Error $index pada vue js

<li v-for="todo in todos">
        <span v-bind:class="{'completed' : todo.done}">{{ todo.text }}</span>
        <button v-on:click="removeTodo($index)"> Hapus </button>
        <button v-on:click="toggleC(todo)">Done</button>
      </li>

pesan error : Property or method "$index" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See

dan yang saya baca di dokumentasi nya kalo untuk $index udh gabisa di gunakan di versi 2.xx jadi untuk pengganti property $index apa ya?