Add a SemiFuture variant of collect

Summary: At least make it possible to do things correctly.

Reviewed By: yfeldblum, LeeHowes

Differential Revision: D14426753

fbshipit-source-id: 63b3572175224dbbda828981a5c9f067a5e35f33

folly/futures/Future-inl.h

@@ -1572,9 +1572,9 @@ collectAll(InputIterator first, InputIterator last) {
 
 // TODO(T26439406): Make return SemiFuture
 template <class InputIterator>
-Future<std::vector<
+SemiFuture<std::vector<
     typename std::iterator_traits<InputIterator>::value_type::value_type>>
-collect(InputIterator first, InputIterator last) {
+collectSemiFuture(InputIterator first, InputIterator last) {
   using F = typename std::iterator_traits<InputIterator>::value_type;
   using T = typename F::value_type;
 
@@ -1611,15 +1611,22 @@ collect(InputIterator first, InputIterator last) {
       }
     });
   }
-  return ctx->p.getSemiFuture().via(&InlineExecutor::instance());
+  return ctx->p.getSemiFuture();
+}
+
+template <class InputIterator>
+Future<std::vector<
+    typename std::iterator_traits<InputIterator>::value_type::value_type>>
+collect(InputIterator first, InputIterator last) {
+  return collectSemiFuture(first, last).toUnsafeFuture();
 }
 
 // collect (variadic)
 
 // TODO(T26439406): Make return SemiFuture
 template <typename... Fs>
-Future<std::tuple<typename remove_cvref_t<Fs>::value_type...>> collect(
-    Fs&&... fs) {
+SemiFuture<std::tuple<typename remove_cvref_t<Fs>::value_type...>>
+collectSemiFuture(Fs&&... fs) {
   using Result = std::tuple<typename remove_cvref_t<Fs>::value_type...>;
   struct Context {
     ~Context() {
@@ -1646,7 +1653,13 @@ Future<std::tuple<typename remove_cvref_t<Fs>::value_type...>> collect(
         });
       },
       static_cast<Fs&&>(fs)...);
-  return ctx->p.getSemiFuture().via(&InlineExecutor::instance());
+  return ctx->p.getSemiFuture();
+}
+
+template <typename... Fs>
+Future<std::tuple<typename remove_cvref_t<Fs>::value_type...>> collect(
+    Fs&&... fs) {
+  return collectSemiFuture(std::forward<Fs>(fs)...).toUnsafeFuture();
 }
 
 // collectAny (iterator)