Use a template template parameter with CRTP in a Concept - c++

Question

I want to write a concept that tests for inheritance from a base class.
My Base class is publicly inherited by Derived classes, using CRTP.
This code works fine:
#include <type_traits>
namespace NS
{
template<typename D>
class Base {
// ...
};
class Derived : public Base<Derived>
{
public:
constexpr Derived() = default;
// ...
};
}
template<typename D>
concept bool inheritsFromB() {
return std::is_base_of<NS::Base<D>, D>::value;
}
template<inheritsFromB b>
void myFunct() {};
int main() {
constexpr auto d = NS::Derived();
using dType = typename std::decay<decltype(d)>::type;
myFunct<dType>();
}
I hit a problem if I want to template Derived. Is this possible?
namespace NS
{
template<typename D, typename T>
class Base { ... };
template<typename T>
class Derived : public Base<Derived<T>, T>
{ // ...
// probably some using declaration for T?
};
}
template<template <typename> class D>
concept bool inheritsFromB() {
return std::is_base_of<NS::B<D<T>,T>, D<T>::value;
}
...
the obvious problem being that I have no T in my concept declaration.
Moreover, I'm pretty sure I can't declare
template<template <typename> class D, typename T>
concept bool inheritsFromB() {
...
}
because a concept requires one template parameter.
Edit - the Working Paper P0121R0 lists in section 8.3.5, p23, template<typename T, typename U> concept bool C3 = true;. Consequently, wherever I read a concept can take only one parameter was either outdated, wrong, or I read it lacking care. end edit
Can I access the other type(s) T that I need here? Is there an alternative way (it seems to me like the template type D would carry the information of what it's type T is, but I also can't use using T = typename D<T>::valueType;, because I need the T to specific the type of D<T>...)

Answer 1

I suspect the following trait should work:
#include <type_traits>
#include <utility>
namespace NS
{
template <typename D, typename T>
class Base {};
template <typename T>
class Derived : public Base<Derived<T>, T> {};
}
namespace detail
{
template <typename T, template <typename> typename D>
std::true_type is_derived_from_base(const ::NS::Base<D<T>,T>*);
std::false_type is_derived_from_base(void*);
}
template <typename T>
using is_derived_from_base = decltype(detail::is_derived_from_base(std::declval<T*>()));
template <typename T>
concept bool inheritsFromB()
{
return is_derived_from_base<T>{};
}
DEMO (without concepts)