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Erik Johnston
86a1e73ef4
The reason for the change is to make it easier to support these checks when porting event class to Rust. Previously, code that needed to access `prev_state_events` had to combine a `room_version.msc4242_state_dags` boolean check with an `isinstance(event, FrozenEventVMSC4242)` cast (or `cast()`) for the type checker. Introduce `supports_msc4242_state_dag()` in a new `synapse/events/py_protocol.py` which does both in one step via `TypeIs[MSC4242Event]`, removing the need to import the concrete `FrozenEventVMSC4242` class at every call site. `MSC4242Event` is an `EventBase` subclass used purely for type narrowing — it's marked with a metaclass that rejects `isinstance()` to make accidental runtime use loud. No behavioural change: callers continue to gate on the same room version flag and access the same `prev_state_events` attribute.