Fix MSVC build

Summary: I broke the build because MSVC versions prior to Visual Studio 2017 Update 7 failed to compile it :)

Reviewed By: yfeldblum

Differential Revision: D8107563

fbshipit-source-id: 0c8f97325fd76a56d335ff71c9b969ad2bfaa5a1

folly/futures/Future-inl.h

@@ -1217,8 +1217,9 @@ collectAllSemiFuture(Fs&&... fs) {
 }
 
 template <typename... Fs>
-auto collectAll(Fs&&... fs) -> Future<typename decltype(
-    collectAllSemiFuture(std::forward<Fs&&>(fs)...))::value_type> {
+Future<typename futures::detail::CollectAllVariadicContext<
+    typename std::decay<Fs>::type::value_type...>::type::value_type>
+collectAll(Fs&&... fs) {
   return collectAllSemiFuture(std::forward<Fs>(fs)...).toUnsafeFuture();
 }
 

folly/futures/helpers.h

@@ -344,8 +344,9 @@ typename futures::detail::CollectAllVariadicContext<
 collectAllSemiFuture(Fs&&... fs);
 
 template <typename... Fs>
-auto collectAll(Fs&&... fs) -> Future<typename decltype(
-    collectAllSemiFuture(std::forward<Fs&&>(fs)...))::value_type>;
+Future<typename futures::detail::CollectAllVariadicContext<
+    typename std::decay<Fs>::type::value_type...>::type::value_type>
+collectAll(Fs&&... fs);
 
 /// Like collectAll, but will short circuit on the first exception. Thus, the
 /// type of the returned Future is std::vector<T> instead of