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 <type_traits>
 #include <utility>
 
 #include "build/build_config.h"
 
+// libc++ uses namespace std { inline namespace <libc++-version> { } }
+// so redeclaration in namespace std does not work.
+#ifdef _LIBCPP_BEGIN_NAMESPACE_STD
+_LIBCPP_BEGIN_NAMESPACE_STD
+#else
+namespace std {

[Nico, 2016/05/25 13:56:59]

defining stuff in namespace std in user code has undefined behavior, we
shouldn't do that 

[alshabalin, 2016/05/26 09:18:46]

Done. Removed in favor of macro from
https://codereview.chromium.org/2001783002/patch/120001/130004

+#endif  // _LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class T>
+struct is_trivially_destructible;
+
+template <class T>
+struct has_trivial_destructor;
+
+#ifdef _LIBCPP_END_NAMESPACE_STD
+_LIBCPP_END_NAMESPACE_STD
+#else
+}  // namespace std
+#endif  // _LIBCPP_END_NAMESPACE_STD
+
 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 51 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> {
 };
 
+// is_trivially_destructible
+
+namespace internal {

[danakj, 2016/05/25 20:02:09]

Sorry I've been thinking about how to do this in a way that isn't defining stuff
in std, which is bad as nico says.

Can you do something like waht this does instead (maybe pull the #if checks out
into a single define so we can use that for both)?

https://codereview.chromium.org/2001783002/diff/100001/base/template_util.h

[alshabalin, 2016/05/26 09:18:46]

Done. Defined a macro CR_USE_FALLBACKS_FOR_OLD_GLIBCXX for that.

+
+// From C++17.
+template <typename... Ts>
+struct make_void {
+  typedef void type;
+};
+
+template <typename... Ts>
+using void_t = typename make_void<Ts...>::type;
+
+template <class T, class = void>
+struct IsTriviallyDestructibleImpl : public std::has_trivial_destructor<T> {};
+
+template <class T>
+struct IsTriviallyDestructibleImpl<
+    T,
+    void_t<typename std::is_trivially_destructible<T>::type>>
+    : public std::is_trivially_destructible<T> {};
+
+}  // namespace internal
+
+// TODO(crbug.com/554293): Remove this when all platforms have this in the std
+// namespace.
+template <class T>
+struct is_trivially_destructible
+    : public internal::IsTriviallyDestructibleImpl<T> {};
+
 }  // namespace base
 
 #endif  // BASE_TEMPLATE_UTIL_H_