git.types.assert_never is documented to raise AssertionError if raise_error is True and exc is None:

But it actually raises ValueError in this situation, its full implementation being:

For comparison. there is an assert_never function in Python's typing module since 3.11 (and in typing_extensions). This function is similar but only has one parameter--which the first parameter of git.types.assert_never corresponds to--and it documents that it raises an exception if called at runtime, but does not document the type of exception raised. At least currently in Python 3.12, it raises AssertionError.

The GitPython function is a public member of git.types, and it is used internally in GitPython. I don't know if the docstring should be fixed to describe the existing behavior, or if that behavior should itself be considered the bug here and changed to match the docstring.