ts2556 is too restrictive when generics are involved

[mkantor]

🔎 Search Terms

"generic", "tuple", "spread", "rest", "ts2556", "a spread argument must either have a tuple type or be passed to a rest parameter"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about spreading into argument lists

⏯ Playground Link

https://www.typescriptlang.org/play?#code/CYUwxgNghgTiAEYD2A7AzgF3gMwFzwAoBKeAXgD54A3JAS2AFgAoZ0SWBZdLAD3zijBUEAJ7wA2gF1m2AgDoFPEgHpl8AO5IYAazTNmXTPAwgjpeAB4AKvBA8TKYGngChKURMnkCfeFZIUzPA48ooqakKm8ChIWJo6QA

💻 Code

declare const f: () => void

declare const x: readonly []
f(...x) // works

const test = <T extends readonly []>(x: T) =>
  f(...x) // does not work

🙁 Actual behavior

Attempting to spread a generically-typed value whose constraint guarantees tuple-ness into a compatible argument list is forbidden.

🙂 Expected behavior

I should be allowed to do this.

Additional information about the issue

Possibly-related issues:

• When trying to use mapped tuples as rest parameters error 'A rest parameter must be of an array type' given #29919
• Spread operator should not have worse ergonomics than apply - unexpected error spreading a union-of-tuples #49802
• Using Parameters in a class member function gets incorrect result while all parameters are optional.  #53541

[Andarist]

It's likely that this one would get fixed by #53615

[KaffinPX]

Thanks a lot @mkantor ;)