Alternative solution for detecting libc++'s version.

[EricWF]

Summary

This patch presents an alternative approach to adding a <version> header and including it from <config>.

Instead of using a <__version> header this patch adds a __libcpp_version *file* not intended for use as a header, and leaves _LIBCPP_VERSION unchanged. This prevents the additional cost of including <__version> from <__config>. To prevent _LIBCPP_VERSION and __libcpp_version from getting out of sync we static_assert that the values match when building the library.

Additionally this patch adds a _LIBCPP_LIBRARY_VERSION macro for reporting the version of the dylib used. This is unrelated to @rsmith's original patch but I've been meaning to implement it for a while.

[rsmith]

This is perfect for my needs, thanks!

[EricWF]

r285382.

include/__config

 #else
 #define _LIBCPP_SAFE_STATIC
 #endif
 
 #if !__has_builtin(__builtin_addressof) && _GNUC_VER < 700
 #define _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
 #endif
 
+_LIBCPP_BEGIN_NAMESPACE_STD
+_LIBCPP_FUNC_VIS _LIBCPP_WEAK int __libcpp_library_version();
+_LIBCPP_END_NAMESPACE_STD
+
+#define _LIBCPP_LIBRARY_VERSION \
+ (_VSTD::__libcpp_library_version ? _VSTD::__libcpp_library_version() : -1)
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG

include/__libcpp_version

+4000
 No newline at end of file

src/libcpp_version.cpp

+#include "__config"
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// Test that _LIBCPP_VERSION and __libcpp_version are in sync.
+// The __libcpp_version file stores only a number representing the libc++
+// version so it can be easily parsed by clang.
+static_assert(_LIBCPP_VERSION ==
+#include "__libcpp_version"
+    , "version file does not match");
+
+int __libcpp_library_version() { return _LIBCPP_VERSION; }
+
+_LIBCPP_END_NAMESPACE_STD

test/libcxx/version.pass.cpp

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Test the _LIBCPP_VERSION and _LIBCPP_LIBRARY_VERSION macros
+
+#include <__config>
+
+#ifndef _LIBCPP_VERSION
+#error _LIBCPP_VERSION must be defined
+#endif
+
+#ifndef _LIBCPP_LIBRARY_VERSION
+#error _LIBCPP_LIBRARY_VERSION must be defined
+#endif
+
+#include <cassert>
+
+int main() {
+  assert(_LIBCPP_VERSION == _LIBCPP_LIBRARY_VERSION);
+  assert(std::__libcpp_library_version);
+  assert(_LIBCPP_LIBRARY_VERSION == std::__libcpp_library_version());
+}