Make base::Optional trivially destructible when possible.

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
+
 namespace base {
 
 template <class T> struct is_non_const_reference : std::false_type {};
 template <class T> struct is_non_const_reference<T&> : std::true_type {};
 template <class T> struct is_non_const_reference<const T&> : std::false_type {};
 
 // is_assignable
 
 namespace internal {
 
@@ skipping 61 matching lines @@
 // is_move_assignable is true if a T&& is assignable to a T&.
 // TODO(crbug.com/554293): Remove this when all platforms have this in the std
 // namespace.
 template <class T>
 struct is_move_assignable
     : public is_assignable<typename std::add_lvalue_reference<T>::type,
                            const typename std::add_rvalue_reference<T>::type> {
 };
 
 // underlying_type produces the integer type backing an enum type.
-// This hacks around libstdc++ 4.6 missing std::underlying_type, while we need
-// to support it.
 // TODO(crbug.com/554293): Remove this when all platforms have this in the std
 // namespace.
-#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)
+#if defined(CR_USE_FALLBACKS_FOR_OLD_GLIBCXX)
 template <typename T>
 struct underlying_type {
   using type = __underlying_type(T);
 };
 #else
 template <typename T>
 using underlying_type = std::underlying_type<T>;
 #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 base
 
+#undef CR_USE_FALLBACKS_FOR_OLD_GLIBCXX
+
 #endif  // BASE_TEMPLATE_UTIL_H_