From c7b2177145e76d09c39d8153e4c4faec20ff852e Mon Sep 17 00:00:00 2001
From: Matt Borland <matt@mattborland.com>
Date: Thu, 26 Jan 2023 09:21:33 -0800
Subject: [PATCH] Add framework for is_constant_evaluated

---
 .../charconv/detail/is_constant_evaluated.hpp | 54 +++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 include/boost/charconv/detail/is_constant_evaluated.hpp

diff --git a/include/boost/charconv/detail/is_constant_evaluated.hpp b/include/boost/charconv/detail/is_constant_evaluated.hpp
new file mode 100644
index 0000000..f5f8fe4
--- /dev/null
+++ b/include/boost/charconv/detail/is_constant_evaluated.hpp
@@ -0,0 +1,54 @@
+//  Copyright John Maddock 2011-2021.
+//  Copyright Matt Borland 2023.
+//  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)
+
+#ifndef BOOST_CHARCONV_TOOLS_IS_CONSTANT_EVALUATED_HPP
+#define BOOST_CHARCONV_TOOLS_IS_CONSTANT_EVALUATED_HPP
+
+#include <boost/charconv/config.hpp>
+
+#ifdef __has_include
+# if __has_include(<version>)
+#  include <version>
+#  ifdef __cpp_lib_is_constant_evaluated
+#   include <type_traits>
+#   define BOOST_CHARCONV_HAS_IS_CONSTANT_EVALUATED
+#  endif
+# endif
+#endif
+
+#ifdef __has_builtin
+#  if __has_builtin(__builtin_is_constant_evaluated) && !defined(BOOST_NO_CXX14_CONSTEXPR)
+#    define BOOST_CHARCONV_HAS_BUILTIN_IS_CONSTANT_EVALUATED
+#  endif
+#endif
+
+//
+// MSVC also supports __builtin_is_constant_evaluated if it's recent enough:
+//
+#if defined(_MSC_FULL_VER) && (_MSC_FULL_VER >= 192528326)
+#  define BOOST_CHARCONV_HAS_BUILTIN_IS_CONSTANT_EVALUATED
+#endif
+
+//
+// As does GCC-9:
+//
+#if !defined(BOOST_NO_CXX14_CONSTEXPR) && (__GNUC__ >= 9) && !defined(BOOST_CHARCONV_HAS_BUILTIN_IS_CONSTANT_EVALUATED)
+#  define BOOST_CHARCONV_HAS_BUILTIN_IS_CONSTANT_EVALUATED
+#endif
+
+#if defined(BOOST_CHARCONV_HAS_IS_CONSTANT_EVALUATED) && !defined(BOOST_NO_CXX14_CONSTEXPR)
+#  define BOOST_CHARCONV_IS_CONSTANT_EVALUATED(x) std::is_constant_evaluated()
+#elif defined(BOOST_CHARCONV_HAS_BUILTIN_IS_CONSTANT_EVALUATED)
+#  define BOOST_CHARCONV_IS_CONSTANT_EVALUATED(x) __builtin_is_constant_evaluated()
+#elif !defined(BOOST_NO_CXX14_CONSTEXPR) && (__GNUC__ >= 6)
+#  define BOOST_CHARCONV_IS_CONSTANT_EVALUATED(x) __builtin_constant_p(x)
+#  define BOOST_CHARCONV_USING_BUILTIN_CONSTANT_P
+#else
+#  define BOOST_CHARCONV_IS_CONSTANT_EVALUATED(x) false
+#  define BOOST_CHARCONV_NO_CONSTEXPR_DETECTION
+#endif
+
+#endif // BOOST_CHARCONV_TOOLS_IS_CONSTANT_EVALUATED_HPP