fix dependent sender computation in write_env's completion signatures

Author: ericniebler

include/stdexec/__detail/__write_env.hpp

@@ -67,11 +67,11 @@ namespace stdexec {
         };
 
       static constexpr auto get_completion_signatures =
-        []<class _Self, class... _Env>(_Self&&, _Env&&...) noexcept
-        -> __completion_signatures_of_t<
-          __child_of<_Self>,
-          __meval<__join_env_t, const __decay_t<__data_of<_Self>>&, _Env...>
-        > {
+          []<class _Self, class... _Env>(_Self &&, _Env &&...) noexcept
+          -> __completion_signatures_of_t<
+              __child_of<_Self>,
+              __meval<__join_env_t, const __decay_t<__data_of<_Self>> &,
+                      _Env>...> {
         static_assert(sender_expr_for<_Self, write_env_t>);
         return {};
       };

test/exec/test_task.cpp

@@ -37,6 +37,8 @@ namespace {
   // This is a work-around for clang-12 bugs in Release mode
   thread_local int __thread_id = 0;
 
+  static_assert(stdexec::sender<exec::task<void>>);
+
   // This is a work-around for apple clang bugs in Release mode
   STDEXEC_WHEN(STDEXEC_APPLE_CLANG(), [[clang::optnone]]) auto get_id() -> int {
     return __thread_id;
@@ -257,6 +259,17 @@ namespace {
     CHECK(count == 3);
   }
 
+  TEST_CASE("task - can co_await task wrapped in write_env", "[types][task]") {
+    stdexec::sync_wait([]() -> exec::task<void> {
+      co_await stdexec::write_env(
+          []() -> exec::task<void> {
+            auto token = co_await stdexec::get_stop_token();
+            assert(!token.stop_possible());
+          }(),
+          stdexec::prop{stdexec::get_stop_token, stdexec::never_stop_token{}});
+    }());
+  }
+
   struct test_domain {
     template <sender_expr_for<then_t> _Sender>
     static constexpr auto transform_sender(_Sender&&) noexcept {