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

まず、リンク先の TypeScript Playground にあるジェネリックなタプルスプレッドの例を再現し、成功する非ジェネリックの場合と比較します。引数リストへのスプレッドに対して TS2556 を報告するコンパイラの経路を追跡し、制約付きジェネリックの場合が受け入れられる一方で、互換性のないスプレッドは引き続き拒否されることを示すテストを追加します。