Bug Report

🔎 Search Terms

contextual, generic, inference, rest parameter, ts2345

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generic inference

⏯ Playground Link

Playground link with relevant code

💻 Code

function foo<A extends any[]>(
  arg: <T extends { a: number }>(t: T, ...rest: A) => number
) { }

foo((t, u: number) => t.a)
/* Argument of type
  '<T extends { a: number; }>(t: T, u: number) => number' is not assignable to parameter of type
  '<T extends { a: number; }>(t: T, u: number) => number'.
  🤔🤔🤔
*/

🙁 Actual behavior

The compiler infers that (t, u: number) => t.a is of type <T extends {a: number}>(t: T, u: number) => number, but then complains that this type is not assignable to itself (or at least an identically written type). The problem seems to be that the two generic T's can't be unified for some reason.

🙂 Expected behavior

The compiler should infer the function type as it's doing now, but without futher complaint.

Comes from this Stack Overflow question. I don't really quite get the use case; presumably a real world application would have more than one site where T appears, like <T extends { a: number }>(t: T, ...rest: A) => T, which has the same problem.

Is there an existing issue that mentions this? Is it intended? Limitation? Genuine bug?