C++ named requirements: LiteralType (since C++11)

#include <cstddef>
#include <iostream>
#include <stdexcept>
 
class conststr // conststr is a literal type
{
    const char* p;
    std::size_t sz;
public:
    template<std::size_t N>
    constexpr conststr(const char(&a)[N]) : p(a), sz(N - 1) {}
 
    constexpr char operator[](std::size_t n) const
    {
        return n < sz ? p[n] : throw std::out_of_range("");
    }
 
    constexpr std::size_t size() const { return sz; }
};
 
constexpr std::size_t count_lower(conststr s)
{
    std::size_t c{};
    for (std::size_t n{}; n != s.size(); ++n)
        if ('a' <= s[n] && s[n] <= 'z')
            ++c;
    return c;
}
 
// An output function that requires a compile-time constant N, for testing
template<int N>
struct constN
{
    constN() { std::cout << N << '\n'; }
};
 
int main()
{
    std::cout << "The number of lowercase letters in \"Hello, world!\" is ";
    constN<count_lower("Hello, world!")>(); // the string literal is implicitly
                                            // converted to conststr
}