#47069 breaks narrowing in @types/has and @types/lodash

[sandersn]

declare function hasOwnProperty<P extends PropertyKey>(target: {}, property: P): target is { [K in P]: unknown };
declare const arrayLikeOrIterable: ArrayLike<any> | Iterable<any>;
if (hasOwnProperty(arrayLikeOrIterable, 'length')) {
    arrayLikeOrIterable; // $ExpectType ArrayLike<any>
}

Expected: arrayLikeOrIterable narrows to ArrayLike<any>
Actual: It doesn't, so instead the compiler intersects the declared type with { length: unknown }.

Broken by #47069

[sandersn]

Also breaks lodash like so:

interface LoDashStatic {
  isArrayLike<T extends { __lodashAnyHack: any }>(t: T): boolean;
  isArrayLike(value: ((...args: any[]) => any) | null | undefined): value is never;
  isArrayLike(value: any): value is { length: number };
}
declare var _: LoDashStatic

declare const anything: any;
const value: string | string[] | { [index: number]: boolean, length: number } | [number, boolean]
  | number | { length: string } | { a: string } | null | undefined
  = anything;

if (_.isArrayLike(value)) {
  const result: string | string[] | { [index: number]: boolean, length: number } | [number, boolean] = value;
} else {
  const result: number | { length: string } | { a: string; } | null | undefined = value;
}

[weswigham]

Relying on subtyping with readonly properties in the wrong direction? :(

How about if you slap a readonly in that mapped type in the OP?

Only "fix" I can think of would be swapping user-defined-typeguard narrowing to use not-subtype because subtype reduction demands ordering between two non-identical types. I wouldn't be happy about it though - if we did it, I think it should only be behind a compat flag, since, while it just didn't matter before, these kinds of guards really should be looking for read-only props if they expect to match them (which becomes extra important with how we have distinct setter types now, too).

[jakebailey]

This seems to be blocking changes to the affected libs; DT runs 4.7 in CI so PRs that modify these stubs fail, e.g. DefinitelyTyped/DefinitelyTyped#58468 which is to revert a bad change in lodash.