Forcing opDispatch error (not SFINAE?)

[deadalnix via Digitalmars-d]

On Tuesday, 17 June 2014 at 20:13:39 UTC, Luís Marques wrote:
> Consider this:
>
>     struct S
>     {
>         void opDispatch(string name, T)(T value)
>         {
>             alias AllowedTypes = SomeComplexTemplate!S;
>
>             //static assert(is(T : AllowedTypes, "wrong type"));
>
>              static if(!is(T : AllowedTypes))
>             {
>                 pragma(msg, "wrong type");
>                 static assert(0);
>             }
>
>         }
>     }
>
> If the static assert fails, then the opDispatch instantiation 
> fails, and so the relevant member is declared not found. 
> Because of SFINAE, I assume (no?), the user/developer will 
> never see the "wrong type" error message. So, instead, you have 
> to use a pragma(msg), saying something like "hey, ignore the 
> message bellow saying 'name' is not found; it's totally there, 
> the problem is that you tried to assign a value of an invalid 
> type".
>
> Is there a way to improve this situation?
>
> (BTW, the documentation for opDispatch is a bit thin on the 
> details.)

No SFINAE please. That is an idiotic C++ idiom. use template
constraints if you want to disable this template for some
parameter sets. static assert is to statically assert. In this
case it fails. As it should !