const r1 = [1, 2, 3].myFilter((v) => v < 2);
// const r1: number[]
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) {
    return v.num % 2 === 1;
  }
);

/*
const r4: {
  num: number
}[]
*//

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