fix `folly::via` for callables returning SemiFuture

Summary:
When passing a callable to `folly::via` that returns a SemiFuture,
via should return a Future. The implementation does the correct thing
but the declaration's return type is not correct. This diff fixes that.

Reviewed By: yfeldblum

Differential Revision: D9219041

fbshipit-source-id: e61544231e7846c713ff43394c3bf5d1aae18233

folly/futures/Future-inl.h

@@ -1297,8 +1297,8 @@ Future<T>::onError(F&& func) {
 }
 
 template <class Func>
-auto via(Executor* x, Func&& func)
-    -> Future<typename isFuture<decltype(std::declval<Func>()())>::Inner> {
+auto via(Executor* x, Func&& func) -> Future<
+    typename isFutureOrSemiFuture<decltype(std::declval<Func>()())>::Inner> {
   // TODO make this actually more performant. :-P #7260175
   return via(x).then(std::forward<Func>(func));
 }

folly/futures/helpers.h

@@ -274,8 +274,8 @@ inline Future<Unit> via(
 /// This is semantically equivalent to via(executor).then(func), but
 /// easier to read and slightly more efficient.
 template <class Func>
-auto via(Executor*, Func&& func)
-    -> Future<typename isFuture<decltype(std::declval<Func>()())>::Inner>;
+auto via(Executor*, Func&& func) -> Future<
+    typename isFutureOrSemiFuture<decltype(std::declval<Func>()())>::Inner>;
 
 /** When all the input Futures complete, the returned Future will complete.
   Errors do not cause early termination; this Future will always succeed

folly/futures/test/ViaTest.cpp

@@ -603,6 +603,11 @@ TEST(ViaFunc, future) {
             .getVia(&x));
 }
 
+TEST(ViaFunc, semi_future) {
+  ManualExecutor x;
+  EXPECT_EQ(42, via(&x, [] { return makeSemiFuture(42); }).getVia(&x));
+}
+
 TEST(ViaFunc, voidFuture) {
   ManualExecutor x;
   int count = 0;