        lookup = lookup_class(lhs, rhs)
        # Interpret '__exact=None' as the sql 'is NULL'; otherwise, reject all
        # uses of None as a query value unless the lookup supports it.
        if lookup.rhs is None and not lookup.can_use_none_as_rhs:
            if lookup_name not in ("exact", "iexact"):
                raise ValueError("Cannot use None as a query value")
            return lhs.get_lookup("isnull")(lhs, True)
…
        require_outer = False
        if lookup_type == "isnull":
            # Both `NOT (field IS NOT NULL)` and `field IS NULL` require an
            # OUTER JOIN to effectively include related `NULL` values.
            # Otherwise an INNER JOIN can be used as it excludes them.
            require_outer = (
                condition.rhs is False if current_negated else condition.rhs is True
            )
        elif current_negated and condition.rhs is not None:
            require_outer = True
            # The condition added below will be SQL like this:
            # `NOT (col IS NOT NULL)`, where the first NOT is added in
            # upper layers of code. The reason for addition is that if col
            # `IS NULL`, then `col != someval` will result in SQL "unknown"
            # which isn't the same as in Python. The Python None handling
            # is wanted, and it can be achieved by
            # `(col != someval OR col IS NULL)` which is equivalent to
            # `NOT (col = someval AND col IS NOT NULL)`. The second form is
            # required here as the condition is negated per `current_negated`.
            if (
                self.is_nullable(targets[0])
                or self.alias_map[join_list[-1]].join_type == LOUTER
            ):
                lookup_class = targets[0].get_lookup("isnull")
                col = self._get_col(targets[0], join_info.targets[0], alias)
                # Use OR + IS NULL when RHS `in` values include None.
                if (
                    lookup_type == "in"
                    # Check containers (not strings or bytes).
                    and isinstance(condition.rhs, Iterable)
                    and not isinstance(condition.rhs, (str, bytes))
                    and any(v is None for v in condition.rhs)
                ):
                    clause.add(lookup_class(col, True), OR)
                else:
                    clause.add(lookup_class(col, False), AND)
            # If someval is a nullable column, someval IS NOT NULL is
            # added.
            if isinstance(value, Col) and self.is_nullable(value.target):
                lookup_class = value.target.get_lookup("isnull")
                clause.add(lookup_class(value, False), AND)
        return clause, used_joins if not require_outer else ()
Warm-up
