Dummy implementation for casting from const std::unique_ptr&

Implementation is missing.
If the instance is already registered, it should be returned with increase refcount.
If not, a new instance must be created, which must not delete.

Author: rhaschke

include/pybind11/detail/smart_holder_type_casters.h

@@ -698,6 +698,9 @@ struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caste
 
         return inst.release();
     }
+    static handle cast(const std::unique_ptr<T, D> &src, return_value_policy policy, handle parent) {
+        throw cast_error("Casting from const std::unique_ptr& not yet implemented.");
+    }
 
     template <typename>
     using cast_op_type = std::unique_ptr<T, D>;