Let makeSemiFutureWith accept functions returning Future

Summary: [Folly] Let `makeSemiFutureWith` accept functions returning `Future`, joining what would otherwise be the resulting `SemiFuture<Future<...>>` into `SemiFuture<...>`.

Reviewed By: LeeHowes

Differential Revision: D8505834

fbshipit-source-id: 10b626fae00b5002877295b59716066f089cb6a8

folly/futures/Future-inl.h

@@ -676,10 +676,11 @@ SemiFuture<typename std::decay<T>::type> makeSemiFuture(T&& t) {
 
 // makeSemiFutureWith(SemiFuture<T>()) -> SemiFuture<T>
 template <class F>
-typename std::
-    enable_if<isSemiFuture<invoke_result_t<F>>::value, invoke_result_t<F>>::type
-    makeSemiFutureWith(F&& func) {
-  using InnerType = typename isSemiFuture<invoke_result_t<F>>::Inner;
+typename std::enable_if<
+    isFutureOrSemiFuture<invoke_result_t<F>>::value,
+    SemiFuture<typename invoke_result_t<F>::value_type>>::type
+makeSemiFutureWith(F&& func) {
+  using InnerType = typename isFutureOrSemiFuture<invoke_result_t<F>>::Inner;
   try {
     return std::forward<F>(func)();
   } catch (std::exception& e) {
@@ -695,7 +696,7 @@ typename std::
 // makeSemiFutureWith(void()) -> SemiFuture<Unit>
 template <class F>
 typename std::enable_if<
-    !(isSemiFuture<invoke_result_t<F>>::value),
+    !(isFutureOrSemiFuture<invoke_result_t<F>>::value),
     SemiFuture<lift_unit_t<invoke_result_t<F>>>>::type
 makeSemiFutureWith(F&& func) {
   using LiftedResult = lift_unit_t<invoke_result_t<F>>;

folly/futures/helpers.h

@@ -143,15 +143,16 @@ SemiFuture<Unit> makeSemiFuture();
 
 // makeSemiFutureWith(SemiFuture<T>()) -> SemiFuture<T>
 template <class F>
-typename std::
-    enable_if<isSemiFuture<invoke_result_t<F>>::value, invoke_result_t<F>>::type
-    makeSemiFutureWith(F&& func);
+typename std::enable_if<
+    isFutureOrSemiFuture<invoke_result_t<F>>::value,
+    SemiFuture<typename invoke_result_t<F>::value_type>>::type
+makeSemiFutureWith(F&& func);
 
 // makeSemiFutureWith(T()) -> SemiFuture<T>
 // makeSemiFutureWith(void()) -> SemiFuture<Unit>
 template <class F>
 typename std::enable_if<
-    !(isSemiFuture<invoke_result_t<F>>::value),
+    !(isFutureOrSemiFuture<invoke_result_t<F>>::value),
     SemiFuture<typename lift_unit<invoke_result_t<F>>::type>>::type
 makeSemiFutureWith(F&& func);
 

folly/futures/test/SemiFutureTest.cpp

@@ -317,6 +317,10 @@ TEST(SemiFuture, makeSemiFuture) {
   EXPECT_NO_THROW(makeSemiFutureWith(failfunf));
   EXPECT_THROW(makeSemiFutureWith(failfunf).value(), eggs_t);
 
+  auto futurefun = [] { return makeFuture<int>(44); };
+  EXPECT_TYPE(makeSemiFutureWith(futurefun), SemiFuture<int>);
+  EXPECT_EQ(44, makeSemiFutureWith(futurefun).value());
+
   EXPECT_TYPE(makeSemiFuture(), SemiFuture<Unit>);
 }