Decouple Invoker from BindState

base/bind_helpers.h

Index: base/bind_helpers.h
diff --git a/base/bind_helpers.h b/base/bind_helpers.h
index dd16fec758a89832b580c232e29f39343b4d5067..bd3f3a58bcd3f2352324b5a1bf81a0b6459ed2cf 100644
--- a/base/bind_helpers.h
+++ b/base/bind_helpers.h
@@ -462,11 +462,11 @@ 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>
+template <bool IsMethod, typename ArgsTuple>

[dcheng, 2016/06/27 07:57:22]

Just for my own understanding, why did this template argument change?

[tzik, 2016/06/27 08:43:11]

That is for utility for the caller, Invoker::RunImpl, which has the tuple of
bound arguments, but doesn't have unpacked tuple elements.

Maybe, it's less confusing to do it in the caller side rather than modifying
IsWeakMethod?

 struct IsWeakMethod : std::false_type {};
 
 template <typename T, typename... Args>
-struct IsWeakMethod<true, T, Args...> : IsWeakReceiver<T> {};
+struct IsWeakMethod<true, std::tuple<T, Args...>> : IsWeakReceiver<T> {};
 
 // Packs a list of types to hold them in a single type.
 template <typename... Types>
@@ -549,19 +549,25 @@ struct MakeFunctionTypeImpl<R, TypeList<Args...>> {
 template <typename R, typename ArgList>
 using MakeFunctionType = typename MakeFunctionTypeImpl<R, ArgList>::Type;
 
-// Used for ExtractArgs.
+// Used for ExtractArgs and ExtractReturnType.
 template <typename Signature>
 struct ExtractArgsImpl;
 
 template <typename R, typename... Args>
 struct ExtractArgsImpl<R(Args...)> {
-  using Type = TypeList<Args...>;
+  using ReturnType = R;
+  using ArgsList = TypeList<Args...>;
 };
 
 // A type-level function that extracts function arguments into a TypeList.
 // E.g. ExtractArgs<R(A, B, C)> is evaluated to TypeList<A, B, C>.
 template <typename Signature>
-using ExtractArgs = typename ExtractArgsImpl<Signature>::Type;
+using ExtractArgs = typename ExtractArgsImpl<Signature>::ArgsList;
+
+// A type-level function that extracts the return type of a function.
+// E.g. ExtractReturnType<R(A, B, C)> is evaluated to R.
+template <typename Signature>
+using ExtractReturnType = typename ExtractArgsImpl<Signature>::ReturnType;
 
 }  // namespace internal