Add std::is_constant_evaluated support.

test/test_boost_is_constant_evaluated.cpp

@@ -0,0 +1,56 @@
+//  (C) Copyright John Maddock 2021.
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/config.hpp>
+#include <type_traits>
+
+#if BOOST_IS_CONSTANT_EVALUATED_VERSION >= 2
+inline constexpr int factorial1(int i)
+{
+   if (!BOOST_IS_CONSTANT_EVALUATED())
+   {
+      std::abort();
+   }
+   else if (i == 1)
+   {
+      return 1;
+   }
+   else
+      return i * factorial1(i - 1);
+}
+#endif
+#if BOOST_IS_CONSTANT_EVALUATED_VERSION >= 1
+inline constexpr int factorial2(int i)
+{
+   if (!BOOST_IS_CONSTANT_EVALUATED_INT(i))
+   {
+      std::abort();
+   }
+   else if (i == 1)
+   {
+      return 1;
+   }
+   else
+      return i * factorial2(i - 1);
+}
+#endif
+
+int main()
+{
+#if BOOST_IS_CONSTANT_EVALUATED_VERSION >= 2
+   constexpr int f1 = factorial1(4);
+   
+   if (f1 != 24) 
+      return 1;
+#endif
+#if BOOST_IS_CONSTANT_EVALUATED_VERSION >= 1
+   constexpr int f2 = factorial2(4);
+   
+   if (f2 != 24) 
+      return 1;
+#endif
+   return 0;
+}
+