Default generic types are unused in ReturnType

[ghost]

Bug Report

🔎 Search Terms

generic, default, unknown, infer

#48480 looked similar, but it was talking about the type of a function parameter's default value not being inferred, whereas this ticket is about a generic parameter with a default type not being used by infer.

🕗 Version & Regression Information

I tested against v4.4.4, 4.6.3, and nightly.

⏯ Playground Link

Playground link

💻 Code

function myFunc<A = number, B = string>(a: A, b: B) {
  return [a, b] as const
}

type myFuncType = typeof myFunc // <A = number, B = string>(a: A, b: B) => readonly [A, B] 

type myFuncReturnType = ReturnType< myFuncType > // readonly [unknown, unknown]

🙁 Actual vs Expected behaviour

I expect the type of myFuncReturnType to be readonly [number, string], but it's actually readonly [unknown, unknown], because the default generic type was not used.

[MartinJohns]

This is working as intended. See #42064 (comment).

[Harpush]

I believe typescript 4.7 will solve it with the new Instantiation expressions?
#47607

[typescript-bot]

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