Enable type_traits fallback for all < gcc 5.0 releases.

base/template_util.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TEMPLATE_UTIL_H_
 #define BASE_TEMPLATE_UTIL_H_
 
 #include <stddef.h>
 #include <iosfwd>
 #include <type_traits>
 #include <utility>
 
 #include "build/build_config.h"
 
 // This hacks around libstdc++ 4.6 missing stuff in type_traits, while we need
 // to support it.
 #define CR_GLIBCXX_4_7_0 20120322
 #define CR_GLIBCXX_4_5_4 20120702
 #define CR_GLIBCXX_4_6_4 20121127
 #if defined(__GLIBCXX__) &&                                               \
     (__GLIBCXX__ < CR_GLIBCXX_4_7_0 || __GLIBCXX__ == CR_GLIBCXX_4_5_4 || \
      __GLIBCXX__ == CR_GLIBCXX_4_6_4)
 #define CR_USE_FALLBACKS_FOR_OLD_GLIBCXX
 #endif
 
 // Some versions of libstdc++ have partial support for type_traits, but misses
-// a smaller subset while removing some of the older non-standard stuff.
-#define CR_GLIBCXX_4_8_4 20141219
-#define CR_GLIBCXX_4_9_2 20150426
+// a smaller subset while removing some of the older non-standard stuff. Assume
+// that all versions below 5.0 fall in this category, along with one 5.0
+// experimental release.
 #define CR_GLIBCXX_5_0_0 20150123
-#if defined(__GLIBCXX__) &&                                                \
-    (__GLIBCXX__ == CR_GLIBCXX_4_8_4 || __GLIBCXX__ == CR_GLIBCXX_4_9_2 || \
-     __GLIBCXX__ == CR_GLIBCXX_5_0_0)
+#if _GNUC_VER < 500 || (defined(__GLIBCXX__) && __GLIBCXX__ == CR_GLIBCXX_5_0_0)

[Mostyn Bramley-Moore, 2017/01/05 10:53:54]

Where is _GNUC_VER defined?  It doesn't appear to be predefined in my tests with
g++ 4.8 and 5.4.0.

Also, I think this feature is libstdc++ thing, which is not strictly coupled to
the compiler version (ie you can use a brand new libstdc++ with an old compiler
just fine).

[sof, 2017/01/05 10:57:38]

It's used below in this file, so I assume it is well-defined and safe to use in
this context?

As we're testing against older libstdc++ here, your concern doesn't seem to
hold. You could use a newer g++, but why would you want to use an old libstdc++
with it?

[Mostyn Bramley-Moore, 2017/01/05 12:26:32]

It looks like _GNUC_VER is a libcxx macro, and since it's undefined here then
the value is 0 in libstdc++ builds.  Which would mean that the __GLIBCXX__
checks are possibly never used.
I don't personally use newer g++ with older libstdc++, but I know that Google
does this (or did at some point), in order to produce chrome builds for eg old
debian releases.

[sof, 2017/01/05 12:35:28]

It's been subsequently updated to only rely on __GNUC__; we're only after the
major version here after all.

 #define CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
 #endif
 
 // This hacks around using gcc with libc++ which has some incompatibilies.
 // - is_trivially_* doesn't work: https://llvm.org/bugs/show_bug.cgi?id=27538
 // TODO(danakj): Remove this when android builders are all using a newer version
 // of gcc, or the android ndk is updated to a newer libc++ that works with older
 // gcc versions.
 #if !defined(__clang__) && defined(_LIBCPP_VERSION)
 #define CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX
@@ skipping 145 matching lines @@
 using is_trivially_copyable = std::is_trivially_copyable<T>;
 #endif
 
 }  // namespace base
 
 #undef CR_USE_FALLBACKS_FOR_OLD_GLIBCXX
 #undef CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX
 #undef CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
 
 #endif  // BASE_TEMPLATE_UTIL_H_