KeyType, ValueType traits for hashes

[Mantis]

24.01.2012 20:49, bearophile пишет:
> Mantis:
>
>> Of course, most likely that user already did type check, but if not,
>> this will give less cryptic error:
>>
>> Error: static assert "Not associative array: int"
>> instantiated from here: KeyType!(int)
>>
>> , instead of:
>>
>> Error: template instance KeyType!(int) KeyType!(int) does not match
>> template declaration KeyType(AA) if (isAssociativeArray!(AA))
> It's a tradeoff. Your custom error message is more readable, but the failure at the template constraint causes a error line at the instantiation point. Sadly I think there is no solution that solves both problems  (we have stack traces for templates, but...).
>
> Bye,
> bearophile
>
True. Maybe something is possible to achieve in this direction?:

template verboseFail(alias Cond, string message)
{
     static if (Cond)
     {
         enum verboseFail = true;
     }
     else
     {
         pragma( msg, message );
         enum verboseFail = false;
     }
}

template KeyType(AA)
     if (verboseFail!(isAssociativeArray!AA, "Not assosiative array: " ~ 
AA.stringof))
{
     static if (is(AA V : V[K], K))
     {
         alias K KeyType;
     }
}

It doesn't work very nicely (the message is logged, but dos not look as 
it is related to error), but perharps it may be improved somehow?