Move C++20 detection to detail/config.hpp; disable C++20 use for Clang < 13.

Author: pdimov

include/boost/describe/detail/config.hpp

@@ -45,4 +45,20 @@
 
 #define BOOST_DESCRIBE_INLINE_CONSTEXPR BOOST_DESCRIBE_INLINE_VARIABLE BOOST_DESCRIBE_CONSTEXPR_OR_CONST
 
+#if __cplusplus >= 202002L || ( defined(_MSVC_LANG) && _MSVC_LANG >= 202002L )
+
+# define BOOST_DESCRIBE_CXX20
+
+// Clang 13.0 is needed for unevaluated lambdas
+# if defined(__clang__) && __clang_major__ < 13
+#  undef BOOST_DESCRIBE_CXX20
+# endif
+
+// Apple Clang 13.1 is Clang 13.0
+# if defined(__clang__) && defined(__apple_build_version__) && __clang_major__ == 13 && __clang_minor__ < 1
+#  undef BOOST_DESCRIBE_CXX20
+# endif
+
+#endif
+
 #endif // #ifndef BOOST_DESCRIBE_DETAIL_CONFIG_HPP_INCLUDED

include/boost/describe/enum.hpp

@@ -43,7 +43,7 @@ template<class... T> auto enum_descriptor_fn_impl( int, T... )
     return list<enum_descriptor<T>...>();
 }
 
-#if __cplusplus >= 202002L || ( defined(_MSVC_LANG) && _MSVC_LANG >= 202002L )
+#if defined(BOOST_DESCRIBE_CXX20)
 
 template<auto V, auto N> struct enum_desc
 {

test/nested_enum_test2.cpp

@@ -7,9 +7,9 @@
 #include <boost/config.hpp>
 #include <boost/config/pragma_message.hpp>
 
-#if BOOST_CXX_VERSION < 202002L
+#if !defined(BOOST_DESCRIBE_CXX20)
 
-BOOST_PRAGMA_MESSAGE("Skipping test because BOOST_CXX_VERSION < 202002L")
+BOOST_PRAGMA_MESSAGE("Skipping test because BOOST_DESCRIBE_CXX20 isn't defined")
 
 #else