fix incorrect libstdc++ from GCC >= 5.1 check

base/bit_cast.h

 // Copyright 2016 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_BIT_CAST_H_
 #define BASE_BIT_CAST_H_
 
 #include <string.h>
 #include <type_traits>
 
@@ skipping 46 matching lines @@
 // Fortunately memcpy() is very fast.  In optimized mode, compilers replace
 // calls to memcpy() with inline object code when the size argument is a
 // compile-time constant.  On a 32-bit system, memcpy(d,s,4) compiles to one
 // load and one store, and memcpy(d,s,8) compiles to two loads and two stores.
 
 template <class Dest, class Source>
 inline Dest bit_cast(const Source& source) {
   static_assert(sizeof(Dest) == sizeof(Source),
                 "bit_cast requires source and destination to be the same size");
 
-#if ((defined(__GLIBCXX__) && (__GLIBCXX__ >= 20150422)) || \

[Mostyn Bramley-Moore, 2016/04/05 22:13:48]

This is the value from G++ 4.8, I believe.

+#if ((defined(__GLIBCXX__) && (__GLIBCXX__ >= 20151010ul)) || \

[Mostyn Bramley-Moore, 2016/04/05 22:13:48]

This is actually the value from G++ 5.2, just a placeholder until I can find the
value corresponding to 5.1.

[JF, 2016/04/05 22:21:29]

IIRC the docs are:

From: https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html

  Incremental bumping of a library pre-defined macro. For releases before 3.4.0,
the macro is __GLIBCPP__. For later releases, it's __GLIBCXX__. (The libstdc++
project generously changed from CPP to CXX throughout its source to allow the
"C" pre-processor the CPP macro namespace.) These macros are defined as the date
the library was released, in compressed ISO date format, as an unsigned long.

  This macro is defined in the file "c++config" in the
"libstdc++-v3/include/bits" directory. (Up to GCC 4.1.0, it was changed every
night by an automated script. Since GCC 4.1.0, it is the same value as
gcc/DATESTAMP.)


And from: https://gcc.gnu.org/releases.html

  GCC 5.1	April 22, 2015


It sounds like that doesn't work for you?

[Mostyn Bramley-Moore, 2016/04/05 22:26:05]

gcc/DATESTAMP in the GCC 4.8.5 tarball is 20150623, larger than the value from
the GCC 5.1 tarball.

      defined(_LIBCPP_VERSION))
   // GCC 5.1 contains the first libstdc++ with is_trivially_copyable.
   // Assume libc++ Just Works: is_trivially_copyable added on May 13th 2011.
   static_assert(std::is_trivially_copyable<Dest>::value,
                 "non-trivially-copyable bit_cast is undefined");
   static_assert(std::is_trivially_copyable<Source>::value,
                 "non-trivially-copyable bit_cast is undefined");
 #elif __has_feature(is_trivially_copyable)
   // The compiler supports an equivalent intrinsic.
   static_assert(__is_trivially_copyable(Dest),
@@ skipping 11 matching lines @@
 #else
   // Do nothing, let the bots handle it.
 #endif
 
   Dest dest;
   memcpy(&dest, &source, sizeof(dest));
   return dest;
 }
 
 #endif  // BASE_BIT_CAST_H_