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

Start by reproducing the generic tuple-spread example in the linked TypeScript Playground and compare it with the non-generic case that succeeds. Trace the compiler path that reports TS2556 for spreads into argument lists, then add coverage showing the constrained generic case is accepted while incompatible spreads remain rejected.