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 590d788b96e0a650e22065884d4c8dc41d63531e..bf9844b1e6d97fa3686e24c6adb666821a68daf7 100644
--- a/base/bind_helpers.h
+++ b/base/bind_helpers.h
@@ -458,15 +458,15 @@ 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 {};
+template <bool IsMethod, typename ArgsTuple>
+struct IsWeakMethod : std::false_type {};
 
 template <typename T, typename... Args>
-struct IsWeakMethod<true, WeakPtr<T>, Args...> : public std::true_type {};
+struct IsWeakMethod<true, std::tuple<WeakPtr<T>, Args...>> : std::true_type {};
 
 template <typename T, typename... Args>
-struct IsWeakMethod<true, ConstRefWrapper<WeakPtr<T>>, Args...>
-    : public std::true_type {};
+struct IsWeakMethod<true, std::tuple<ConstRefWrapper<WeakPtr<T>>, Args...>>
+    : std::true_type {};
 
 
 // Packs a list of types to hold them in a single type.
@@ -550,19 +550,25 @@ 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