Support move-only type on base::Callback::Run

base/callback.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.
 
 #ifndef BASE_CALLBACK_H_
 #define BASE_CALLBACK_H_
 
 #include "base/callback_forward.h"
 #include "base/callback_internal.h"
 #include "base/template_util.h"
@@ skipping 368 matching lines @@
     PolymorphicInvoke invoke_func =
         &internal::BindState<Runnable, BindRunType, BoundArgsType...>
             ::InvokerType::Run;
     polymorphic_invoke_ = reinterpret_cast<InvokeFuncStorage>(invoke_func);
   }
 
   bool Equals(const Callback& other) const {
     return CallbackBase::Equals(other);
   }
 
-  R Run(typename internal::CallbackParamTraits<Args>::ForwardType... args)
-      const {
+  R Run(Args... args) const {

[dcheng, 2016/03/02 00:25:45]

So this costs an extra copy now, since we don't have enough information to do
perfect forwarding: this was determined when we instantiated the callback, when
we don't yet know how Run() will be called.

We can't make this use forwarding references, because we need to instantiate a
reference to this in the ctor. So this is the best we can do.

(Maybe we could have a comment in the CL description or in the code or in both
that documents why the seemingly-obvious perfect forwarding pattern doesn't work
here?)

[tzik, 2016/03/02 01:04:21]

Acknowledged. Updating.

[tzik, 2016/03/02 12:44:51]

Done.

     PolymorphicInvoke f =
         reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
-
-    return f(bind_state_.get(), internal::CallbackForward(args)...);
+    return f(bind_state_.get(), std::forward<Args>(args)...);
   }
 
  private:
-  using PolymorphicInvoke =
-      R(*)(internal::BindStateBase*,
-           typename internal::CallbackParamTraits<Args>::ForwardType...);
+  using PolymorphicInvoke = R (*)(internal::BindStateBase*, Args&&...);
 };
 
 }  // namespace base
 
 #endif  // BASE_CALLBACK_H_