Manipulated tuple type not allowed to be spread/not recognized as iterable

[majellin24]

Bug Report

🔎 Search Terms

tuple type spread, symbol iterator spread

🕗 Version & Regression Information

• This changed between versions 4.6.4 and 4.7.4

⏯ Playground Link

Playground link

💻 Code

type Mapped<T extends any[]> = { [ P in keyof T ] : T[P] }

function fn<T extends (...args: any[]) => any>(cb: T, ...rest: Mapped<Parameters<T>>): ReturnType<T> {
  return cb(...rest);
  //           ^?
}

🙁 Actual behavior

The code failed to compile with the error

---

Type 'Mapped<Parameters>' must have a 'Symbol.iterator' method that returns an iterator.

---

🙂 Expected behavior

Prior to 4.7.4 this compiled. I included the types "paramsOfTest" and "paramsOfNone" to demonstrate that the resulting types do evaluate to tuples, even without any parameters. It seems to me that the type should always evaluate to an iterable tuple since the "Parameters" type should evaluate to a tuple and the types ReplaceIndexWithUndefined and FilterUndefined should also evaluate to iterable types due to their type constraints.

Edited to use the simpler repro from comments to simplify.

Given that this was working previously and I was not able to find more information I assumed this was a bug but I apologize if I have missed something.

[whzx5byb]

A simpler repo:

type Mapped<T extends any[]> = { [ P in keyof T ] : T[P] }

function fn<T extends (...args: any[]) => any>(cb: T, ...rest: Mapped<Parameters<T>>): ReturnType<T> {
  return cb(...rest); //<- unexpected error here. works in 4.6.4
}

Note that it works when remove the constraint T extends any[] in Mapped<T extends any[]>.

[Andarist]

Likely a duplicate of #29919

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.