is_constexpr implementation for function returning structural type (integral or enumeration)

is_constexpr.h

#include <type_traits>
// is_constexpr implementation
// Live example https://godbolt.org/z/79adTjdax
template<auto F>
struct constexpr_true : std::true_type {};

template<auto F>
static auto test_constexpr(int) -> constexpr_true<F()>;

template<auto F>
static auto test_constexpr(long) -> std::false_type;

template<auto F>
struct is_constexpr : decltype(test_constexpr<F>(0)) {};

// is_constexpr test 

struct A {
  static constexpr int c() {
    return 1;
  }
};

struct B {
  static int c() noexcept {
    return cc;
  }
  static inline int cc = 1;
};

static_assert(is_constexpr<&A::c>::value);
static_assert(!is_constexpr<&B::c>::value);