Begin C++20 support.

include/boost/config/stdlib/dinkumware.hpp

@@ -192,6 +192,26 @@
 #  define BOOST_NO_CXX17_STD_INVOKE
 #endif
 
+// C++20 features
+#if !defined(_MSVC_STL_UPDATE) || (_MSVC_STL_UPDATE < 202008L)
+#  define BOOST_NO_CXX20_HDR_BARRIER
+#  define BOOST_NO_CXX20_HDR_BIT
+#  define BOOST_NO_CXX20_HDR_LATCH
+#  define BOOST_NO_CXX20_HDR_SPAN
+#  define BOOST_NO_CXX20_HDR_COMPARE
+#  define BOOST_NO_CXX20_HDR_NUMBERS
+#  define BOOST_NO_CXX20_HDR_CONCEPTS
+#  define BOOST_NO_CXX20_HDR_COROUTINE
+#  define BOOST_NO_CXX20_HDR_SEMAPHORE
+#endif
+// C++20 features not yet implemented:
+#  define BOOST_NO_CXX20_HDR_FORMAT
+#  define BOOST_NO_CXX20_HDR_SOURCE_LOCATION
+#  define BOOST_NO_CXX20_HDR_STOP_TOKEN
+#  define BOOST_NO_CXX20_HDR_SYNCSTREAM
+// Incomplete:
+#  define BOOST_NO_CXX20_HDR_RANGES
+
 #if !(!defined(_CPPLIB_VER) || (_CPPLIB_VER < 650) || !defined(BOOST_MSVC) || (BOOST_MSVC < 1912) || !defined(_HAS_CXX17) || (_HAS_CXX17 == 0))
 // Deprecated std::iterator:
 #  define BOOST_NO_STD_ITERATOR
@@ -227,6 +247,12 @@
 #    define BOOST_NO_CXX98_BINDERS
 #  endif
 #endif
+//
+// Things deprecated in C++20:
+//
+#if defined(_HAS_CXX20)
+#  define BOOST_NO_CXX11_ATOMIC_SMART_PTR
+#endif
 
 
 //

include/boost/config/stdlib/libstdcpp3.hpp

@@ -351,6 +351,17 @@ extern "C" char *gets (char *__s);
 #  define BOOST_NO_CXX17_HDR_CHARCONV
 #endif
 
+#if BOOST_LIBSTDCXX_VERSION < 110000
+//
+// Header <bit> may be present but lacks std::bit_cast:
+//
+#define BOOST_NO_CXX20_HDR_BIT
+#endif
+
+#ifndef __cpp_impl_coroutine
+#  define BOOST_NO_CXX20_HDR_COROUTINE
+#endif
+
 //
 // Headers not present on Solaris with the Oracle compiler:
 #if defined(__SUNPRO_CC) && (__SUNPRO_CC < 0x5140)