Disable class specialization using concepts

You can use type traits to check whether some type is a specialization of span or std::array. This works for me:

#include <type_traits>

template<typename, std::ptrdiff_t> class span;

template <typename T>
struct is_array : std::false_type { };
template <typename T, size_t N>
struct is_array<std::array<T, N>> : std::true_type { };

template <typename T>
struct is_span : std::false_type { };
template <typename T, std::ptrdiff_t P>
struct is_span<span<T, P>> : std::true_type { };

template <typename T>
concept bool NotSpanNotArray = !is_array<T>::value && !is_span<T>::value;

template<typename, std::ptrdiff_t> class span {
public:
  template<NotSpanNotArray T> constexpr span(T& cont);
  // template<NotSpanNotArray T> constexpr span(const T& cont);
};

Working demo: https://wandbox.org/permlink/M0n60U8Hl4mpacuI

Just I am not 100% sure whether such a solution meets that participate in overload resolution if and only if requirement. Some language-lawyer might clarify this.

---

UPDATE

I just realized that std::is_array works only for "ordinary" arrays, not std::array. Therefore I added a custom is_array type trait as well.

First, create a trait to check for specializations. array and span look the same in the sense that they take a type parameter and a non-type parameter:

template <typename T, template <typename, auto> class Z>
struct is_specialization : std::false_type { };
template <typename A, auto V, template <typename, auto> class Z>
struct is_specialization<Z<A,V>, Z> : std::true_type { };

template <typename T, template <typename, auto> class Z>
inline constexpr bool is_specialization_v = is_specialization<T, Z>::value;

And then we can build up a concept from that:

// the last bullet point
template <typename T, typename E>
concept ValidForElement =
    ConvertibleTo<std::remove_pointer_t<T>(*)[], E(*)[]>;

template <typename T, typename E>
concept AllowedContainer =
    // not a specialization of span (note: requires forward declaration of span)
    !is_specialization_v<std::remove_cv_t<T>, std::span>
    // not a specialization of array
    && !is_specialization_v<std::remove_cv_t<T>, std::array>
    // not a raw array
    && !std::is_array_v<std::remove_cv_t<T>>
    && requires (T cont) {
        // data(cont) is well-formed and has a valid type
        { data(cont); } -> ValidForElement<E>
        // size(cont) is well-formed
        { size(cont); }
    };

Which you would use like:

template <typename Element, std::ptrdiff_t Extent = -1>
struct span {
    template <typename C> requires AllowedContainer<C, Element>
    span(C&);
    template <typename C> requires AllowedContainer<C const, Element>
    span(C const&);
};

---

The const-ness requirement there prevents the nice partial-concept-id syntax, but we could just add another concept for that I guess:

template <typename T, typename E>
concept ConstAllowedContainer = AllowedContainer<T const, E>;

template <typename Element, std::ptrdiff_t Extent = -1>
struct span {
    template <AllowedContainer<E> C>      span(C&);
    template <ConstAllowedContainer<E> C> span(C const&);
};

Not sure if there's a cleverer approach here yet.

---

But really this whole pair-of-constructor thing is probably a mistake and you want to do a forwarding reference:

template <typename Element, std::ptrdiff_t Extent = -1>
struct span {
    template <AllowedContainer<E> C>
    span(C&&);
};

This last approach requires a few tweaks to the concept (all the remove_cv_t's should become remove_cvref_t's).