fix(run_async): keep allocator arguments out of handler deduction

include/boost/capy/ex/run_async.hpp

@@ -34,6 +34,19 @@ namespace boost {
 namespace capy {
 namespace detail {
 
+/** Match types usable as `run_async` completion handlers.
+
+    Excludes the types meaningful to the other `run_async` parameters,
+    so a stop token, memory resource pointer, or allocator argument
+    selects its dedicated overload by conversion instead of deducing
+    as an exact-match handler.
+*/
+template<class H>
+concept RunAsyncHandler =
+    !std::is_convertible_v<H, std::pmr::memory_resource*> &&
+    !std::is_convertible_v<H, std::stop_token> &&
+    !Allocator<H>;
+
 /// Function pointer type for type-erased frame deallocation.
 using dealloc_fn = void(*)(void*, std::size_t);
 
@@ -517,6 +530,7 @@ run_async(Ex ex)
     @see executor
 */
 template<Executor Ex, class H1>
+    requires detail::RunAsyncHandler<H1>
 [[nodiscard]] auto
 run_async(Ex ex, H1 h1)
 {
@@ -560,6 +574,7 @@ run_async(Ex ex, H1 h1)
     @see executor
 */
 template<Executor Ex, class H1, class H2>
+    requires (detail::RunAsyncHandler<H1> && detail::RunAsyncHandler<H2>)
 [[nodiscard]] auto
 run_async(Ex ex, H1 h1, H2 h2)
 {
@@ -626,6 +641,7 @@ run_async(Ex ex, std::stop_token st)
     @see executor
 */
 template<Executor Ex, class H1>
+    requires detail::RunAsyncHandler<H1>
 [[nodiscard]] auto
 run_async(Ex ex, std::stop_token st, H1 h1)
 {
@@ -653,6 +669,7 @@ run_async(Ex ex, std::stop_token st, H1 h1)
     @see executor
 */
 template<Executor Ex, class H1, class H2>
+    requires (detail::RunAsyncHandler<H1> && detail::RunAsyncHandler<H2>)
 [[nodiscard]] auto
 run_async(Ex ex, std::stop_token st, H1 h1, H2 h2)
 {

test/unit/ex/run_async.cpp

@@ -19,6 +19,7 @@
 #include <atomic>
 #include <cstddef>
 #include <cstdio>
+#include <memory_resource>
 #include <memory>
 #include <queue>
 #include <stdexcept>
@@ -507,6 +508,33 @@ struct run_async_test
         BOOST_TEST(result);
     }
 
+    static task<void>
+    set_flag(bool& flag)
+    {
+        flag = true;
+        co_return;
+    }
+
+    void
+    testDerivedResourcePointer()
+    {
+        int dispatch_count = 0;
+        sync_executor d(dispatch_count);
+        std::pmr::monotonic_buffer_resource arena;
+        bool ran = false;
+
+        run_async(d, &arena)(set_flag(ran));
+        BOOST_TEST(ran);
+
+        ran = false;
+        run_async(d, std::stop_token{}, &arena)(set_flag(ran));
+        BOOST_TEST(ran);
+
+        ran = false;
+        run_async(d, &arena, [&] { ran = true; })(returns_void());
+        BOOST_TEST(ran);
+    }
+
     //----------------------------------------------------------
     // Sync Dispatcher
     //----------------------------------------------------------
@@ -775,6 +803,7 @@ struct run_async_test
 
         // Allocator Propagation
         testAllocatorPropagation();
+        testDerivedResourcePointer();
 
         // Sync Dispatcher
         testSyncDispatcherBasic();