removed requirement for executor const refs.

include/boost/cobalt/detail/fork.hpp

@@ -56,7 +56,7 @@ struct fork
 
     bool outstanding_work() {return use_count != 0u;}
 
-    const executor * exec = nullptr;
+    std::optional<executor> exec = nullptr;
     bool wired_up() {return exec != nullptr;}
 
     using executor_type = executor;

include/boost/cobalt/detail/gather.hpp

@@ -163,7 +163,7 @@ struct gather_variadic_impl
 #if defined(BOOST_ASIO_ENABLE_HANDLER_TRACKING)
       this->loc = loc;
 #endif
-      this->exec = &cobalt::detail::get_executor(h);
+      this->exec = cobalt::detail::get_executor(h);
       last_forked.release().resume();
       while (last_index < tuple_size)
         impls[last_index++](*this).release();
@@ -362,7 +362,7 @@ struct gather_ranged_impl
 #if defined(BOOST_ASIO_ENABLE_HANDLER_TRACKING)
       this->loc = loc;
 #endif
-      exec = &detail::get_executor(h);
+      exec =  detail::get_executor(h);
 
       last_forked.release().resume();
       while (last_index < cancel.size())

include/boost/cobalt/detail/handler.hpp

@@ -73,6 +73,24 @@ struct completion_handler_noop_executor
 
 };
 
+template<typename Promise>
+executor
+get_executor(std::coroutine_handle<Promise> h)
+{
+  if constexpr (requires {h.promise().get_executor();})
+  {
+    return h.promise().get_executor();
+  }
+  else
+    return this_thread::get_executor();
+}
+
+inline executor
+get_executor(std::coroutine_handle<>)
+{
+  return this_thread::get_executor();
+}
+
 
 struct completion_handler_base
 {
@@ -84,7 +102,7 @@ struct completion_handler_base
   }
 
   using executor_type = executor;
-  const executor_type & executor_ ;
+  executor_type executor_ ;
   const executor_type & get_executor() const noexcept
   {
     return executor_ ;
@@ -113,11 +131,10 @@ struct completion_handler_base
   }
 
   template<typename Promise>
-    requires (requires (Promise p) {{p.get_executor()} -> std::same_as<const executor&>;})
   completion_handler_base(std::coroutine_handle<Promise> h,
                           completed_immediately_t * completed_immediately = nullptr)
           : cancellation_slot(asio::get_associated_cancellation_slot(h.promise())),
-            executor_(h.promise().get_executor()),
+            executor_(cobalt::detail::get_executor(h)),
 #if !defined(BOOST_COBALT_NO_PMR)
             allocator(asio::get_associated_allocator(h.promise(), this_thread::get_allocator())),
 #else
@@ -128,24 +145,22 @@ struct completion_handler_base
   }
 #if !defined(BOOST_COBALT_NO_PMR)
   template<typename Promise>
-    requires (requires (Promise p) {{p.get_executor()} -> std::same_as<const executor&>;})
   completion_handler_base(std::coroutine_handle<Promise> h,
                           pmr::memory_resource * resource,
                           completed_immediately_t * completed_immediately = nullptr)
           : cancellation_slot(asio::get_associated_cancellation_slot(h.promise())),
-            executor_(h.promise().get_executor()),
+            executor_(cobalt::detail::get_executor(h)),
             allocator(resource),
             completed_immediately(completed_immediately)
   {
   }
 #else
   template<typename Promise>
-  requires (requires (Promise p) {{p.get_executor()} -> std::same_as<const executor&>;})
   completion_handler_base(std::coroutine_handle<Promise> h,
                           detail::sbo_resource * resource,
                           completed_immediately_t * completed_immediately = nullptr)
       : cancellation_slot(asio::get_associated_cancellation_slot(h.promise())),
-        executor_(h.promise().get_executor()),
+        executor_(cobalt::detail::get_executor(h)),
         allocator(resource),
         completed_immediately(completed_immediately)
   {
@@ -166,27 +181,6 @@ void assign_cancellation(std::coroutine_handle<Promise> h, Handler && func)
       h.promise().get_cancellation_slot().assign(std::forward<Handler>(func));
 }
 
-template<typename Promise>
-const executor &
-get_executor(std::coroutine_handle<Promise> h)
-{
-  if constexpr (requires {h.promise().get_executor();})
-  {
-    static_assert(std::same_as<decltype(h.promise().get_executor()),
-                               const executor &>,
-                  "for performance reasons, the get_executor function on a promise must return a const reference");
-    return h.promise().get_executor();
-  }
-  else
-    return this_thread::get_executor();
-}
-
-inline const executor &
-get_executor(std::coroutine_handle<>)
-{
-  return this_thread::get_executor();
-}
-
 }
 
 template<typename ... Args>

include/boost/cobalt/detail/join.hpp

@@ -192,7 +192,7 @@ struct join_variadic_impl
 #if defined(BOOST_ASIO_ENABLE_HANDLER_TRACKING)
       this->loc = loc;
 #endif
-      this->exec = &detail::get_executor(h);
+      this->exec = detail::get_executor(h);
       last_forked.release().resume();
       while (last_index < tuple_size)
         impls[last_index++](*this).release();
@@ -450,7 +450,7 @@ struct join_ranged_impl
 #if defined(BOOST_ASIO_ENABLE_HANDLER_TRACKING)
       this->loc = loc;
 #endif
-      exec = &detail::get_executor(h);
+      exec = detail::get_executor(h);
 
       last_forked.release().resume();
       while (last_index < cancel.size())

include/boost/cobalt/detail/race.hpp

@@ -305,7 +305,7 @@ struct race_variadic_impl
     {
       this->loc = loc;
 
-      this->exec = &cobalt::detail::get_executor(h);
+      this->exec = cobalt::detail::get_executor(h);
       last_forked.release().resume();
 
       if (!this->outstanding_work()) // already done, resume rightaway.
@@ -617,7 +617,7 @@ struct race_ranged_impl
                        const boost::source_location & loc = BOOST_CURRENT_LOCATION)
     {
       this->loc = loc;
-      this->exec = &detail::get_executor(h);
+      this->exec = detail::get_executor(h);
       last_forked.release().resume();
 
       if (!this->outstanding_work()) // already done, resume rightaway.