gc: double-check usage of ...

Fixes #423.

R=ken2
https://golang.org/cl/180045

src/cmd/gc/dcl.c

@@ -961,7 +961,7 @@ findtype(NodeList *l)
 }
 
 NodeList*
-checkarglist(NodeList *all)
+checkarglist(NodeList *all, int input)
 {
 	int named;
 	Node *n, *t, *nextt;
@@ -1015,8 +1015,12 @@ checkarglist(NodeList *all)
 		if(n != N)
 			n = newname(n->sym);
 		n = nod(ODCLFIELD, n, t);
-		if(l->next != nil && n->right != N && n->right->op == OTYPE && isddd(n->right->type))
-			yyerror("only last argument can have type ...");
+		if(n->right != N && n->right->op == OTYPE && isddd(n->right->type)) {
+			if(!input)
+				yyerror("cannot use ... in output argument list");
+			else if(l->next != nil)
+				yyerror("can only use ... as final argument in list");
+		}
 		l->n = n;
 	}
 	return all;

src/cmd/gc/go.h

@@ -955,7 +955,7 @@ Type*	newtype(Sym*);
 Type*	oldtype(Sym*);
 void	fninit(NodeList*);
 Node*	nametodcl(Node*, Type*);
-NodeList*	checkarglist(NodeList*);
+NodeList*	checkarglist(NodeList*, int);
 void	checkwidth(Type*);
 void	defercheckwidth(void);
 void	resumecheckwidth(void);

src/cmd/gc/go.y

@@ -1070,6 +1070,7 @@ fndcl:
 	{
 		Node *n;
 
+		$3 = checkarglist($3, 1);
 		$$ = nod(ODCLFUNC, N, N);
 		$$->nname = $1;
 		if($3 == nil && $5 == nil)
@@ -1085,6 +1086,8 @@ fndcl:
 	{
 		Node *rcvr, *t;
 
+		$2 = checkarglist($2, 0);
+		$6 = checkarglist($6, 1);
 		$$ = N;
 		if($2 == nil) {
 			yyerror("method has no receiver");
@@ -1113,6 +1116,7 @@ fndcl:
 fntype:
 	LFUNC '(' oarg_type_list_ocomma ')' fnres
 	{
+		$3 = checkarglist($3, 1);
 		$$ = nod(OTFUNC, N, N);
 		$$->list = $3;
 		$$->rlist = $5;
@@ -1140,6 +1144,7 @@ fnres:
 	}
 |	'(' oarg_type_list_ocomma ')'
 	{
+		$2 = checkarglist($2, 0);
 		$$ = $2;
 	}
 
@@ -1280,6 +1285,7 @@ indcl:
 	'(' oarg_type_list_ocomma ')' fnres
 	{
 		// without func keyword
+		$2 = checkarglist($2, 0);
 		$$ = nod(OTFUNC, fakethis(), N);
 		$$->list = $2;
 		$$->rlist = $4;
@@ -1320,7 +1326,7 @@ oarg_type_list_ocomma:
 	}
 |	arg_type_list ocomma
 	{
-		$$ = checkarglist($1);
+		$$ = $1;
 	}
 
 /*

test/fixedbugs/bug228.go

+// errchk $G -e $D/$F.go
+
+// Copyright 2009 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 f(x int, y ...)	// ok
+
+func g(x int, y float) (...)	// ERROR "[.][.][.]"
+
+func h(x, y ...)		// ERROR "[.][.][.]"
+
+func i(x int, y ..., z float)	// ERROR "[.][.][.]"
+
+var x ...;		// ERROR "[.][.][.]|syntax"
+
+type T ...;		// ERROR "[.][.][.]|syntax"