Answer by HolyBlackCat for how to know an enum class is sorted

There's no way to check the declaration order. Best you can do is check that the constants are contiguous (i.e. there are no gaps in the numbering).

First you start with __PRETTY_FUNCTION__/__FUNSIG__. This lets you check if the enum has a constant with a specific value:

template <auto X>constexpr const auto &PrettyFuncForValue(){    #ifdef _MSC_VER    return __FUNCSIG__;    #else    return __PRETTY_FUNCTION__;    #endif}

enum class E{    a = 10,    b = 20,    c = 21,    d = 22,};// Those outputs here are from Clang, other compilers have something similar.std::cout << PrettyFuncForValue<E::a>() << '\n'; // const auto &PrettyFuncForValue() [X = E::a]std::cout << PrettyFuncForValue<E(10)>() << '\n'; // const auto &PrettyFuncForValue() [X = E::a]std::cout << PrettyFuncForValue<E(11)>() << '\n'; // const auto &PrettyFuncForValue() [X = (E)11]

As you can see, values with associated names result in a different string pattern.

You can then search for this pattern:

template <auto X>inline constexpr bool enum_constant_is_valid =    std::string_view(PrettyFuncForValue<X>()).rfind(        #ifdef _MSC_VER"<(enum "        #else"X = ("        #endif    ) == std::string_view::npos;

std::cout << enum_constant_is_valid<E::a> << '\n'; // 1std::cout << enum_constant_is_valid<E(10)> << '\n'; // 1std::cout << enum_constant_is_valid<E(11)> << '\n'; // 0

Lastly you slap std::integer_sequence onto it to check a range of values:

template <auto A, auto B>requires std::same_as<decltype(A), decltype(B)>inline constexpr bool enum_range_is_valid =    []<std::underlying_type_t<decltype(A)> ...I>(std::integer_sequence<std::underlying_type_t<decltype(A)>, I...>){        return (enum_constant_is_valid<decltype(A)(std::to_underlying(A) + I)> && ...);    }(std::make_integer_sequence<std::underlying_type_t<decltype(A)>, std::to_underlying(B) - std::to_underlying(A) + 1>{});std::cout << enum_range_is_valid<E::a, E::a> << '\n'; // 1std::cout << enum_range_is_valid<E::a, E::b> << '\n'; // 0std::cout << enum_range_is_valid<E::b, E::d> << '\n'; // 1

Godbolt link with the full code.