Disallow redundant Bind calls.

base/bind_internal.h

Index: base/bind_internal.h
diff --git a/base/bind_internal.h b/base/bind_internal.h
index 17f78a3d67b37f07a87df2385c1dd5ca08fff6bf..d16109e4243d174528c26b372aa07b6eacd792e8 100644
--- a/base/bind_internal.h
+++ b/base/bind_internal.h
@@ -422,6 +422,23 @@ struct CancellationChecker<BindState<Callback<Signature>, BoundArgs...>> {
   }
 };
 
+// Template helpers to detect using Bind() on a base::Callback without any
+// additional arguments. In that case, the original base::Callback object should
+// just be directly used.
+template <typename Functor, typename... BoundArgs>
+struct BindingCallbackWithNoArgs {
+  static constexpr bool value = false;
+};
+
+template <typename Signature,
+          typename... BoundArgs,
+          CopyMode copy_mode,
+          RepeatMode repeat_mode>
+struct BindingCallbackWithNoArgs<Callback<Signature, copy_mode, repeat_mode>,
+                                 BoundArgs...> {
+  static constexpr bool value = sizeof...(BoundArgs) == 0;
+};
+
 // BindState<>
 //
 // This stores all the state passed into Bind().
@@ -439,7 +456,12 @@ struct BindState final : BindStateBase {
       : BindState(IsCancellable{},
                   invoke_func,
                   std::forward<ForwardFunctor>(functor),
-                  std::forward<ForwardBoundArgs>(bound_args)...) {}
+                  std::forward<ForwardBoundArgs>(bound_args)...) {
+    static_assert(!BindingCallbackWithNoArgs<Functor, BoundArgs...>::value,
+                  "Attempting to bind a base::Callback with no additional "
+                  "arguments: save a heap allocation and use the original "
+                  "base::Callback object");
+  }
 
   Functor functor_;
   std::tuple<BoundArgs...> bound_args_;