Difference in parameter inference between arrow functions and classic functions

In the example code below the arrow function has correct type inference while the function example is unable to infer the parameter type and falls back to any.

declare function monitor<T extends (...args: any) => any>(
  extractor: (...args: Parameters<T>) => Record<string, unknown>,
  executor: T,
): (...args: Parameters<T>) => ReturnType<T>;

monitor((p) => ({ p }), (p: number) => p);
monitor((p) => ({ p }), function (p: number) { return p; });

Note that if we specify type of this (this: void) the function variant is also able to infer the type:

Duplicate #47599

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Difference in parameter inference between arrow functions and classic functions #50258

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Difference in parameter inference between arrow functions and classic functions #50258

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knutkj commented on Aug 10, 2022

RyanCavanaugh commented on Aug 10, 2022

typescript-bot commented on Aug 13, 2022

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