create explicit instantiations for folly::Unit

Summary:
move template instantiations for folly::Unit of several types into explicit instantiations in cpp files.

`folly::Future<folly::Unit>`
`folly::SemiFuture<folly::Unit>`
`folly::Promise<folly::Unit>`
`folly::SharedPromise<folly::Unit>`
`folly::futures::detail::FutureBase<folly::Unit>`
`folly::futures::detail::Core<folly::Unit>`
`folly::Try<folly::Unit>`

Reviewed By: yfeldblum, ericniebler

Differential Revision: D22092858

fbshipit-source-id: b8faa1c8aedfc193c17e2c3bc2e5b0f429ae6b0f

folly/Try-inl.h

@@ -346,4 +346,7 @@ auto unwrapTryTuple(Tuple&& instance) {
   return try_detail::unwrapTryTupleImpl(Seq{}, std::forward<Tuple>(instance));
 }
 
+// limited to the instances unconditionally forced by the futures library
+extern template class Try<Unit>;
+
 } // namespace folly

folly/Try.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/Try.h>
+
+namespace folly {
+
+template class Try<Unit>;
+
+} // namespace folly

folly/futures/Future-inl.h

@@ -2571,4 +2571,14 @@ SemiFuture<Unit> Timekeeper::at(std::chrono::time_point<Clock> when) {
   return after(std::chrono::duration_cast<HighResDuration>(when - now));
 }
 
+#if FOLLY_USE_EXTERN_FUTURE_UNIT
+// limited to the instances unconditionally forced by the futures library
+namespace futures {
+namespace detail {
+extern template class FutureBase<Unit>;
+} // namespace detail
+} // namespace futures
+extern template class Future<Unit>;
+extern template class SemiFuture<Unit>;
+#endif
 } // namespace folly

folly/futures/Future.cpp

@@ -79,4 +79,16 @@ SemiFuture<Unit> wait(std::shared_ptr<fibers::Baton> baton) {
 #endif
 
 } // namespace futures
+
+#if FOLLY_USE_EXTERN_FUTURE_UNIT
+namespace futures {
+namespace detail {
+template class FutureBase<Unit>;
+} // namespace detail
+} // namespace futures
+
+template class Future<Unit>;
+template class SemiFuture<Unit>;
+#endif
+
 } // namespace folly

folly/futures/Promise-inl.h

@@ -159,4 +159,9 @@ bool Promise<T>::isFulfilled() const noexcept {
   return true;
 }
 
+#if FOLLY_USE_EXTERN_FUTURE_UNIT
+// limited to the instances unconditionally forced by the futures library
+extern template class Promise<Unit>;
+#endif
+
 } // namespace folly

folly/futures/Promise.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/futures/Promise.h>
+
+namespace folly {
+
+#if FOLLY_USE_EXTERN_FUTURE_UNIT
+template class Promise<Unit>;
+#endif
+
+} // namespace folly

folly/futures/SharedPromise-inl.h

@@ -105,4 +105,9 @@ bool SharedPromise<T>::isFulfilled() const {
   return hasResult();
 }
 
+#if FOLLY_USE_EXTERN_FUTURE_UNIT
+// limited to the instances unconditionally forced by the futures library
+extern template class SharedPromise<Unit>;
+#endif
+
 } // namespace folly

folly/futures/SharedPromise.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/futures/SharedPromise.h>
+
+namespace folly {
+
+#if FOLLY_USE_EXTERN_FUTURE_UNIT
+template class SharedPromise<Unit>;
+#endif
+
+} // namespace folly

folly/futures/detail/Core.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/futures/detail/Core.h>
+
+namespace folly {
+namespace futures {
+namespace detail {
+
+#if FOLLY_USE_EXTERN_FUTURE_UNIT
+template class Core<folly::Unit>;
+#endif
+
+} // namespace detail
+} // namespace futures
+} // namespace folly

folly/futures/detail/Core.h

@@ -1027,6 +1027,12 @@ class Core final {
   std::unique_ptr<exception_wrapper> interrupt_{};
   std::function<void(exception_wrapper const&)> interruptHandler_{nullptr};
 };
+
+#if FOLLY_USE_EXTERN_FUTURE_UNIT
+// limited to the instances unconditionally forced by the futures library
+extern template class Core<folly::Unit>;
+#endif
+
 } // namespace detail
 } // namespace futures