Custom type caster for "pointer only" type?

[Holt59]

I'm trying to adapt some code that used Boost.Python with custom type converters for "pointer only" types, in particular Qt types such as QObject, QWidget, etc., by interfacing with sip. It's not the cleanest code but it worked fine with Boost.Python converter.

With pybind11, I cannot get the casters to work... I tried writing a custom type caster for QWidget* (the pointer, not the object), but I feel like it's never used... If I try to wrap something like:

m.def("foo", [](QWidget* w) { ... });

And call it from Python, I get:

TypeError: foo(): incompatible function arguments. The following argument types are supported:
1. (arg0: QWidget) -> None

And if I accept a raw object and try to .cast<QWidget*>(), I get an error

RuntimeError: Unable to cast Python instance to C++ type (compile in debug mode for details)

Is there any documentation on how to write "pointer only" type like that? I already know how to extract the proper pointer from the python handle and back if needed, but I simply cannot get pybind11 to use the type casters I write.