I realize there are a number of related issues. However, some where about ReadonlyArray, some didn't see directly related at all even though it was pointed there.

Also, this is a much smaller example, just a single line of code really:

Playground link

function demo(it: Iterable<number>): number[] {
    // THIS LINE: "array" is any[]
    const array = Array.isArray(it) ? it : [...it];
    // Deliberate: n.length is a string method, but no error because of type "any" for n
    return array.map(n => n.length);
}

I know people will point to this ir that implementation or design decision detail — honestly guys, you got it REVERSED. The tool should serve the purpose, not the other way around!

That the array loses all it's type information in such a simple example is A BUG.

And it is possible to do much better: See here! I'm only pointing this out because at times the responses sound like "We tried everything but it is just not possible" when the competition shows that it is actually quite possible.