Add deduction guide to co_result

Reviewed By: yfeldblum, lewissbaker

Differential Revision: D23240860

fbshipit-source-id: 74aab6151c9c44159a2d3fc4e041b475b69e29e7

folly/experimental/coro/Task.h

@@ -70,6 +70,9 @@ class co_result final {
   Try<T> result_;
 };
 
+template <class T>
+co_result(Try<T>)->co_result<T>;
+
 namespace detail {
 
 class TaskPromiseBase {