Knockout.mapping tests rely on the old signature relation rules. The repro is fairly small:

interface Kow<T> {
    (value: T): void;
}
declare var src: Kow<true> | Kow<false>;
var tgt: Kow<boolean> = src;

Expected: src assignable to target.
Actual: Error, type 'boolean' is not assignable to type 'false'.