Reorganise library version detection, adds tentative fix for clang with libstdc++. See issues 10600 and 7473

include/boost/config/stdlib/libstdcpp3.hpp

@@ -91,6 +91,14 @@
 # endif
 #endif
 
+//
+// Decide whether we have C++11 support turned on:
+//
+#if defined(__GXX_EXPERIMENTAL_CXX0X__) || (__cplusplus >= 201103)
+#  define BOOST_LIBSTDCXX11
+#endif
+//
+//  Decide which version of libstdc++ we have, normally
 //  stdlibc++ C++0x support is detected via __GNUC__, __GNUC_MINOR__, and possibly
 //  __GNUC_PATCHLEVEL__ at the suggestion of Jonathan Wakely, one of the stdlibc++
 //  developers. He also commented:
@@ -102,10 +110,46 @@
 //
 //  Another resource for understanding stdlibc++ features is:
 //  http://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#manual.intro.status.standard.200x
+//
+//  However, using the GCC version number fails when the compiler is clang since this
+//  only ever claims to emulate GCC-4.2, see https://svn.boost.org/trac/boost/ticket/7473
+//  for a long discussion on this issue.  What we can do though is use clang's __has_include
+//  to detect the presence of a C++11 header that was introduced with a specific GCC release.
+//  We still have to be careful though as many such headers were buggy and/or incomplete when
+//  first introduced, so we only check for headers that were fully featured from day 1, and then
+//  use that to infer the underlying GCC version:
+//
+#ifdef __clang__
+
+#if __has_include(<array>)
+#  define BOOST_LIBSTDCXX_VERSION 40300
+#endif
+#if __has_include(<ratio>)
+#  define BOOST_LIBSTDCXX_VERSION 40400
+#endif
+#if __has_include(<future>)
+#  define BOOST_LIBSTDCXX_VERSION 40500
+#endif
+#if __has_include(<typeindex>)
+#  define BOOST_LIBSTDCXX_VERSION 40600
+#endif
+#if __has_include(<chrono>)
+#  define BOOST_LIBSTDCXX_VERSION 40700
+#endif
+//
+//  GCC 4.8 and 9 add working versions of <atomic> and <regex> respectively.
+//  However, we have no test for these as the headers were present but broken
+//  in early GCC versions.
+//
+#endif
+
+#if !defined(BOOST_LIBSTDCXX_VERSION)
+#  define BOOST_LIBSTDCXX_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
+#endif
 
 //  C++0x headers in GCC 4.3.0 and later
 //
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 3) || !defined(__GXX_EXPERIMENTAL_CXX0X__)
+#if (BOOST_LIBSTDCXX_VERSION < 40300) || !defined(BOOST_LIBSTDCXX11)
 #  define BOOST_NO_CXX11_HDR_ARRAY
 #  define BOOST_NO_CXX11_HDR_TUPLE
 #  define BOOST_NO_CXX11_HDR_UNORDERED_MAP
@@ -115,7 +159,7 @@
 
 //  C++0x headers in GCC 4.4.0 and later
 //
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 4) || !defined(__GXX_EXPERIMENTAL_CXX0X__)
+#if (BOOST_LIBSTDCXX_VERSION < 40400) || !defined(BOOST_LIBSTDCXX11)
 #  define BOOST_NO_CXX11_HDR_CONDITION_VARIABLE
 #  define BOOST_NO_CXX11_HDR_FORWARD_LIST
 #  define BOOST_NO_CXX11_HDR_INITIALIZER_LIST
@@ -135,7 +179,7 @@
 
 //  C++0x features in GCC 4.5.0 and later
 //
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 5) || !defined(__GXX_EXPERIMENTAL_CXX0X__)
+#if (BOOST_LIBSTDCXX_VERSION < 40500) || !defined(BOOST_LIBSTDCXX11)
 #  define BOOST_NO_CXX11_NUMERIC_LIMITS
 #  define BOOST_NO_CXX11_HDR_FUTURE
 #  define BOOST_NO_CXX11_HDR_RANDOM
@@ -143,26 +187,26 @@
 
 //  C++0x features in GCC 4.6.0 and later
 //
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 6) || !defined(__GXX_EXPERIMENTAL_CXX0X__)
+#if (BOOST_LIBSTDCXX_VERSION < 40600) || !defined(BOOST_LIBSTDCXX11)
 #  define BOOST_NO_CXX11_HDR_TYPEINDEX
 #  define BOOST_NO_CXX11_ADDRESSOF
 #endif
 
 //  C++0x features in GCC 4.7.0 and later
 //
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7) || !defined(__GXX_EXPERIMENTAL_CXX0X__)
+#if (BOOST_LIBSTDCXX_VERSION < 40700) || !defined(BOOST_LIBSTDCXX11)
 // Note that although <chrono> existed prior to 4.7, "steady_clock" is spelled "monotonic_clock"
 // so 4.7.0 is the first truely conforming one.
 #  define BOOST_NO_CXX11_HDR_CHRONO
 #  define BOOST_NO_CXX11_ALLOCATOR
 #endif
-//  C++0x features in GCC 4.7.0 and later
+//  C++0x features in GCC 4.8.0 and later
 //
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8) || !defined(__GXX_EXPERIMENTAL_CXX0X__)
+#if (BOOST_LIBSTDCXX_VERSION < 40800) || !defined(BOOST_LIBSTDCXX11)
 // Note that although <atomic> existed prior to gcc 4.8 it was largely unimplemented for many types:
 #  define BOOST_NO_CXX11_HDR_ATOMIC
 #endif
-#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 9) || !defined(__GXX_EXPERIMENTAL_CXX0X__)
+#if (BOOST_LIBSTDCXX_VERSION < 40900) || !defined(BOOST_LIBSTDCXX11)
 // Although <regex> is present and compilable against, the actual implementation is not functional
 // even for the simplest patterns such as "\d" or "[0-9]". This is the case at least in gcc up to 4.8, inclusively.
 #  define BOOST_NO_CXX11_HDR_REGEX