Hello,
I miss these macros in boost/config.hpp:

• BOOST_NO_CXX20/BOOST_NO_CXX23 (like BOOST_NO_CXX14, BOOST_NO_CXX17)
• BOOST_CXX17_CONSTEXPR/BOOST_CXX20_CONSTEXPR/BOOST_CXX23_CONSTEXPR (like BOOST_CXX14_CONSTEXPR)
• BOOST_CXX20_CONSTEVAL/BOOST_CXX23_CONSTEVAL

This could be done like this:
// C++17
#if defined(__cpp_constexpr)
#if (__cpp_constexpr < 201603L)
#define BOOST_NO_CXX17_CONSTEXPR
#endif
#else
#define BOOST_NO_CXX17_CONSTEXPR
#endif

#if defined(BOOST_NO_CXX17_CONSTEXPR)
#define BOOST_CXX17_CONSTEXPR
#else
#define BOOST_CXX17_CONSTEXPR constexpr
#endif

// C++20
#if (BOOST_CXX_VERSION < 202002L)
#define BOOST_NO_CXX20
#endif

#if defined(__cpp_constexpr)
#if (__cpp_constexpr < 202002L)
#define BOOST_NO_CXX20_CONSTEXPR
#endif
#else
#define BOOST_NO_CXX20_CONSTEXPR
#endif

#if defined(BOOST_NO_CXX20_CONSTEXPR)
#define BOOST_CXX20_CONSTEXPR
#else
#define BOOST_CXX20_CONSTEXPR constexpr
#endif

#if defined(__cpp_consteval)
#if (__cpp_consteval < 201811L)
#define BOOST_CXX20_NO_CONSTEVAL
#endif
#else
#define BOOST_CXX20_NO_CONSTEVAL
#endif

#if defined(BOOST_CXX20_NO_CONSTEVAL)
#define BOOST_CXX20_CONSTEVAL
#else
#define BOOST_CXX20_CONSTEVAL consteval
#endif

// C++23
#if (BOOST_CXX_VERSION < 202100L)
#define BOOST_NO_CXX23
#endif

#if defined(__cpp_constexpr)
#if (__cpp_constexpr < 202211L)
#define BOOST_NO_CXX23_CONSTEXPR
#endif
#else
#define BOOST_NO_CXX23_CONSTEXPR
#endif

#if defined(BOOST_NO_CXX23_CONSTEXPR)
#define BOOST_CXX23_CONSTEXPR
#else
#define BOOST_CXX23_CONSTEXPR constexpr
#endif

#if defined(__cpp_consteval)
#if (__cpp_consteval < 202211L)
#define BOOST_CXX23_NO_CONSTEVAL
#endif
#else
#define BOOST_CXX23_NO_CONSTEVAL
#endif

#if defined(BOOST_CXX23_NO_CONSTEVAL)
#define BOOST_CXX23_CONSTEVAL
#else
#define BOOST_CXX23_CONSTEVAL consteval
#endif

thx
Gero