tweak is_coroutine_handle detail trait

Summary: Tweak `is_coroutine_handle` detail trait. No need for leading `_` and let it be a trait variable.

Reviewed By: iahs

Differential Revision: D25864424

fbshipit-source-id: 2f0723fcb3c97c6a0972056999f1d9574695c30b

folly/experimental/coro/Traits.h

@@ -41,14 +41,11 @@ using remove_reference_wrapper_t = typename remove_reference_wrapper<T>::type;
 namespace detail {
 
 template <typename T>
-using _is_coroutine_handle =
-    folly::detail::is_instantiation_of<std::experimental::coroutine_handle, T>;
+inline constexpr bool is_coroutine_handle_v =
+    folly::detail::is_instantiation_of_v< //
+        std::experimental::coroutine_handle,
+        T>;
 
-template <typename T>
-struct _is_valid_await_suspend_return_type : folly::Disjunction<
-                                                 std::is_void<T>,
-                                                 std::is_same<bool, T>,
-                                                 _is_coroutine_handle<T>> {};
 } // namespace detail
 
 /// is_awaiter<T>::value

folly/experimental/coro/ViaIfAsync.h

@@ -229,8 +229,7 @@ class StackAwareViaIfAsyncAwaiter {
     viaCoroutine_.setContinuation(h);
     viaCoroutine_.setAsyncFrame(asyncFrame);
 
-    if constexpr (!detail::_is_coroutine_handle<
-                      await_suspend_result_t>::value) {
+    if constexpr (!detail::is_coroutine_handle_v<await_suspend_result_t>) {
       viaCoroutine_.saveContext();
     }
 
@@ -316,8 +315,7 @@ class ViaIfAsyncAwaiter {
       -> await_suspend_result_t {
     viaCoroutine_.setContinuation(continuation);
 
-    if constexpr (!detail::_is_coroutine_handle<
-                      await_suspend_result_t>::value) {
+    if constexpr (!detail::is_coroutine_handle_v<await_suspend_result_t>) {
       viaCoroutine_.saveContext();
     }