arg := MySlice{1, 2, 3}
	want := MySlice{2, 4, 6}
	got := _DoubleElems[MySlice, int](arg)
	if !reflect.DeepEqual(got, want) {
		panic(fmt.Sprintf("got %s, want %s", got, want))
	}

	// constraint type inference
	got = _DoubleElems[MySlice](arg)
	if !reflect.DeepEqual(got, want) {
		panic(fmt.Sprintf("got %s, want %s", got, want))
	}

	got = _DoubleElems(arg)
	if !reflect.DeepEqual(got, want) {