Type inference on tuple intersection types broken

リンク先の TypeScript Playground から始め、交差されたタプルを含む各条件型について推論された結果を比較します。条件型の推論中にタプルの交差型がどのように処理されるか、特に先頭と末尾のパターンについて追跡します。完了条件は、一覧にある推論ケースが期待されるタプル要素を生成し、既存の動作にリグレッションを発生させないことです。

🔎 Search Terms

infer tuple intersection

🕗 Version & Regression Information

• Version 5.5.4

⏯ Playground Link

https://www.typescriptlang.org/play/?#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

💻 Code

type Target = [number, string, boolean] & { att: string; };

type A = Readonly<Target>; // wishful thinking

type Head1 = Target extends [infer H, ...infer Tail] ? H : never; // doesn't work
type Head2 = Target extends [infer H, ...any[]] ? H : never; // works

type Tail1 = Target extends [infer H, ...infer Tail] ? Tail : never; // doesn't work
type Tail2 = Target extends [any, ...infer Tail] ? Tail : never; // doesn't work

type ReverseHead1 = Target extends [...infer RTail, infer RH] ? RH : never; // doesn't work
type ReverseHead2 = Target extends [...any[], infer RH] ? RH : never; // doesn't work

type ReverseTail1 = Target extends [...infer RTail, infer RH] ? RTail : never; // doesn't work
type ReverseTail2 = Target extends [...infer RTail, any] ? RTail : never; // doesn't work

🙁 Actual behavior

Doesn't work

🙂 Expected behavior

Works

Additional information about the issue

I would expect Readonly<[number, string, boolean] & { att: string }> to be readonly [number, string, boolean] & { readonly att: string }, but that's wishful thinking. I'm sure you guys got a myriad of reasons and design limitations that don't allow it; weak types, homomorphic, variance, instantiation and whatnot.

In any case, I thought, whatever, I'll do it myself with some infers... but it looks like I won't be able to do the general case because infers break on tuple types that are intersected.

So, just letting you know in case you care... here are some unexpected behaviors ✌️