Remove nullary continuation form of Future::then

Summary:
Remove the form of Future::then that could take a continuation with no parameters from elsewhere in Future.h as it is deprecated, and pending deletion.

In particular this required splitting the multi forms of the functions which still allow flexible typing at this point in time.

Reviewed By: yfeldblum

Differential Revision: D10206118

fbshipit-source-id: bb2ae241d3bfd5b89cae077e667cb39c4cc62797

folly/futures/Future.h

@@ -1238,16 +1238,26 @@ class Future : private futures::detail::FutureBase<T> {
   /// - Calling code should act as if `valid() == false`,
   ///   i.e., as if `*this` was moved into RESULT.
   /// - `RESULT.valid() == true`
-  template <class Arg, class... Args>
-  auto then(Executor* x, Arg&& arg, Args&&... args) && {
+  template <class Arg>
+  auto then(Executor* x, Arg&& arg) && {
     auto oldX = this->getExecutor();
     this->setExecutor(x);
-    FOLLY_PUSH_WARNING
-    FOLLY_GNU_DISABLE_WARNING("-Wdeprecated-declarations")
+
+    // TODO(T29171940): thenImplementation here is ambiguous
+    // as then used to be but that is better than keeping then in the public
+    // API.
+    using R = futures::detail::callableResult<T, Arg&&>;
     return std::move(*this)
-        .then(std::forward<Arg>(arg), std::forward<Args>(args)...)
+        .thenImplementation(std::forward<Arg>(arg), R{})
         .via(oldX);
-    FOLLY_POP_WARNING
+  }
+
+  template <class R, class Caller, class... Args>
+  auto then(Executor* x, R (Caller::*func)(Args...), Caller* instance) && {
+    auto oldX = this->getExecutor();
+    this->setExecutor(x);
+
+    return std::move(*this).then(func, instance).via(oldX);
   }
 
   template <class Arg, class... Args>
@@ -1809,8 +1819,13 @@ class Future : private futures::detail::FutureBase<T> {
   template <class Callback, class... Callbacks>
   auto thenMulti(Callback&& fn, Callbacks&&... fns) && {
     // thenMulti with two callbacks is just then(a).thenMulti(b, ...)
+
+    // TODO(T29171940): Switch to thenImplementation here. It is ambiguous
+    // as then used to be but that is better than keeping then in the public
+    // API.
+    using R = futures::detail::callableResult<T, decltype(fn)>;
     return std::move(*this)
-        .then(std::forward<Callback>(fn))
+        .thenImplementation(std::forward<Callback>(fn), R{})
         .thenMulti(std::forward<Callbacks>(fns)...);
   }
 
@@ -1828,7 +1843,12 @@ class Future : private futures::detail::FutureBase<T> {
   template <class Callback>
   auto thenMulti(Callback&& fn) && {
     // thenMulti with one callback is just a then
-    return std::move(*this).then(std::forward<Callback>(fn));
+
+    // TODO(T29171940): Switch to thenImplementation here. It is ambiguous
+    // as then used to be but that is better than keeping then in the public
+    // API.
+    using R = futures::detail::callableResult<T, decltype(fn)>;
+    return std::move(*this).thenImplementation(std::forward<Callback>(fn), R{});
   }
 
   template <class Callback>
@@ -1861,8 +1881,12 @@ class Future : private futures::detail::FutureBase<T> {
     // via(x).then(a).thenMulti(b, ...).via(oldX)
     auto oldX = this->getExecutor();
     this->setExecutor(x);
+    // TODO(T29171940): Switch to thenImplementation here. It is ambiguous
+    // as then used to be but that is better than keeping then in the public
+    // API.
+    using R = futures::detail::callableResult<T, decltype(fn)>;
     return std::move(*this)
-        .then(std::forward<Callback>(fn))
+        .thenImplementation(std::forward<Callback>(fn), R{})
         .thenMulti(std::forward<Callbacks>(fns)...)
         .via(oldX);
   }

folly/futures/Retrying.h

@@ -181,7 +181,7 @@ retryingPolicyCappedJitteredExponentialBackoff(
           }
           auto backoff = detail::retryingJitteredExponentialBackoffDur(
               n, backoff_min, backoff_max, jitter_param, rngp);
-          return futures::sleep(backoff).then([] { return true; });
+          return futures::sleep(backoff).thenValue([](auto&&) { return true; });
         });
   };
 }