cannot pass argument `& callback` of type `extern (C) int function()` to parameter `extern (C) int function()`

[Daas]

> That defines a function pointer with old K&R C-style variadic 
> functions. Try:
>
> ```C
> typedef int(*fp)(void);
> ```
>
> The conversion from K&R variadic to void parameter list should 
> probably be allowed by the compiler, and if not, at the very 
> least that uninformative error message should be improved. 
> Filed as https://github.com/dlang/dmd/issues/23177

Thank you!