, len(x), y, z)
+	}
+}
+
+/*
+issue 1712
+
+bug331.go:12: cannot use "hello" (type string) as type float64 in assignment
+bug331.go:12: cannot use 0 (type float64) as type os.Error in assignment:
+	float64 does not implement os.Error (missing String method)
+bug331.go:12: error in shape across RETURN
+*/