Type inference on tuple intersection types broken

🔎 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 ✌️

Start with the linked TypeScript Playground and compare the inferred results for each conditional type involving the intersected tuple. Trace how tuple intersection types are handled during conditional type inference, especially for head and tail patterns. Done means the listed inference cases produce the expected tuple elements without regressing existing behavior.