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Source provides no match for required element at position 0 in target despite source extending [any, ...any[]] #48401
Description
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.
Activity
MartinJohns commentedon Mar 24, 2022
Simple workaround:
...tail: [infer TH, ...infer TT]andRecursiveType<[TH, ...TT]>Roaders commentedon Mar 24, 2022
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:
Playground
RyanCavanaugh commentedon Mar 28, 2022
We don't have a way to indirectly propagate the facts of the parent constraint into the
inferresult in a way that is still generic but allows it to directly pass constraint checks. #48112 should help with this use caseRoaders commentedon Mar 29, 2022
Yes, that would have definitely helped in several places. I look forward to it being merged. Thanks
50an6xy06r6n commentedon Aug 4, 2023
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[]] | []) => {}