no coroutine_traits

Summary: Use of `std::coroutine_traits` and `std::experimental::coroutine_traits` can get awkward and we do not actually need the flexibility. Just define member type aliases `promise_type`.

Reviewed By: andriigrynenko

Differential Revision: D26706363

fbshipit-source-id: 0cdb59d542a64ff85d8a126003a014b7b6be3134

folly/Expected.h

@@ -87,6 +87,8 @@ using ExpectedErrorType =
 // Details...
 namespace expected_detail {
 
+template <typename Value, typename Error>
+struct Promise;
 template <typename Value, typename Error>
 struct PromiseReturn;
 
@@ -821,6 +823,8 @@ class Expected final : expected_detail::ExpectedStorage<Value, Error> {
   template <class U>
   using rebind = Expected<U, Error>;
 
+  using promise_type = expected_detail::Promise<Value, Error>;
+
   static_assert(
       !std::is_reference<Value>::value,
       "Expected may not be used with reference types");
@@ -1393,14 +1397,4 @@ expected_detail::Awaitable<Value, Error>
   return expected_detail::Awaitable<Value, Error>{std::move(o)};
 }
 } // namespace folly
-
-// This makes folly::Expected<Value> useable as a coroutine return type...
-namespace std {
-namespace experimental {
-template <typename Value, typename Error, typename... Args>
-struct coroutine_traits<folly::Expected<Value, Error>, Args...> {
-  using promise_type = folly::expected_detail::Promise<Value, Error>;
-};
-} // namespace experimental
-} // namespace std
 #endif // FOLLY_HAS_COROUTINES

folly/Optional.h

@@ -76,6 +76,8 @@ class Optional;
 
 namespace detail {
 template <class Value>
+struct OptionalPromise;
+template <class Value>
 struct OptionalPromiseReturn;
 } // namespace detail
 
@@ -101,6 +103,8 @@ class Optional {
  public:
   typedef Value value_type;
 
+  using promise_type = detail::OptionalPromise<Value>;
+
   static_assert(
       !std::is_reference<Value>::value,
       "Optional may not be used with reference types");
@@ -676,13 +680,4 @@ detail::OptionalAwaitable<Value>
 }
 } // namespace folly
 
-// This makes folly::Optional<Value> useable as a coroutine return type..
-namespace std {
-namespace experimental {
-template <typename Value, typename... Args>
-struct coroutine_traits<folly::Optional<Value>, Args...> {
-  using promise_type = folly::detail::OptionalPromise<Value>;
-};
-} // namespace experimental
-} // namespace std
 #endif // FOLLY_HAS_COROUTINES