import dataclasses
from enum import Enum

class NodeStatus(Enum):
    Offline = 1
    Running = 2
    Available = 3

@dataclasses.dataclass
class Node:
    status: NodeStatus = NodeStatus.Offline

    def set_free(self) -> None:
        self.status = NodeStatus.Available

def process_node(node: Node) -> None:
    if node.status != NodeStatus.Offline:
        return                                  # This narrows node.status to Literal[NodeStatus.Offline] forever

    node.set_free()                             # <<< mypy ignores side effects on .status from this
                                                #     This should restore node.status to any Nodestatus
    if node.status != NodeStatus.Running:       # error:  Non-overlapping equality check (left operand type: "Literal[NodeStatus.Offline]", right operand type: "Literal[NodeStatus.Running]")  [comparison-overlap]
        node.status = NodeStatus.Running