UDA inheritance

[Joseph Rushton Wakeling]

On Thursday, 10 September 2020 at 13:14:47 UTC, drug wrote:
> Just a thought - couldn't you use classes for this? Get an UDA 
> and check if it is a descendant of the specific class.

Yes, I did wonder about that, but it doesn't allow all the 
inference that I'm looking for.  For example:

     class First
     {
         enum A;
         enum B;
     }

     class Second : First
     {
     }


     @(Second.A)
     struct MySecond
     {
     }


     import std.traits : hasUDA;

     static assert(hasUDA!(MySecond, Second.A));  // OK!
     static assert(hasUDA!(MySecond, First.A));   // OK!
     static assert(hasUDA!(MySecond, Second));    // fails
     static assert(hasUDA!(MySecond, First));     // fails

It's obvious _why_, of course, given how I set the above up.  And 
this contrasts with what one can do with enums:

     enum Something { A, B }

     @(Something.A)
     struct MySomething { }

     import std.traits : hasUDA;

     static assert(hasUDA!(MySomething, Something.A));  // OK!
     static assert(hasUDA!(MySomething, Something));    // OK!

... where I can check for the enum specialization _or_ the 
general enum type and have things just work.

I'm looking, ideally, to be able to do both.  I did try something 
like:

     enum First { A, B }

     enum Second : First { A = First.A, B = First.B }

... but that doesn't work.  Hence why I thought it might be worth 
making a forum post.