When using magic_enum in combination with fmtlib, compilation errors occur with older versions of compilers.

[WtDMaO0]

Hello everyone, I'm currently using magic_enum in combination with the fmtlib repository. In my current usage scenario, some machines still have very old compiler versions, specifically using gcc7. During usage, I discovered compilation errors occurring, with the issue located in the enum_integer module.

https://github.com/WtDMaO0/magic_enum/blob/2e6e8bfff361c2c40990426c462b44af6c27820b/include/magic_enum/magic_enum.hpp#L1270

// Returns integer value from enum value.
template <typename E>
[[nodiscard]] constexpr auto enum_integer(E value) noexcept -> detail::enable_if_t<E, underlying_type_t<E>> {
  return static_cast<underlying_type_t<E>>(value);
}

enum_interger is used here
https://github.com/WtDMaO0/magic_enum/blob/2e6e8bfff361c2c40990426c462b44af6c27820b/include/magic_enum/magic_enum_format.hpp#L55

template <typename E, std::enable_if_t<std::is_enum_v<std::decay_t<E>>, int> = 0>
std::string format_as(E e) {
  using D = std::decay_t<E>;
  static_assert(std::is_same_v<char, magic_enum::string_view::value_type>, "magic_enum::formatter requires string_view::value_type type same as char.");
  if constexpr (magic_enum::detail::supported<D>::value) {
    if constexpr (magic_enum::detail::subtype_v<D> == magic_enum::detail::enum_subtype::flags) {
      if (const auto name = magic_enum::enum_flags_name<D>(e); !name.empty()) {
        return {name.data(), name.size()};
      }
    } else {
      if (const auto name = magic_enum::enum_name<D>(e); !name.empty()) {
        return {name.data(), name.size()};
      }
    }
  }
  return std::to_string(magic_enum::enum_integer<D>(e));
}

As can be seen here, enum_integer is used inside format_as for scenarios not supported by magic_enum. However, the deduced type of enum_integer is detail::enable_if_t<E, underlying_type_t>, which triggers compiler compatibility checks and consequently causes the compilation error.

[WtDMaO0]

soon will add a example.

[WtDMaO0]

fixed on master 905ba74