Fix evaluation-order assumption in futures varidic collect

Summary: [Folly] Fix evaluation-order assumption in futures varidic collect - argument evaluation order is unspecified so it must not be relied on when side-effects must occur in a specific order.

Reviewed By: lewissbaker

Differential Revision: D19347873

fbshipit-source-id: db4326d995a241619f96ff2bacdf5a5fa2332091

folly/futures/Future-inl.h

@@ -1348,13 +1348,13 @@ template <typename... Ts>
 void stealDeferredExecutorsVariadic(
     std::vector<futures::detail::DeferredWrapper>& executors,
     Ts&... ts) {
-  auto foreach = [&](auto& future) {
+  foreach(
+      [&](auto, auto& future) {
         if (auto executor = stealDeferredExecutor(future)) {
           executors.push_back(std::move(executor));
         }
-    return folly::unit;
-  };
-  [](...) {}(foreach(ts)...);
+      },
+      ts...);
 }
 
 template <class InputIterator>