As in, for arguments that have __index__() methods, do the conversion to a true Python integer eagerly when the slice is built rather than lazily when slice.indices() (or the C-level equivalent) is called?

That actually seems like a potentially plausible future approach to me, but isn't a change I'd want to make hastily - those values are visible as the start, stop and step attributes on the slice, and https://docs.python.org/3/reference/datamodel.html#types currently describes those as "These attributes can have any type."

Given that folks do a lot of arcane things with the subscript notation, I wouldn't want to break working code if we have less intrusive alternatives.