Move decay_rvalue_reference_t metafunction to folly/experimental/coro/detail/Traits.h

Summary:
Make this metafunction available for use outside of the
folly::coro::blockingWait() implementation.

I plan to reuse this metafunction for other operators such as `when_all()`.

Reviewed By: yfeldblum

Differential Revision: D13778687

fbshipit-source-id: f766f93da21bf19e89262cd60048b0b7b1b76940

folly/experimental/coro/BlockingWait.h

@@ -20,6 +20,7 @@
 #include <folly/experimental/coro/Task.h>
 #include <folly/experimental/coro/Traits.h>
 #include <folly/experimental/coro/ViaIfAsync.h>
+#include <folly/experimental/coro/detail/Traits.h>
 #include <folly/fibers/Baton.h>
 #include <folly/synchronization/Baton.h>
 
@@ -251,23 +252,12 @@ BlockingWaitPromise<void>::get_return_object() noexcept {
       BlockingWaitPromise<void>>::from_promise(*this)};
 }
 
-template <typename T>
-struct decay_rvalue_reference {
-  using type = T;
-};
-
-template <typename T>
-struct decay_rvalue_reference<T&&> : std::decay<T> {};
-
-template <typename T>
-using decay_rvalue_reference_t = typename decay_rvalue_reference<T>::type;
-
 template <
     typename Awaitable,
     typename Result = await_result_t<Awaitable>,
     std::enable_if_t<!std::is_lvalue_reference<Result>::value, int> = 0>
 auto makeBlockingWaitTask(Awaitable&& awaitable)
-    -> BlockingWaitTask<decay_rvalue_reference_t<Result>> {
+    -> BlockingWaitTask<detail::decay_rvalue_reference_t<Result>> {
   co_return co_await static_cast<Awaitable&&>(awaitable);
 }
 
@@ -276,7 +266,7 @@ template <
     typename Result = await_result_t<Awaitable>,
     std::enable_if_t<std::is_lvalue_reference<Result>::value, int> = 0>
 auto makeBlockingWaitTask(Awaitable&& awaitable)
-    -> BlockingWaitTask<decay_rvalue_reference_t<Result>> {
+    -> BlockingWaitTask<detail::decay_rvalue_reference_t<Result>> {
   co_yield co_await static_cast<Awaitable&&>(awaitable);
 }
 

folly/experimental/coro/detail/Traits.h

+/*
+ * Copyright 2019-present Facebook, Inc.
+ *
+ * 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.
+ */
+#pragma once
+
+#include <folly/Traits.h>
+
+namespace folly {
+namespace coro {
+namespace detail {
+
+/**
+ * A type trait to decay rvalue-reference types to a prvalue.
+ */
+template <typename T>
+struct decay_rvalue_reference {
+  using type = T;
+};
+
+template <typename T>
+struct decay_rvalue_reference<T&&> : remove_cvref<T> {};
+
+template <typename T>
+using decay_rvalue_reference_t = typename decay_rvalue_reference<T>::type;
+
+} // namespace detail
+} // namespace coro
+} // namespace folly