FutureBase::assign() consumes its arg, so pass that arg via rvalue-ref

Summary:
future/semifuture move-assignment operators call this->assign(). The latter's arg was passed as an lvalue-ref. Result: now that arg is an rvalue-ref.

Note: I adjusted the assign() calling convention to avoid a forwarding ref. It would have been okay but it's easier to avoid a forwarding-ref than to explain why it happens to always be an rvalue-ref.

Reviewed By: yfeldblum

Differential Revision: D7831539

fbshipit-source-id: cf1c66cd498f768905ac3d21e39f693d7a652b39

folly/futures/Future-inl.h

@@ -161,9 +161,9 @@ FutureBase<T>::FutureBase(in_place_t, Args&&... args)
 }
 
 template <class T>
-template <class FutureType>
-void FutureBase<T>::assign(FutureType& other) noexcept {
-  std::swap(core_, other.core_);
+void FutureBase<T>::assign(FutureBase<T>&& other) noexcept {
+  detach();
+  core_ = exchange(other.core_, nullptr);
 }
 
 template <class T>
@@ -662,14 +662,14 @@ SemiFuture<T>::SemiFuture(Future<T>&& other) noexcept
 template <class T>
 SemiFuture<T>& SemiFuture<T>::operator=(SemiFuture<T>&& other) noexcept {
   releaseDeferredExecutor(this->core_);
-  this->assign(other);
+  this->assign(std::move(other));
   return *this;
 }
 
 template <class T>
 SemiFuture<T>& SemiFuture<T>::operator=(Future<T>&& other) noexcept {
   releaseDeferredExecutor(this->core_);
-  this->assign(other);
+  this->assign(std::move(other));
   // SemiFuture should not have an executor on construction
   if (this->core_) {
     this->setExecutor(nullptr);
@@ -816,7 +816,7 @@ Future<T>::Future(Future<T>&& other) noexcept
 
 template <class T>
 Future<T>& Future<T>::operator=(Future<T>&& other) noexcept {
-  this->assign(other);
+  this->assign(std::move(other));
   return *this;
 }
 

folly/futures/Future.h

@@ -197,8 +197,7 @@ class FutureBase {
 
   void throwIfInvalid() const;
 
-  template <class FutureType>
-  void assign(FutureType&) noexcept;
+  void assign(FutureBase<T>&& other) noexcept;
 
   Executor* getExecutor() const {
     return getCore().getExecutor();