Better propagation of type predicate signatures

declare function isNumber(value: {}): value is number;
declare const values: {}[];
declare function filterAs<a, b extends a>(values: a[], isIt: (value: a) => value is b): b[];
const yayNumbers = filterAs(values, isNumber); // number[]
const ohNoNotAgain = filterAs(values, x => isNumber(x)); /* <-- expected to work,
actual: Argument of type '(x: {}) => boolean' is not assignable to parameter of type '(value: {}) => value is {}'.
        Signature '(x: {}): boolean' must have a type predicate. */

[gcnew]

@aleksey-bykov Implementing features that are useful but not entirely thorough attracts a lot of criticism and is oftentimes even questionable. e.g. https://github.com/Microsoft/TypeScript/issues/13002

[ascott18]

Another use case here that seems like it should work but doesn’t - direct type assertions in a predicate (as opposed to a type assertion being done by a subroutine of a predicate):

Playground

class Extension { }
class ImageExtension extends Extension {
  imageUrl: string | null = null;
}

const extensions: Extension[] = [];
extensions.push(new ImageExtension());

// Doesn't work
const imageExtension: ImageExtension | undefined = extensions.find(e => e instanceof ImageExtension);

// Does work, but is unnecessarily verbose
const imageExtension2: ImageExtension | undefined = extensions.find((e): e is ImageExtension => e instanceof ImageExtension);