casperversteeg/fuckyducky

fuckyducky (compile w/ -std=c++17)

Basically, I have the inheritance graph:

Base
 |
 |-- A<T>
 |    |
 |    |- DerivA<T>
 |
 |-- B<T,U>
     |
     |- DerivB<T,U>

I want to create a type helper what_type_t which will be passed either a DerivA<T> or DerivB<T,U> (of which there will be many) which will either return the type T if it is std::arithmetic, or TypeInvalid if not, recursing through as many nested DerivA and DerivB as necessary to get to a final type.

The most logical thing, to me, is to identify the many DerivA and DerivB by their base classes A and B, which is what the code is currently attempting to do. However, in the template specializations of WhatType:

template <typename T> struct WhatType<A<T>> {
  using type = typename WhatType<T>::type;
};

template <typename T, typename U> struct WhatType<B<T, U>> {
  using type =
      typename WhatType<std::common_type_t<typename WhatType<T>::type,
                                           typename WhatType<U>::type>>::type;
};

the types A and B will not be matched with DerivA and DerivB respectively, even though I very much would like it to...