Add detail/is_fixed_enum.hpp

include/boost/endian/detail/is_fixed_enum.hpp

@@ -0,0 +1,51 @@
+#ifndef BOOST_ENDIAN_DETAIL_IS_FIXED_ENUM_HPP_INCLUDED
+#define BOOST_ENDIAN_DETAIL_IS_FIXED_ENUM_HPP_INCLUDED
+
+// Copyright 2020 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/endian/detail/is_scoped_enum.hpp>
+#include <boost/type_traits/is_enum.hpp>
+#include <boost/type_traits/remove_cv.hpp>
+#include <boost/config.hpp>
+
+namespace boost
+{
+namespace endian
+{
+namespace detail
+{
+
+#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+
+// Thanks to Will Wray for the T{0} idea, which unfortunately only works in C++17+
+//
+// VS2017 works, VS2019 works from 19.20 to 19.22, then in 19.23 starts accepting
+// T{0} for unscoped non-fixed enums as well. Sad!
+
+template<class T, class E = T> struct is_fixed_enum_impl: boost::false_type
+{
+};
+
+template<class T> struct is_fixed_enum_impl<T, decltype(T{0})>: boost::is_enum<T>
+{
+};
+
+template<class T> struct is_fixed_enum: is_fixed_enum_impl< boost::remove_cv_t<T> >
+{
+};
+
+#else
+
+template<class T> struct is_fixed_enum: is_scoped_enum<T>
+{
+};
+
+#endif
+
+} // namespace detail
+} // namespace endian
+} // namespace boost
+
+#endif  // BOOST_ENDIAN_DETAIL_IS_FIXED_ENUM_HPP_INCLUDED