Make onTimeout use thenError which maintains executor.

Summary: Make onTimeout call thenError which maintains the executor rather than onError which does not.

Reviewed By: yfeldblum

Differential Revision: D8051906

fbshipit-source-id: 41ca5f657a441ff8eaacdfab6d4cfb45d5e5f0c2

folly/futures/Future-inl.h

@@ -1067,8 +1067,8 @@ Future<T> Future<T>::ensure(F&& func) {
 template <class T>
 template <class F>
 Future<T> Future<T>::onTimeout(Duration dur, F&& func, Timekeeper* tk) {
-  return within(dur, tk).onError(
-      [funcw = std::forward<F>(func)](FutureTimeout const&) mutable {
+  return within(dur, tk).template thenError<FutureTimeout>(
+      [funcw = std::forward<F>(func)](auto const&) mutable {
         return std::forward<F>(funcw)();
       });
 }

folly/futures/test/SemiFutureTest.cpp

@@ -946,7 +946,7 @@ TEST(SemiFuture, onError) {
   {
     auto f = makeSemiFuture()
                  .defer([] { return 42; })
-                 .deferError<eggs_t>([&](eggs_t& /* e */) {
+                 .deferError<eggs_t>([&](auto& /* e */) {
                    flag();
                    return -1;
                  });