Doesn't using directives work with c++20 concepts?

Question

Does using directives not work with concepts? why?

The example below doesn't work and I get a compiler error saying that it expected a type.

#include <concepts>

namespace A::X {

  struct BaseA {};

  template < typename AType >
    concept DerivedFromA = std::derived_from < AType, BaseA >;

}

namespace A {

  using DerivedFromA = X::DerivedFromA;

}

I want to be able to access concept DerivedFromA from namespace A as DerivedFromA rather than X::DerivedFromA

Answer 1

Does using directives not work with concepts? why?

Because the way you're employing using declares a type alias. And concepts are not type templates.

What you intend to do is to make a name from one namespace available in a different one. That looks like this:

namespace A
{
  using X::DerivedFromA;
}

This works on all kinds of names.