write_env: No Evaluated Context for Environment Type

include/stdexec/__detail/__write_env.hpp

@@ -61,10 +61,10 @@ namespace stdexec {
           stdexec::get_env(__child));
       };
 
-      static constexpr auto get_env =
-        [](__ignore, const auto& __state, const auto& __rcvr) noexcept {
-          return __env::__join(__state, stdexec::get_env(__rcvr));
-        };
+      static constexpr auto get_env = [](__ignore, const auto& __state, const auto& __rcvr) noexcept
+        -> decltype(__env::__join(__state, stdexec::get_env(__rcvr))) {
+        return __env::__join(__state, stdexec::get_env(__rcvr));
+      };
 
       static constexpr auto get_completion_signatures =
           []<class _Self, class... _Env>(_Self &&, _Env &&...) noexcept

test/CMakeLists.txt

@@ -55,6 +55,7 @@ set(stdexec_test_sources
     stdexec/algos/adaptors/test_stopped_as_optional.cpp
     stdexec/algos/adaptors/test_stopped_as_error.cpp
     stdexec/algos/adaptors/test_ensure_started.cpp
+    stdexec/algos/adaptors/test_write_env.cpp
     stdexec/algos/consumers/test_start_detached.cpp
     stdexec/algos/consumers/test_sync_wait.cpp
     stdexec/algos/other/test_execute.cpp

test/stdexec/algos/adaptors/test_write_env.cpp

@@ -0,0 +1,66 @@
+/*
+ * SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+ *                         Copyright (c) 2025 Robert Leahy. All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ * Licensed under the Apache License, Version 2.0 with LLVM Exceptions (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <catch2/catch.hpp>
+
+#include <utility>
+#include <type_traits>
+
+#include <stdexec/execution.hpp>
+#include <test_common/receivers.hpp>
+
+namespace {
+
+  template <typename T>
+  struct receiver : expect_void_receiver<> {
+    constexpr ::stdexec::env<> get_env() const noexcept {
+      return state_->get_env();
+    }
+    T* state_;
+  };
+
+  struct state;
+
+  static_assert(!std::is_same_v<
+                void,
+                decltype(::stdexec::connect(
+                  ::stdexec::just()
+                    | ::stdexec::write_env(::stdexec::prop{
+                      ::stdexec::get_stop_token,
+                      std::declval<::stdexec::inplace_stop_source&>().get_token()}),
+                  receiver<state>{{}, nullptr}))>);
+
+  struct state {
+    constexpr ::stdexec::env<> get_env() const noexcept {
+      return {};
+    }
+  };
+
+  TEST_CASE(
+    "write_env works when the actual environment is sourced from a type which was initially "
+    "incomplete but has since been completed",
+    "[adaptors][write_env]") {
+    ::stdexec::inplace_stop_source source;
+    state s;
+    auto op = ::stdexec::connect(
+      ::stdexec::just()
+        | ::stdexec::write_env(::stdexec::prop{::stdexec::get_stop_token, source.get_token()}),
+      receiver<state>{{}, &s});
+    ::stdexec::start(op);
+  }
+} // namespace