core(basic sender): fix trailing return type (#2081)

* core(basic sender): fix trailing return type
---------

Signed-off-by: romintomasetti <romin.tomasetti@gmail.com>

Author: romintomasetti

include/stdexec/__detail/__basic_sender.hpp

@@ -164,7 +164,8 @@ namespace STDEXEC
 
       static constexpr auto __get_state =  //
         []<class _Sender, class _Receiver>(_Sender&& __sndr, _Receiver&& __rcvr) noexcept(
-          __nothrow_decay_copyable<__data_of<_Sender>>) -> decltype(auto)
+          __nothrow_decay_copyable<__data_of<_Sender>>)
+        -> __state<std::remove_cvref_t<_Receiver>, std::remove_cvref_t<__data_of<_Sender>>>
       {
         return __state{static_cast<_Receiver&&>(__rcvr),
                        STDEXEC::__get<1>(static_cast<_Sender&&>(__sndr))};

test/stdexec/algos/adaptors/test_write_env.cpp

@@ -21,6 +21,8 @@
 #include <type_traits>
 #include <utility>
 
+#include <exec/completion_signatures.hpp>
+#include <exec/single_thread_context.hpp>
 #include <stdexec/execution.hpp>
 #include <test_common/receivers.hpp>
 
@@ -68,4 +70,114 @@ namespace
                                  receiver<state>{{}, &s});
     ::STDEXEC::start(op);
   }
+
+  template <class IncompleteType, class Env = STDEXEC::env_of_t<IncompleteType>>
+  struct ReceiverIncomplete
+  {
+    using receiver_concept = STDEXEC::receiver_tag;
+
+    IncompleteType* m_ptr;
+
+    void set_value() && noexcept
+    {
+      STDEXEC::set_value(std::move(m_ptr->rcvr));
+    }
+
+    template <typename Error>
+    void set_error(Error&& error) && noexcept
+    {
+      STDEXEC::set_error(std::move(m_ptr->rcvr), std::forward<Error>(error));
+    }
+
+    [[nodiscard]]
+    constexpr auto get_env() const noexcept -> Env
+    {
+      return STDEXEC::get_env(*m_ptr);
+    }
+  };
+
+  template <STDEXEC::sender Sndr, STDEXEC::receiver Rcvr>
+  struct OpStateIncomplete
+  {
+    using operation_state_concept = STDEXEC::operation_state_tag;
+
+    using rcvr_t          = ReceiverIncomplete<OpStateIncomplete, STDEXEC::env_of_t<Rcvr>>;
+    using inner_opstate_t = STDEXEC::connect_result_t<Sndr, rcvr_t>;
+
+    Rcvr            rcvr;
+    inner_opstate_t inner_opstate;
+
+    OpStateIncomplete(Sndr&& sndr, Rcvr rcvr_)
+      : rcvr(std::move(rcvr_))
+      , inner_opstate(STDEXEC::connect(std::forward<Sndr>(sndr), rcvr_t{this}))
+    {}
+
+    void start() & noexcept
+    {
+      STDEXEC::start(inner_opstate);
+    }
+
+    [[nodiscard]]
+    constexpr auto get_env() const noexcept -> STDEXEC::env_of_t<Rcvr>
+    {
+      return STDEXEC::get_env(rcvr);
+    }
+  };
+
+  template <STDEXEC::sender Sndr>
+  struct SenderIncomplete
+  {
+    using sender_concept = STDEXEC::sender_tag;
+
+    template <class Self, class... Env>
+    static consteval auto get_completion_signatures()
+    {
+      return exec::get_child_completion_signatures<Self, Sndr, Env...>();
+    }
+
+    template <STDEXEC::receiver Rcvr>
+    auto connect(Rcvr rcvr) && -> OpStateIncomplete<Sndr, Rcvr>
+    {
+      return {std::forward<Sndr>(sndr), std::move(rcvr)};
+    }
+
+    Sndr sndr;
+  };
+
+  struct incomplete_t
+  {
+    constexpr auto operator()() const noexcept
+    {
+      return STDEXEC::__closure(*this);
+    }
+
+    template <typename Sndr>
+    constexpr auto operator()(Sndr&& sndr) const -> SenderIncomplete<Sndr>
+    {
+      return {std::forward<Sndr>(sndr)};
+    }
+  };
+
+  inline constexpr incomplete_t incomplete{};
+
+  TEST_CASE("write_env with a receiver pointing to an incomplete operation state",
+            "[adaptors][write_env]")
+  {
+    exec::single_thread_context stc{};
+
+    int value = 0;
+
+    STDEXEC::sender auto sndr =
+      STDEXEC::read_env(STDEXEC::get_allocator)
+      | STDEXEC::then([](auto allocator)
+                      { static_assert(std::same_as<decltype(allocator), std::allocator<int>>); })
+      | STDEXEC::continues_on(stc.get_scheduler()) | incomplete()
+      | STDEXEC::write_env(STDEXEC::prop{STDEXEC::get_allocator, std::allocator<int>{}})
+      | STDEXEC::then([&value]() { ++value; });
+
+    STDEXEC::sync_wait(std::move(sndr));
+
+    CHECK(value == 1);
+  }
+
 }  // namespace