move is_flags to customize::enum_range

force set the enum-flag true or false

doc/limitations.md

@@ -7,11 +7,13 @@
 
 * Enum can't reflect if the enum is a forward declaration.
 
-* For enum-flags add specialization `is_flags_enum` for necessary enum type. Specialization of `is_flags_enum` must be injected in `namespace magic_enum::customize`.
+* For enum-flags add `is_flags` to specialization `enum_range` for necessary enum type. Specialization of `enum_range` must be injected in `namespace magic_enum::customize`.
   ```cpp
   enum class Directions { Up = 1 << 1, Down = 1 << 2, Right = 1 << 3, Left = 1 << 4 };
   template <>
-  struct magic_enum::customize::is_flags_enum<Directions> : std::true_type {};
+  struct magic_enum::customize::enum_range<Directions> {
+    static constexpr bool is_flags = true;
+  };
   ```
 
 * Enum value must be in range `[MAGIC_ENUM_RANGE_MIN, MAGIC_ENUM_RANGE_MAX]`.

example/enum_flag_example.cpp

@@ -26,9 +26,11 @@
 #include <magic_enum.hpp>
 
 enum class AnimalFlags : std::uint64_t { HasClaws = 1 << 10, CanFly = 1 << 20, EatsFish = 1 << 30, Endangered = std::uint64_t{1} << 40 };
-// Add specialization `is_flags_enum` to force define that enum are flags.
+// Add specialization `is_flags` to force define that enum are flags.
-// template <>
+//template <>
-// struct magic_enum::customize::is_flags_enum<AnimalFlags> : std::true_type {};
+//struct magic_enum::customize::enum_range<AnimalFlags> {
+//  static constexpr bool is_flags = true;
+//};
 
 int main() {
   // Enum-flags variable to string name.

include/magic_enum.hpp

@@ -142,9 +142,6 @@ constexpr string_view enum_name(E) noexcept {
   return {};
 }
 
-template <typename E>
-struct is_flags_enum : std::false_type {};
-
 } // namespace magic_enum::customize
 
 namespace detail {
@@ -157,6 +154,12 @@ struct supported
     : std::false_type {};
 #endif
 
+template <typename T, typename = void>
+struct has_is_flags : std::false_type {};
+
+template <typename T>
+struct has_is_flags<T, std::void_t<decltype(customize::enum_range<T>::is_flags)>> : std::bool_constant<std::is_same_v<bool, std::decay_t<decltype(customize::enum_range<T>::is_flags)>>> {};
+
 struct char_equal_to {
   constexpr bool operator()(char lhs, char rhs) const noexcept {
     return lhs == rhs;
@@ -472,14 +475,14 @@ template <typename E, typename U = std::underlying_type_t<E>>
 constexpr bool is_flags_enum() noexcept {
   static_assert(is_enum_v<E>, "magic_enum::detail::is_flags_enum requires enum type.");
 
-#if defined(MAGIC_ENUM_NO_CHECK_FLAGS)
+  if constexpr (has_is_flags<E>::value) {
-  return customize::is_flags_enum<E>::value;
+    return customize::enum_range<E>::is_flags;
-#else
+  } else if constexpr (std::is_same_v<U, bool>) { // bool special case
-  if constexpr (std::is_same_v<U, bool>) { // bool special case
     return false;
-  } else if constexpr (customize::is_flags_enum<E>::value) {
-    return true;
   } else {
+#if defined(MAGIC_ENUM_NO_CHECK_FLAGS)
+    return false;
+#else
     constexpr auto flags_values = values<E, true>();
     constexpr auto default_values = values<E, false>();
     if (flags_values.size() == 0 || default_values.size() > flags_values.size()) {
@@ -492,8 +495,8 @@ constexpr bool is_flags_enum() noexcept {
       }
     }
     return flags_values.size() > 0;
-  }
 #endif
+  }
 }
 
 template <typename E>

test/test_flags.cpp

@@ -40,8 +40,6 @@
 #include <sstream>
 
 enum class Color { RED = 1, GREEN = 2, BLUE = 4 };
-template <>
-struct magic_enum::customize::is_flags_enum<Color> : std::true_type {};
 
 enum class Numbers : int {
   one = 1 << 1,