Use __builtin_bit_cast when available

test/Jamfile.v2

@@ -304,6 +304,7 @@ run bit_byteswap_test.cpp
 compile-fail bit_width_fail.cpp
   : <warnings>off ;
 
+compile bit_cast_test_cx.cpp ;
 compile bit_rotate_test_cx.cpp ;
 compile bit_countr_test_cx.cpp ;
 compile bit_countl_test_cx.cpp ;

test/bit_cast_test_cx.cpp

@@ -0,0 +1,24 @@
+// constexpr test for boost/core/bit.hpp (bit_cast)
+//
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX11_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped because BOOST_NO_CXX11_CONSTEXPR is defined" )
+
+#else
+
+#include <boost/core/bit.hpp>
+#include <boost/cstdint.hpp>
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+STATIC_ASSERT( boost::core::bit_cast<boost::uint32_t>( 1.0f ) == 0x3F800000u );
+STATIC_ASSERT( boost::core::bit_cast<boost::uint64_t>( 1.0 ) == 0x3FF0000000000000ull );
+
+#endif