class MyArray<T> {
    constructor(arr: Array<T>) { };
    filter<S extends T>(callbackfn: (value: T, index: number, array: ReadonlyArray<T>) => value is S, thisArg?: any): MyArray<S> { return (null as any); };
    // Do not include fallback case! This is how MyArray differs from Array's filter type.
    // filter(callbackfn: (value: T, index: number, array: ReadonlyArray<T>) => any, thisArg?: any): T[];
}

declare function isBoolean(a: any): a is boolean;

declare const numArr: Array<number>;
const test1: Array<boolean> = numArr.filter(isBoolean);
// Here I'd expect Array<never>
// The actual result is Array<number>
// This is because the type guarded case is falling through completely

declare const myNumArray: MyArray<number>;
const test2: Array<boolean> = myNumArray.filter(isBoolean);
// When there is no such case, the error becomes clear