Require that execute() work with const executors, as per specification.
diff --git a/asio/include/asio/execution/executor.hpp b/asio/include/asio/execution/executor.hpp
index 558f6c4..1072b94 100644
--- a/asio/include/asio/execution/executor.hpp
+++ b/asio/include/asio/execution/executor.hpp
@@ -59,7 +59,7 @@
 template <typename T, typename F>
 struct is_executor_of_impl :
   conditional<
-    can_execute<T, F>::value,
+    can_execute<const T, F>::value,
     is_executor_of_impl_base<T, F>,
     false_type
   >::type