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. */