The `chr` function does accept objects that implement the
`SupportsIndex` protocol. The [implementation] of the builtin method
calls `PyLong_AsLongAndOverflow` on the argument, which in turn calls
`__index__()`, if present. The parameter of the `chr` function can
therefore be annotated with `int | SupportsIndex`.
It seems to me like `SupportsIndex` alone would be enough, since `int`
implements `SupportIndex`, but I chose `int | SupportIndex` to make it
consistent with the annotations on `hex`, `oct` and `bin`.
[implementation]: https://github.com/python/cpython/blob/b05fa90b21dd01bb836285cdd41920320b09e681/Python/bltinmodule.c#L725
[PyLong_AsLongAndOverflow]: https://docs.python.org/3.13/c-api/long.html#c.PyLong_AsLongAndOverflow
David Peter