Determine if a type if derived from a template parameter

[Alex]

On Thursday, 11 April 2019 at 08:57:10 UTC, Basile B. wrote:
> On Wednesday, 10 April 2019 at 19:28:38 UTC, Alex wrote:
>> e.g.,
>>
>> class X(T,S)
>> {
>>    T x;
>>    S y;
>> }
>>
>> Somehow determine if x's type is derived from the template 
>> parameter.
>>
>> I doubt D has this capability but it would be nice for certain 
>> things. In my reflect library the types must be specified such 
>> as Reflect!(X!(int,float)) and ideally I would like to do 
>> Reflect!(X!(T,S)) to get generic reflection information. This 
>> helps reduce overhead as one could reflect once on the generic 
>> type, cache the results, then simply modify the results for 
>> specifics.
>
> You can detect a match between a member and a template argument 
> but you cant know if the match is a coincidence or on purpose. 
> For example:
>
>   class X(T)
>   {
>       T x;
>       int y;
>   }
>
>   alias X1 = X!int;
>
> here X1.y matches to T but it's a coincidence.
> The problem that you'll be faced to is that the template 
> parameters are replaced by the symbol or the type.

Yes, which is my point, so one isn't detecting anything. It would 
be very error prone to assume if a type is the same as the type 
of the parameter then it is that parameter.

This is why I asked if there was a way to do this. The compiler 
easily knows this information since it has to do the replacement. 
The only way to hack it is to read the source unless there is 
some way way in D.