Work around an MSVC bug in C++20 mode (facebookexperimental#492)

* Work around an MSVC bug in C++20 mode

PR facebookexperimental#489 seems to have found a bug in MSVC when compiling
`variant_sender` in C++20 mode.  This diff adds a test that I expect to
pass everywhere but fail to build with MSVC.  The next diff will fix the
problem with a work-around.

* Actually work around MSVC's bug

This diff works around the bug revealed by the test added in the
previous diff.

include/unifex/variant_sender.hpp

@@ -107,13 +107,19 @@ class _sender<Senders...>::type {
   friend auto tag_invoke(tag_t<connect>, This&& that, Receiver&& r)
     noexcept(std::conjunction_v<unifex::is_nothrow_connectable<match_reference_t<This, Senders>, Receiver>...>)
   {
+    // MSVC needs this type alias declared outside the lambda below to reliably compile
+    // the visit() expression as C++20
+    using op_t = operation<connect_result_t<Senders, Receiver>...>;
     return std::visit(
         [&r](auto&& sender) noexcept(
             unifex::is_nothrow_connectable_v<decltype(sender), Receiver>) {
-          return operation<connect_result_t<Senders, Receiver>...>{
-             static_cast<decltype(sender)&&>(sender), static_cast<Receiver&&>(r)};
+          // MSVC doesn't like static_cast<Receiver&&>(r) in some cases when compiling
+          // as C++20, but seems to reliably do the right thing with
+          // static_cast<decltype(r)>(r)
+          return op_t{
+             static_cast<decltype(sender)&&>(sender), static_cast<decltype(r)>(r)};
         },
-        std::move(static_cast<decltype(that)>(that).senderVariant_));
+        static_cast<decltype(that)>(that).senderVariant_);
   }
 };
 } // namespace _variant_sender

test/variant_sender_test.cpp

@@ -14,17 +14,19 @@
  * limitations under the License.
  */
 #include <unifex/any_sender_of.hpp>
+#include <unifex/async_manual_reset_event.hpp>
 #include <unifex/defer.hpp>
 #include <unifex/dematerialize.hpp>
 #include <unifex/just.hpp>
-#include <unifex/just_error.hpp>
 #include <unifex/just_done.hpp>
+#include <unifex/just_error.hpp>
+#include <unifex/let_value.hpp>
 #include <unifex/materialize.hpp>
-#include <unifex/variant_sender.hpp>
 #include <unifex/scheduler_concepts.hpp>
 #include <unifex/sync_wait.hpp>
 #include <unifex/then.hpp>
 #include <unifex/timed_single_thread_context.hpp>
+#include <unifex/variant_sender.hpp>
 
 #include <chrono>
 #include <iostream>
@@ -231,3 +233,4 @@ TEST(Variant, TestNoexcept_RvalueRef) {
   auto rvalue_no_except = is_noexcept<false, true, false>::value;
   EXPECT_TRUE(rvalue_no_except);
 }
+