add enum concept

example/example.cpp

@@ -26,6 +26,12 @@
 
 enum class Color : int { RED = -10, BLUE = 0, GREEN = 10 };
 
+template <typename E>
+auto to_integer(magic_enum::Enum<E> value) {
+  // magic_enum::Enum<E> - C++17 Concept for enum type.
+  return static_cast<std::underlying_type_t<E>>(value);
+}
+
 int main() {
   // Enum variable to string name.
   Color c1 = Color::RED;
@@ -44,7 +50,7 @@ int main() {
   // String name to enum value.
   auto c2 = magic_enum::enum_cast<Color>("BLUE");
   if (c2.has_value() && c2.value() == Color::BLUE) {
-    std::cout << "BLUE = " << static_cast<int>(c2.value()) << std::endl; // BLUE = 0
+    std::cout << "BLUE = " << to_integer(c2.value()) << std::endl; // BLUE = 0
   }
 
   // Integer value to enum value.
@@ -55,7 +61,7 @@ int main() {
 
   // Enum value to integer value.
   auto color_integer = magic_enum::enum_integer(Color::RED);
-  if (color_integer == static_cast<std::underlying_type_t<Color>>(Color::RED)) {
+  if (color_integer == to_integer(Color::RED)) {
     std::cout << "RED = " << color_integer << std::endl; // RED = -10
   }
 

include/magic_enum.hpp

@@ -97,7 +97,9 @@ inline constexpr bool is_enum_v = std::is_enum_v<T> && std::is_same_v<T, std::de
 
 template <std::size_t N>
 struct static_string {
-  constexpr static_string(std::string_view str) noexcept : static_string{str, std::make_index_sequence<N>{}} {}
+  constexpr static_string(std::string_view str) noexcept : static_string{str, std::make_index_sequence<N>{}} {
+    assert(str.size() == N);
+  }
 
   constexpr const char* data() const noexcept { return chars.data(); }
 
@@ -109,7 +111,7 @@ struct static_string {
   template <std::size_t... I>
   constexpr static_string(std::string_view str, std::index_sequence<I...>) noexcept : chars{{str[I]...}} {}
 
-  const std::array<char, N> chars;
+  const std::array<const char, N> chars;
 };
 
 template <>
@@ -308,6 +310,9 @@ constexpr auto entries(std::index_sequence<I...>) noexcept {
 template <typename T, typename R>
 using enable_if_enum_t = std::enable_if_t<std::is_enum_v<std::decay_t<T>>, R>;
 
+template <typename T, typename Enable = std::enable_if_t<std::is_enum_v<std::decay_t<T>>>>
+using enum_concept = T;
+
 template <typename T, bool = std::is_enum_v<T>>
 struct is_scoped_enum : std::false_type {};
 
@@ -392,6 +397,9 @@ struct enum_traits<E, std::enable_if_t<is_enum_v<E>>> {
 // Checks is magic_enum supported compiler.
 inline constexpr bool is_magic_enum_supported = detail::supported<void>::value;
 
+template <typename T>
+using Enum = detail::enum_concept<T>;
+
 // Checks whether T is an Unscoped enumeration type.
 // Provides the member constant value which is equal to true, if T is an [Unscoped enumeration](https://en.cppreference.com/w/cpp/language/enum#Unscoped_enumeration) type. Otherwise, value is equal to false.
 template <typename T>