When

```
TypeError: __weakref__ slot disallowed: either we already got one, or __itemsize__ != 0
```

is raised, the second condition `base->tp_itemsize != 0` (i.e. the base is a *variable-length* type, like `int`, `tuple` and `bytes`) in the error message is impossible since using a non-empty `__slots__` (e.g. `__slots__ = ('__weakref__',)`) for a subtype of a variable-length type raises a

```
TypeError: nonempty __slots__ not supported for subtype of '…'
```

earlier in the `type.__new__` implementation.