The related issue is vuejs/language-tools#591.
Execute
npm install
npm run typecheck
and then you get the following error message
src/test.vue:5:29 - error TS2345: Argument of type 'number | keyof Data[]' is not assignable to parameter of type 'number'.
Type 'string' is not assignable to type 'number'.
5 <div>{{ formatIndex(index) }}</div>
~~~~~