const r1 = [1, 2, 3].myFilter((v): v is number => v < 2);
const r2 = [1, 2, 3].myFilter((v, i, a): v is never => false);
const r3 = ['1', 2, '3'].myFilter((v): v is string => typeof v === 'string');
const r4 = [{ num: 1 }, { num: 2 }, { num: 3 }].myFilter(
  function(v): v is { num: number } {
    return v.num % 2 === 1;
  }
);

interface Array<T> {
  myFilter<S extends T>(callback: (v: T, i: number, a: T[]) => v is S, thisArg?: any): S[];
}