Why can't I declare a union of functions with different parameter types?

[disog]

🔎 Search Terms

merge function param union

🕗 Version & Regression Information

• This is reproducible in the playground

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAYgrgOwMYB4AqA+KBeKAKMAQwCdCBbALijQEocs0BuAWACgAbCYKAMyvmQoAzsGIBLBAHMsAH1iJUCOGQBGEYllx5CVQghB1sWQi1Y88ARhqmgA

💻 Code

type Func<T> = (param: T) => T;
let f: Func<string> | Func<number> = (a: any) => a;
f(1);

🙁 Actual behavior

The following error appears in the "Errors" tab:

Argument of type 'number' is not assignable to parameter of type 'never'.

🙂 Expected behavior

I'd expect the type of the function parameter be a union of the original types, just like it is done for the return type.

Additional information about the issue

No response

[RyanCavanaugh]

f might be a Func<string>, in which case it would receive a number.

[disog]

Fair enough. I'll close this issue then. Thanks!

[fatcerberus]

contravariance strikes again