constexpr type_name()

typename.hpp

#include <iostream>

#include <array>
#include <utility>
#include <string_view>

template<std::size_t...Idxs>
constexpr auto substring_as_array(std::string_view str, std::index_sequence<Idxs...>) {
    return std::array{str[Idxs]..., '\n'};
}

template <typename T>
constexpr auto type_name_array() {
#if defined(__clang__)
    constexpr auto prefix   = std::string_view{"[T = "};
    constexpr auto suffix   = std::string_view{"]"};
    constexpr auto function = std::string_view{__PRETTY_FUNCTION__};
#elif defined(__GNUC__)
    constexpr auto prefix   = std::string_view{"with T = "};
    constexpr auto suffix   = std::string_view{"]"};
    constexpr auto function = std::string_view{__PRETTY_FUNCTION__};
#elif defined(_MSC_VER)
    constexpr auto prefix   = std::string_view{"type_name_array<"};
    constexpr auto suffix   = std::string_view{">(void)"};
    constexpr auto function = std::string_view{__FUNCSIG__};
#else
# error Unsupported compiler
#endif
    constexpr auto double_colon = std::string_view{"::"};

    constexpr auto start = function.find(prefix) + prefix.size();
    constexpr auto end = function.rfind(suffix);
    static_assert(start < end);

    constexpr auto name = function.substr(start, (end - start));
    constexpr auto double_colon_pos = name.find(double_colon);
    if constexpr ( double_colon_pos != std::string_view::npos ) {
        constexpr auto name2 = name.substr(double_colon_pos + double_colon.size()
            ,end - (double_colon_pos + double_colon.size()));
        return substring_as_array(name2, std::make_index_sequence<name2.size()>{});
    }
    
    return substring_as_array(name, std::make_index_sequence<name.size()>{});
}

template <typename T>
struct type_name_holder {
    static inline constexpr auto value = type_name_array<T>();
};

template <typename T>
constexpr auto type_name() -> std::string_view {
    constexpr auto& value = type_name_holder<T>::value;
    return std::string_view{value.data(), value.size()};
}

int main()
{
    std::cout << type_name<int>() << std::endl;
}