Allow type narrowing for arrays using Array#forEach and assertion function

lib Update Request

Configuration Check

My compilation target is ES5 and my lib is the default.

Missing / Incorrect Definition

Array type narrowing using Array#forEach with assertion function, in a fashion similar to Array#every, has not been defined. Also applies to ReadonlyArray.

Sample Code

const foo: (number | string)[] = ['aaa'];

function assertString(x: unknown): asserts x is string {
  if (typeof x !== 'string') throw new Error('Must be a string!');
}

foo.forEach(assertString);
foo[0].slice(0); // foo should be string[] from here

Documentation Link

Based on Array#every<S extends T> from lib.es5.d.ts.

A pull request for implementing this definition has been created #52388.

Start in lib/lib.es5.d.ts, using the existing Array#every definition as the reference. Check the Array and ReadonlyArray forEach declarations against the provided assertion-function example. Done means the example narrows the array element type after forEach without errors and both mutable and readonly arrays are covered.