ts2556 is too restrictive when generics are involved

🔎 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:

• https://github.com/microsoft/TypeScript/issues/29919
• https://github.com/microsoft/TypeScript/issues/49802
• https://github.com/microsoft/TypeScript/issues/53541

Inizia riproducendo l’esempio generico di espansione di tuple nel TypeScript Playground collegato e confrontalo con il caso non generico che ha esito positivo. Traccia il percorso del compilatore che segnala TS2556 per le espansioni nelle liste di argomenti, quindi aggiungi test che mostrino che il caso generico vincolato viene accettato, mentre le espansioni incompatibili continuano a essere rifiutate.