"Concepts" should not be part of function headers

[Paul Backus]

On Wednesday, 22 October 2025 at 14:40:58 UTC, H. S. Teoh wrote:
> The user-facing function signature should describe the 
> *logical* API, rather than implementation details such as "if 
> input is a float, if input is an enum, ..." ad nauseaum.  
> Logically, std.conv.to is supposed to accept *all* types (even 
> if the current implementation may or may not accept 
> *everything*), so the function signature should be:
>
>     T to(T,U)(U data) { ... }
>
> *without* any signature constraints.  All the stuff like "if U 
> is floating-point, if U is enum, if U is struct, ...", etc., 
> are implementation details, that should be left to static if's 
> inside the function body rather than clutter the API with an 
> incomprehensibly large overload set.

In the specific case of `std.conv.to`, the user-facing template 
*is* unconstrained. Its signature is:

     template to(T)

However, the internal function that `to` forwards to, `toImpl`, 
does use template constraints to create an incomprehensible 
overload set.