make exec::variant_sender conditionally non-dependent

fixes #1843

Author: ericniebler

include/exec/variant_sender.hpp

@@ -51,9 +51,9 @@ namespace exec {
 
   template <class... _Senders>
   struct variant_sender {
-    template <class _Self, class _Env>
+    template <class _Self, class... _Env>
     using __completions_t = STDEXEC::__mtry_q<STDEXEC::__concat_completion_signatures_t>::__f<
-      STDEXEC::__completion_signatures_of_t<STDEXEC::__copy_cvref_t<_Self, _Senders>, _Env>...
+      STDEXEC::__completion_signatures_of_t<STDEXEC::__copy_cvref_t<_Self, _Senders>, _Env...>...
     >;
 
     template <std::size_t _Index>
@@ -125,8 +125,8 @@ namespace exec {
     }
     STDEXEC_EXPLICIT_THIS_END(connect)
 
-    template <STDEXEC::__decays_to<variant_sender> _Self, class _Env>
-    static consteval auto get_completion_signatures() -> __completions_t<_Self, _Env> {
+    template <STDEXEC::__decays_to<variant_sender> _Self, class... _Env>
+    static consteval auto get_completion_signatures() -> __completions_t<_Self, _Env...> {
       return {};
     }
   };

test/exec/test_variant_sender.cpp

@@ -33,12 +33,19 @@ namespace {
 
   using just_int_t = decltype(just(0));
   using just_void_t = decltype(just());
+  using just_then_void_t = decltype(then(just(), [] { }));
 
   TEST_CASE("variant_sender - default constructible", "[types][variant_sender]") {
     variant_sender<just_void_t, just_int_t> variant{just()};
+    STATIC_REQUIRE(sender_in<decltype(variant)>);
     CHECK(variant.index() == 0);
   }
 
+  TEST_CASE("variant_sender - sender_of concept", "[types][variant_sender]") {
+    variant_sender<just_void_t, just_then_void_t> variant{just()};
+    STATIC_REQUIRE(sender_of<decltype(variant), set_value_t()>);
+  }
+
   TEST_CASE("variant_sender - using an overloaded then adaptor", "[types][variant_sender]") {
     variant_sender<just_void_t, just_int_t> variant = just();
     int index = -1;