ValueError: Cell is empty with very specific code

[smarie]

Consider this horrible and useless code:

@attr.s(slots=True)
class C(object):
    field = attr.ib()

    def f(self, a):
        super(C, self).__init__()

You get a nice error:

        # The following is a fix for
        # https://github.com/python-attrs/attrs/issues/102.  On Python 3,
        # if a method mentions `__class__` or uses the no-arg super(), the
        # compiler will bake a reference to the class in the method itself
        # as `method.__closure__`.  Since we replace the class with a
        # clone, we rewrite these references so it keeps working.
        for item in cls.__dict__.values():
            if isinstance(item, (classmethod, staticmethod)):
                # Class- and staticmethods hide their functions inside.
                # These might need to be rewritten as well.
                closure_cells = getattr(item.__func__, "__closure__", None)
            else:
                closure_cells = getattr(item, "__closure__", None)
    
            if not closure_cells:  # Catch None or the empty list.
                continue
            for cell in closure_cells:
>               if cell.cell_contents is self._cls:
E               ValueError: Cell is empty

..\src\attr\_make.py:639: ValueError

I'll push a PR right away