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 (defined(__GNUC__) && __GNUC_ < 5) || \

[danakj, 2017/01/05 16:30:15]

I agree that it's odd to check the compiler version not the library version
here. If some toolchain is using an older gcc but a newer library, then they
will fall back to our own implementation of is_trivially_copyable. But I think
a) using a new lib with older compiler is probably not supported and wouldn't
work anyways b) falling back in this case still does something reasonable..ish.

Can you improve the comment to clarify about the mismatch between checking
compiler vs library versions.

[sof, 2017/01/05 18:59:44]

ok, tried to do so.

ABI skew is one reason why it makes some sense to compile with a consistent
(compiler,library) pairing, esp so with gcc5's changes in that area (
https://gcc.gnu.org/gcc-5/changes.html#libstdcxx )

The version testing here mirrors how we do it on the clang side, btw --

 http://llvm.org/docs/doxygen/html/type__traits_8h_source.html#l00030

 

+    (defined(__GLIBCXX__) && __GLIBCXX__ == CR_GLIBCXX_5_0_0)
 #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_