Infer Type Guard Return Values

[Gaelan]

If a function returns a type guard, either the result of an instanceofor the return value of another type guard function, it could be inferred as being a type guard itself. However, it is simply inferred as a boolean.

This change would allow the following code to work:

// The interface below should probably be merged into lib.d.ts and friends.
interface Array<T> {
    filter<O extends T>(callbackfn: (value: T, index: number, array: T[]) => value is O, thisArg?: any): O[];
}

declare class Foo { bar: number }

declare var blah: Object[];

blah.filter(object => object instanceof Foo)[0].bar // Property bar does not exist on type object

[mhegazy]

We should add the overload to the library regardless of this suggestion. @Gaelan feel free to send us PR for updating the library.

[RyanCavanaugh]

Accepting PRs for the lib.d.ts overload.

The more complex portion of inferring type guards based on return statements was declined based on complexity. It's fun and easy to write a curried version of this that works today (with the additional overload for filter):

declare class Foo { bar: number }

declare var blah: Object[];

function isInstance<T>(ctor: new(...args: any[]) => T): (x: any) => x is T {
    return <(x: any) => x is T>(x => x instanceof ctor);
} 
blah.filter(isInstance(Foo))[0].bar // Property bar does exist :)

[zpdDG4gta8XKpMCd]

can't quite see how changing libs.d.ts is able to fix this problem, mind sharing idea?