Array.isArray refinement loses the array's type

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

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.

Related(?): #17002

まず、リンク先の TypeScript Playground にある 1 行の例を再現し、Array.isArray による絞り込みの前後で推論される型を比較します。コンテキストを把握するため、関連する issue #17002 と既存の議論を読んでください。この例および関連するケースで、絞り込みによって配列の型情報が保持されれば完了です。