go/types: permit f(nil...) for variadic arguments

This code may be pointless, but it is legal.

Fixes golang/go#18268

Change-Id: Ibacae583606e1a6fdf0c0f01abe2e22e9e608393
Reviewed-on: https://go-review.googlesource.com/34194Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/call.go

@@ -250,7 +250,7 @@ func (check *Checker) argument(fun ast.Expr, sig *Signature, i int, x *operand,
 			check.errorf(ellipsis, "can only use ... with matching parameter")
 			return
 		}
-		if _, ok := x.typ.Underlying().(*Slice); !ok {
+		if _, ok := x.typ.Underlying().(*Slice); !ok && x.typ != Typ[UntypedNil] { // see issue #18268
 			check.errorf(x.pos(), "cannot use %s as parameter of type %s", x, typ)
 			return
 		}

src/go/types/testdata/builtins.src

@@ -19,6 +19,7 @@ func append1() {
 	_ = append(nil /* ERROR not a slice */ , s)
 	_ = append(x /* ERROR not a slice */ , s)
 	_ = append(s)
+	_ = append(s, nil...)
 	append /* ERROR not used */ (s)
 
 	_ = append(s, b)