Source provides no match for required element at position 0 in target despite source extending [any, ...any[]]

[Roaders]

Bug Report

🔎 Search Terms

tuple, conditional type

🕗 Version & Regression Information

4.6.2

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Conditional Types

⏯ Playground Link

Playground link with relevant code

💻 Code

type RecursiveType<TList extends [any, ...any[]]> = ((
    ...list: TList
) => unknown) extends (h: infer Head, ...tail: infer Tail) => unknown
    ? ((h: Head) => unknown) extends (h: Array<infer R>) => unknown
        ? R
        : ((h: Head, ...tail: Tail) => unknown) extends (
              h: any,
              ...tail: [any, ...any[]] // ensure that Tail extends [any, ...any[]]
          ) => unknown
        ? RecursiveType<Tail> // Error: Source provides no match for required element at position 0 in target
        : unknown
    : unknown;

🙁 Actual behavior

Comile error:
Error: Source provides no match for required element at position 0 in target

🙂 Expected behavior

We should be able to pass Tail back into RecursiveType as we are ensuring that Tail extends [any, ...any[]]. I have tried this a few different ways, by re-inferring the type of the tail and by switching the Tail and [any, ...any[]] around so we check that [any, ...any[]] extends Tail instead.

[MartinJohns]

Simple workaround: ...tail: [infer TH, ...infer TT] and RecursiveType<[TH, ...TT]>

[Roaders]

yes, it does. Thanks very much for the workaround. It doesn't work when I specify the type of the items in the tuple though:

type RecursiveType<TList extends [Item, ...Item[]]> = ((
    ...list: TList
) => unknown) extends (h: infer Head, ...tail: infer Tail) => unknown
    ? ((h: Head) => unknown) extends (h: Array<infer R>) => unknown
        ? R
        : ((h: Head, ...tail: [Item, ...Item[]]) => unknown) extends (
              h: any,
              ...tail: [infer TH, ...infer TT] // ensure that Tail extends [Item, ...Item[]]
          ) => unknown
        ? RecursiveType<[TH, ...TT]> // Error: Type 'TH' is not assignable to type 'Item'
        : unknown
    : unknown;

Playground

[RyanCavanaugh]

We don't have a way to indirectly propagate the facts of the parent constraint into the infer result in a way that is still generic but allows it to directly pass constraint checks. #48112 should help with this use case

[Roaders]

Yes, that would have definitely helped in several places. I look forward to it being merged. Thanks

[50an6xy06r6n]

Is this why T[] is not assignable to [T, ...T[]] | []?

That's a degenerate case, but I have some use for things like this:
const f = <T,>(p: readonly [G<T>, ...G[]] | []) => {}