Imported Optional from Chromium with some modifications.

webrtc/base/template_util.h

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 // Borrowed from Chromium's src/base/template_util.h.
 
 #ifndef WEBRTC_BASE_TEMPLATE_UTIL_H_
 #define WEBRTC_BASE_TEMPLATE_UTIL_H_
 
+#include <type_traits>  // For is_trivially_destructible
 #include <stddef.h>  // For size_t.
 
 namespace rtc {
 
 // Template definitions from tr1.
 
 template<class T, T v>
 struct integral_constant {
   static const T value = v;
   typedef T value_type;
@@ skipping 89 matching lines @@
                         sizeof(internal::YesType)> {
 };
 
 template <typename T>
 struct is_class
     : integral_constant<bool,
                         sizeof(internal::IsClassHelper::Test<T>(0)) ==
                             sizeof(internal::YesType)> {
 };
 
+// This hacks around libstdc++ 4.6 missing stuff in type_traits, while we need

[ossu, 2016/06/17 14:51:48]

These are necessary to implement the partially specialized versions of
OptionalStorage. If we don't care enough about propagating trivial
destructability with optionals (I'm guessing it's useful if I use Optional a
lot) I can remove this as well. For now, I think it should remain in.

+// 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
+
+// TODO(crbug.com/554293): Remove this when all platforms have this in the std
+// namespace.
+#if defined(CR_USE_FALLBACKS_FOR_OLD_GLIBCXX)
+template <class T>
+using is_trivially_destructible = std::has_trivial_destructor<T>;
+#else
+template <class T>
+using is_trivially_destructible = std::is_trivially_destructible<T>;
+#endif
+
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_TEMPLATE_UTIL_H_