boost::python map_indexing_suite for boost::container::flat_map<const T*, V>

Hello,

I've tried to adapt

`using myfm = boost::container::flat_map;`

with map_indexing_suite

```
class_< myfm, myfm* >("myfm")
.def( map_indexing_suite{} )
;
```

I get a compile errors:
/usr/include/boost/python/detail/invoke.hpp:75:14: error: no match for call to ‘(const boost::python::detail::specify_a_return_value_policy_to_wrap_functions_returning) (const T*)’

what does it mean?