Why does tsc inline generic constraint?

Question

I have a library where I added this function length:

const length = <
  T extends AnyArray,
  S extends LengthComparison,
  R extends ToTuple<ElementOf<T>, ExtractLength<S>>
>(
  array: T,
  condition: S
): array is R extends T ? R : never

The whole implementation is added to this Playground

When I check it in Playground, *.d.ts will look like:

declare const length: <T extends AnyArray, S extends `>= ${number}`, R extends _ToTuple<ElementOf<T>, ExtractLength<S>, []>>(array: T, condition: S) => array is R extends T ? R : never;

However if I compile it with tsc, I will see a different picture:

declare const length: <T extends AnyArray, S extends `>= ${number}`, R extends ExtractLength<S> extends infer T_1 ? T_1 extends ExtractLength<S> ? T_1 extends never ? never : ElementOf<T>[] : never : never>(array: T, condition: S) => array is R extends T ? R : never;

So the generic constrain for R is inlined and the implementation looks definitely wrong.

How come? What am I doing wrong?

Additional information:

1. npm package – https://github.com/Beraliv/ttuple
2. Playground with ttuple – https://tsplay.dev/mpnZbw
3. Playground with completely different result – https://tsplay.dev/mAKkkw
4. Twitter thread, where I explained how I found it – https://twitter.com/beraliv/status/1553833952418439170