self,
        upstream: Operator,
        ti_count: int | None,
        *,
        session: Session,
    ) -> int | range | None:
        """Infer the map indexes of an upstream "relevant" to this ti.
        The bulk of the logic mainly exists to solve the problem described by
        the following example, where 'val' must resolve to different values,
        depending on where the reference is being used::
            @task
            def this_task(v):  # This is self.task.
                return v * 2
            @task_group
            def tg1(inp):
                val = upstream(inp)  # This is the upstream task.
                this_task(val)  # When inp is 1, val here should resolve to 2.
                return val
            # This val is the same object returned by tg1.
            val = tg1.expand(inp=[1, 2, 3])
            @task_group
            def tg2(inp):