cmd/gc: reject use of ... with multiple-valued expressions.

Fixes #3334.

R=golang-dev, r
CC=golang-dev, remy
https://golang.org/cl/6350103

src/cmd/gc/typecheck.c

@@ -929,7 +929,7 @@ reswitch:
 			goto doconv;
 		}
 
-		if(count(n->list) == 1)
+		if(count(n->list) == 1 && !n->isddd)
 			typecheck(&n->list->n, Erv | Efnstruct);
 		else
 			typechecklist(n->list, Erv);

test/ddd1.go

@@ -22,6 +22,16 @@ var (
 	_ = sum([]int{1}) // ERROR "\[\]int literal.*as type int|incompatible"
 )
 
+func sum3(int, int, int) int { return 0 }
+func tuple() (int, int, int) { return 1, 2, 3 }
+
+var (
+	_ = sum(tuple())
+	_ = sum(tuple()...) // ERROR "multiple-value"
+	_ = sum3(tuple())
+	_ = sum3(tuple()...) // ERROR "multiple-value" "not enough"
+)
+
 type T []T
 
 func funny(args ...T) int { return 0 }