styleguide: Allow enable_if, conditional, and other type_traits.

base/callback_internal.h

 // Copyright (c) 2012 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.
 
 // This file contains utility functions and classes that help the
 // implementation, and management of the Callback objects.
 
 #ifndef BASE_CALLBACK_INTERNAL_H_
 #define BASE_CALLBACK_INTERNAL_H_
 
 #include <stddef.h>
+#include <type_traits>
 
 #include "base/atomic_ref_count.h"
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/template_util.h"
 
 namespace base {
 namespace internal {
@@ skipping 76 matching lines @@
   template <typename U>
   static YesType Test(const typename U::MoveOnlyTypeForCPP03*);
 
   template <typename U>
   static NoType Test(...);
 
   static const bool value = sizeof((Test<T>(0))) == sizeof(YesType) &&
                             !is_const<T>::value;
 };
 
-// Returns |Then| as SelectType::Type if |condition| is true. Otherwise returns
-// |Else|.
-template <bool condition, typename Then, typename Else>
-struct SelectType {
-  typedef Then Type;
-};
-
-template <typename Then, typename Else>
-struct SelectType<false, Then, Else> {
-  typedef Else Type;
-};
-
 template <typename>
 struct CallbackParamTraitsForMoveOnlyType;
 
 template <typename>
 struct CallbackParamTraitsForNonMoveOnlyType;
 
 // TODO(tzik): Use a default parameter once MSVS supports variadic templates
 // with default values.
 // http://connect.microsoft.com/VisualStudio/feedbackdetail/view/957801/compilation-error-with-variadic-templates
 //
 // This is a typetraits object that's used to take an argument type, and
 // extract a suitable type for storing and forwarding arguments.
 //
 // In particular, it strips off references, and converts arrays to
 // pointers for storage; and it avoids accidentally trying to create a
 // "reference of a reference" if the argument is a reference type.
 //
 // This array type becomes an issue for storage because we are passing bound
 // parameters by const reference. In this case, we end up passing an actual
 // array type in the initializer list which C++ does not allow.  This will
 // break passing of C-string literals.
 template <typename T>
 struct CallbackParamTraits
-    : SelectType<IsMoveOnlyType<T>::value,
+    : std::conditional<IsMoveOnlyType<T>::value,
          CallbackParamTraitsForMoveOnlyType<T>,
-         CallbackParamTraitsForNonMoveOnlyType<T> >::Type {
+         CallbackParamTraitsForNonMoveOnlyType<T>>::type {
 };
 
 template <typename T>
 struct CallbackParamTraitsForNonMoveOnlyType {
   typedef const T& ForwardType;
   typedef T StorageType;
 };
 
 // The Storage should almost be impossible to trigger unless someone manually
 // specifies type of the bind parameters.  However, in case they do,
@@ skipping 66 matching lines @@
 
 template <typename T>
 typename enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward(T& t) {
   return t.Pass();
 }
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_INTERNAL_H_