from typing import TypeVar, Protocol


NodePassthrough = TypeVar("NodePassthrough")


class Node(Protocol):
    def iter(self) -> list["Node"]:
        """Returns a list of child nodes."""
        ...


class DfsEnterNodeFn(Protocol[NodePassthrough]):
    def __call__(self, node: Node) -> NodePassthrough:
        """Called when entering a node during depth-first traversal."""
        ...


class DfsLeaveNodeFn(Protocol[NodePassthrough]):
    def __call__(self, node: Node, node_passthrough: NodePassthrough) -> None:
        """Called before leaving a node during depth-first traversal."""
        ...


def dfs(node: Node, upon_entering: DfsEnterNodeFn[NodePassthrough], before_leaving: DfsLeaveNodeFn[NodePassthrough]) -> None:
    """Depth-first search of a tree of nodes, calling upon_entering and before_leaving for each node encountered."""
    node_passthrough = upon_entering(node)

    for child in node.iter():
        dfs(child, upon_entering, before_leaving)

    before_leaving(node, node_passthrough)

example.py:12: error:
Invariant type variable "NodePassthrough" used in protocol where covariant one is expected  [misc]
    class DfsEnterNodeFn(Protocol[NodePassthrough]):
    ^

example.py:18: error:
Invariant type variable "NodePassthrough" used in protocol where contravariant one is expected  [misc]
    class DfsLeaveNodeFn(Protocol[NodePassthrough]):
    ^