Template constraints

[Atila Neves via Digitalmars-d]

Even then this "hides" some errors and debugging isn't easy 
(figuring out why the template constraint failed). I've been 
planning on creating a DIP addressing this for ages, I should 
probably get around to that.

Atila

On Saturday, 14 February 2015 at 17:00:33 UTC, Andrei 
Alexandrescu wrote:
> There's been recurring discussion about failing constraints not 
> generating nice error messages.
>
> void fun(T)(T x) if (complicated_condition) { ... }
> struct Type(T)(T x) if (complicated_condition) { ... }
>
> If complicated_condition is not met, the symbol simply 
> disappears and the compiler error message just lists is as a 
> possible, but not viable, candidate.
>
> I think one simple step toward improving things is pushing the 
> condition in a static_assert inside type definitions:
>
> void fun(T)(T x) if (complicated_condition) { ... } // no change
> struct Type(T)(T x)
> {
>   static assert(complicated_condition, "Informative message.");
>   ...
> }
>
> This should improve error messages for types (only). The 
> rationale is that it's okay for types to refuse compilation 
> because types, unlike functions, don't overload. The major 
> reason for template constraints in functions is allowing for 
> good overloading.
>
>
> Andrei