set_next doesn't detect tag_invoke overloads

[glebov-andrey]

Hi,

There is an issue with set_next not finding tag_invoke(Receiver &, ...) overloads, while member functions work fine.

Here is a patch which fixes the issue:

diff --git a/include/unifex/receiver_concepts.hpp b/include/unifex/receiver_concepts.hpp
index 8a4af7b..2a415e9 100644
--- a/include/unifex/receiver_concepts.hpp
+++ b/include/unifex/receiver_concepts.hpp
@@ -65,20 +65,20 @@ inline const struct _set_next_fn {
 private:
   template <typename Receiver, typename... Values>
   using set_next_member_result_t =
-      decltype(UNIFEX_DECLVAL(Receiver&).set_next(UNIFEX_DECLVAL(Values)...));
+      decltype(UNIFEX_DECLVAL(Receiver).set_next(UNIFEX_DECLVAL(Values)...));
   template <typename Receiver, typename... Values>
   using _result_t = typename conditional_t<
       tag_invocable<_set_next_fn, Receiver&, Values...>,
       meta_tag_invoke_result<_set_next_fn>,
       meta_quote1_<set_next_member_result_t>>::
-      template apply<Receiver, Values...>;
+      template apply<Receiver&, Values...>;
 
 public:
   template(typename Receiver, typename... Values)                  //
-      (requires tag_invocable<_set_next_fn, Receiver, Values...>)  //
+      (requires tag_invocable<_set_next_fn, Receiver&, Values...>)  //
       auto
       operator()(Receiver& r, Values&&... values) const
-      noexcept(is_nothrow_tag_invocable_v<_set_next_fn, Receiver, Values...>)
+      noexcept(is_nothrow_tag_invocable_v<_set_next_fn, Receiver&, Values...>)
           -> _result_t<Receiver, Values...> {
     return unifex::tag_invoke(_set_next_fn{}, r, (Values &&) values...);
   }