cmd/gc: properly set variadic flag on method values.

Fixes #5231.

R=golang-dev, daniel.morsing, adg
CC=golang-dev
https://golang.org/cl/8275044

src/cmd/gc/closure.c

@@ -280,12 +280,12 @@ typecheckpartialcall(Node *fn, Node *sym)
 static Node*
 makepartialcall(Node *fn, Type *t0, Node *meth)
 {
-	Node *ptr, *n, *call, *xtype, *xfunc, *cv;
+	Node *ptr, *n, *fld, *call, *xtype, *xfunc, *cv;
 	Type *rcvrtype, *basetype, *t;
 	NodeList *body, *l, *callargs, *retargs;
 	char *p;
 	Sym *sym;
-	int i;
+	int i, ddd;
 
 	// TODO: names are not right
 	rcvrtype = fn->left->type;
@@ -309,6 +309,7 @@ makepartialcall(Node *fn, Type *t0, Node *meth)
 	i = 0;
 	l = nil;
 	callargs = nil;
+	ddd = 0;
 	xfunc = nod(ODCLFUNC, N, N);
 	for(t = getinargx(t0)->type; t; t = t->down) {
 		snprint(namebuf, sizeof namebuf, "a%d", i++);
@@ -316,7 +317,12 @@ makepartialcall(Node *fn, Type *t0, Node *meth)
 		n->class = PPARAM;
 		xfunc->dcl = list(xfunc->dcl, n);
 		callargs = list(callargs, n);
-		l = list(l, nod(ODCLFIELD, n, typenod(t->type)));
+		fld = nod(ODCLFIELD, n, typenod(t->type));
+		if(t->isddd) {
+			fld->isddd = 1;
+			ddd = 1;
+		}
+		l = list(l, fld);
 	}
 	xtype->list = l;
 	i = 0;
@@ -362,6 +368,7 @@ makepartialcall(Node *fn, Type *t0, Node *meth)
 
 	call = nod(OCALL, nod(OXDOT, ptr, meth), N);
 	call->list = callargs;
+	call->isddd = ddd;
 	if(t0->outtuple == 0) {
 		body = list(body, call);
 	} else {

test/fixedbugs/issue5231.go

+// compile
+
+// Copyright 2013 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 5231: method values lose their variadic property.
+
+package p
+
+type T int
+
+func (t T) NotVariadic(s []int) int {
+	return int(t) + s[0]
+}
+
+func (t T) Variadic(s ...int) int {
+	return int(t) + s[0]
+}
+
+type I interface {
+	NotVariadic(s []int) int
+	Variadic(s ...int) int
+}
+
+func F() {
+	var t T
+	var p *T = &t
+	var i I = p
+
+	nv := t.NotVariadic
+	nv = p.NotVariadic
+	nv = i.NotVariadic
+	var s int = nv([]int{1, 2, 3})
+
+	v := t.Variadic
+	v = p.Variadic
+	v = i.Variadic
+	s = v(1, 2, 3)
+
+	var f1 func([]int) int = nv
+	var f2 func(...int) int = v
+
+	_, _, _ = f1, f2, s
+}