Add deprecation of l-value form of Future<T>::then()

Summary:
Separates l-value and r-value versions of parameterless then.

Deprecates l-value version.

Reviewed By: yfeldblum

Differential Revision: D9222345

fbshipit-source-id: 3dd95b3e35bf1b06d426334c30150b049a5e0b70

folly/futures/Future-inl.h

@@ -1127,7 +1127,7 @@ Future<T> Future<T>::thenError(F&& func) && {
 }
 
 template <class T>
-Future<Unit> Future<T>::then() {
+Future<Unit> Future<T>::then() && {
   return std::move(*this).then([]() {});
 }
 

folly/futures/Future.h

@@ -1375,7 +1375,12 @@ 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`
-  Future<Unit> then();
+  Future<Unit> then() &&;
+
+  [[deprecated("must be rvalue-qualified, e.g., std::move(future).then()")]]
+  Future<Unit> then() & {
+    return std::move(*this).then();
+  }
 
   /// Convenience method for ignoring the value and creating a Future<Unit>.
   /// Exceptions still propagate.