Added static_assert for ref in make_observable

Author: cschreib

include/oup/observable_unique_ptr.hpp

@@ -799,6 +799,8 @@ class basic_observable_ptr final {
  */
 template<typename T, typename Policy, typename... Args>
 auto make_observable(Args&&... args) {
+    static_assert(!std::is_reference_v<T>, "cannot create a pointer to a reference");
+
     using observer_policy    = typename Policy::observer_policy;
     using control_block_type = basic_control_block<observer_policy>;
     using decayed_type       = std::decay_t<T>;