added exec.prop

Author: dietmarkuehl

include/beman/execution/detail/prop.hpp

@@ -0,0 +1,28 @@
+// include/beman/execution/detail/prop.hpp                            -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef INCLUDED_INCLUDE_BEMAN_EXECUTION_DETAIL_PROP
+#define INCLUDED_INCLUDE_BEMAN_EXECUTION_DETAIL_PROP
+
+#include <type_traits>
+
+// ----------------------------------------------------------------------------
+
+namespace beman::execution {
+    template <typename Query, typename Value>
+    struct prop {
+        [[no_unique_address]] Query query_;
+        Value value_;
+
+        auto operator=(prop const&) = delete;
+
+        constexpr auto query(Query) const noexcept -> Value { return this->value_; }
+    };
+
+    template <typename Query, typename Value>
+    prop(Query, Value) -> prop<Query, ::std::unwrap_reference_t<Value>>; 
+}
+
+// ----------------------------------------------------------------------------
+
+#endif

src/beman/execution/CMakeLists.txt

@@ -135,6 +135,7 @@ target_sources(
          ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/operation_state.hpp
          ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/operation_state_task.hpp
          ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/product_type.hpp
+         ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/prop.hpp
          ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/query_with_default.hpp
          ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/queryable.hpp
          ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/read_env.hpp

tests/beman/execution/CMakeLists.txt

@@ -11,6 +11,7 @@ endif()
 list(
   APPEND
   execution_tests
+  exec-prop.test
   exec-scope-simple-counting.test
   exec-spawn-future.test
   exec-scope-concepts.test

tests/beman/execution/exec-prop.test.cpp

@@ -0,0 +1,39 @@
+// tests/beman/execution/exec-prop.test.cpp                           -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <beman/execution/detail/prop.hpp>
+#include <beman/execution/detail/forwarding_query.hpp>
+#include <type_traits>
+#include <concepts>
+#include <test/execution.hpp>
+
+// ----------------------------------------------------------------------------
+
+namespace {
+    constexpr struct test_query_t {
+    template <typename Env>
+        requires requires(const test_query_t& self, const Env& e) { e.query(self); }
+    decltype(auto) operator()(const Env& e) const noexcept(noexcept(e.query(*this))) {
+        return e.query(*this);
+    }
+    constexpr auto query(const test_std::forwarding_query_t&) const noexcept -> bool { return true; }
+    } test_query{};
+
+    struct env {
+        auto query(test_query_t const&) const noexcept { return 42; }
+    };
+
+    template <typename Env, typename Value>
+    auto test_prop(Env&& env, Value&& value) {
+        static_assert(requires{ { test_query(env) } noexcept; });
+        ASSERT(test_query(env) == value);
+    }
+}
+
+TEST(exec_prop) {
+    test_prop(env{}, 42);
+    test_prop(test_std::prop(test_query, 42), 42);
+    [[maybe_unused]] auto p{test_std::prop(test_query, 2.5)};
+    static_assert(not std::is_assignable_v<decltype(p), decltype(p)>);
+}
+