Running Daniels code interactively, with a debug build on Windows,
additionally prints 'XXX undetected error' before the Runtime error is
raised.

I cannot see anything that is wrong with my code.  Maybe this, and the
test failure, has to do with the fact that the new
'type___subclasscheck__' function is sometimes called with the error
indicator already set?  Maybe some PyErr_Fetch() and PyErr_Restore()
calls are needed inside this function, similar to what
PyObject_IsSubclass() does?  I must admit that I do not really
understand the purpose of these calls...

Anyway, I attach a new, cleaner patch although this one still behaves in
the same, buggy, way: type_instancecheck-2.diff.