Modify thenTry to fail correctly with a parameterless lambda

Summary: Removes support for thenTry to accept a lambda with no parameter list and fail to compile with no arguments.

Reviewed By: yfeldblum

Differential Revision: D9011934

fbshipit-source-id: ec29b0c3d9475e7d2c566469b0ee333f0dc05738

Author: LeeHowes

folly/futures/Future-inl.h

@@ -1078,7 +1078,10 @@ template <class T>
 template <typename F>
 Future<typename futures::detail::tryCallableResult<T, F>::value_type>
 Future<T>::thenTry(F&& func) && {
-  return std::move(*this).then(std::forward<F>(func));
+  return std::move(*this).then(
+      [f = std::forward<F>(func)](folly::Try<T>&& t) mutable {
+        return std::forward<F>(f)(std::move(t));
+      });
 }
 
 template <class T>

folly/futures/Future.h

@@ -1232,6 +1232,11 @@ class Future : private futures::detail::FutureBase<T> {
   Future<typename futures::detail::tryCallableResult<T, F>::value_type> thenTry(
       F&& func) &&;
 
+  template <typename R, typename... Args>
+  auto thenTry(R (&func)(Args...)) && {
+    return std::move(*this).thenTry(&func);
+  }
+
   /// When this Future has completed, execute func which is a function that
   /// can be called with `T&&` (often a lambda with parameter type
   /// `auto&&` or `auto`).

folly/futures/test/FutureTest.cpp

@@ -950,8 +950,8 @@ static folly::Future<std::string> doWorkStaticTry(Try<std::string>&& t) {
 
 TEST(Future, thenTrythenValue) {
   auto f =
-      makeFuture<std::string>("0")
-          .thenTry([]() { return makeFuture<std::string>("1"); })
+      makeFuture()
+          .thenTry([](auto&&) { return makeFuture<std::string>("1"); })
           .thenTry(
               [](Try<std::string>&& t) { return makeFuture(t.value() + ";2"); })
           .thenTry([](const Try<std::string>&& t) {