TS 5.8: False positives for conditional expression in return type check

[AlCalzone]

🔎 Search Terms

5.8, return, type

🕗 Version & Regression Information

• This changed between versions 5.7.3 and 5.8.2

⏯ Playground Link

No response

💻 Code

export type ReturnTypeOrStatic<T> = T extends (...args: any[]) => infer R ? R
	: T;

export function evalOrStatic<T>(
	fnOrConst: T,
	...args: any[]
): ReturnTypeOrStatic<T> {
	return typeof fnOrConst === "function" ? fnOrConst(...args) : fnOrConst;
}

🙁 Actual behavior

Error under fnOrConst at the end of the conditional:

Type 'T' is not assignable to type 'ReturnTypeOrStatic<T>'

🙂 Expected behavior

no error. If fnOrConst is a function, its return value (type R) is returned, otherwise it is returned directly (type T).

Additional information about the issue

No response

[Andarist]

If fnOrConst is a function, its return value (type R) is returned, otherwise it is returned directly (type T).

This is true but TS doesn't have a concept of negated types today so it can't quite reason about the "else" branch of this.

This is an expected breaking change. It was announced here. Note that your code has never worked in a similar situation (that logical-wise is actually the same):

export type ReturnTypeOrStatic<T> = T extends (...args: any[]) => infer R
  ? R
  : T;

export function evalOrStatic2<T>(
  fnOrConst: T,
  ...args: any[]
): ReturnTypeOrStatic<T> {
  if (typeof fnOrConst === "function") {
    return fnOrConst(...args);
  }
  return fnOrConst; // error
}

You can see it on the playground here