Introduce base::IsWeakReceiver for base::Bind to support external weak pointers

base/bind_helpers.h

Index: base/bind_helpers.h
diff --git a/base/bind_helpers.h b/base/bind_helpers.h
index 590d788b96e0a650e22065884d4c8dc41d63531e..1e0d8f3f6b60005310a39dec414cd190046d5f25 100644
--- a/base/bind_helpers.h
+++ b/base/bind_helpers.h
@@ -170,6 +170,10 @@
 #include "build/build_config.h"
 
 namespace base {
+
+template <typename T>
+struct IsWeakReceiver;
+
 namespace internal {
 
 // Use the Substitution Failure Is Not An Error (SFINAE) trick to inspect T
@@ -459,15 +463,10 @@ T Unwrap(PassedWrapper<T>& o) {
 // The first argument should be the type of the object that will be received by
 // the method.
 template <bool IsMethod, typename... Args>
-struct IsWeakMethod : public std::false_type {};
+struct IsWeakMethod : std::false_type {};
 
 template <typename T, typename... Args>
-struct IsWeakMethod<true, WeakPtr<T>, Args...> : public std::true_type {};
-
-template <typename T, typename... Args>
-struct IsWeakMethod<true, ConstRefWrapper<WeakPtr<T>>, Args...>
-    : public std::true_type {};
-
+struct IsWeakMethod<true, T, Args...> : IsWeakReceiver<T> {};
 
 // Packs a list of types to hold them in a single type.
 template <typename... Types>
@@ -627,6 +626,25 @@ void DeletePointer(T* obj) {
   delete obj;
 }
 
+// An injection point to control |this| pointer behavior on a method invocation.
+// If IsWeakReceiver<> is true_type for |T| and |T| is used for a receiver of a
+// method, base::Bind cancels the method invocation if the receiver is null.

[danakj, 2016/06/10 23:36:18]

"if the receiver is tested as false" may be more accurate. Or something similar.

[tzik, 2016/06/14 12:06:10]

Done.

+// E.g. Foo::bar() is not called:
+//   struct Foo : base::SupportsWeakPtr<Foo> {
+//     void bar() {}
+//   };
+//
+//   WeakPtr<Foo> oo = nullptr;
+//   base::Bind(&Foo::bar, oo).Run();
+template <typename T>
+struct IsWeakReceiver : std::false_type {};
+
+template <typename T>
+struct IsWeakReceiver<WeakPtr<T>> : std::true_type {};
+
+template <typename T>
+struct IsWeakReceiver<internal::ConstRefWrapper<T>> : IsWeakReceiver<T> {};

[danakj, 2016/06/10 23:36:17]

nit: Does it compile if you put this above the WeakPtr one? I'm guess you intend
to add other things to the list, so grouping them at the bottom of the set of
defns might be nice.

[tzik, 2016/06/14 12:06:10]

Done.

+
 }  // namespace base
 
 #endif  // BASE_BIND_HELPERS_H_