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Source provides no match for required element at position 0 in target despite source extending [any, ...any[]] #48401
Description
Activity
Simple workaround:
...tail: [infer TH, ...infer TT]andRecursiveType<[TH, ...TT]>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;
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 caseYes, that would have definitely helped in several places. I look forward to it being merged. Thanks
Reacted by Noman GulIs 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[]] | []) => {}Reacted by Matt Oskamp, Christian Rishøj and Jacob Eggers
Bug Report
🔎 Search Terms
tuple, conditional type
🕗 Version & Regression Information
4.6.2
⏯ Playground Link
Playground link with relevant code
💻 Code
🙁 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
Tailback intoRecursiveTypeas we are ensuring thatTailextends[any, ...any[]]. I have tried this a few different ways, by re-inferring the type of the tail and by switching theTailand[any, ...any[]]around so we check that[any, ...any[]]extendsTailinstead.