The regression fixed by #12410 was not spotted because we annotated args as a Dict[str, Any] but then added args["limit"] as an int. This is fine because int is a subtype of Any.

We pass args to get_json, which expects args: Optional[QueryArgs], which is:

This isn't found by mypy because args: Dict[str, Any] means that we do no typechecking of the values in args.

All functions which accept args: QueryArgs forward args as the query attribute of MatrixFederationRequest. That attribute is typed more weakly as Dict[Any, Any].

The query attribute is seems to only ever be passed to encode_query_args. That expects a type similar to QueryArgs, and converts its argument list in a form that can be passed to urlencode.

We also have a QueryParams type elsewhere, which again is only forwarded to or passed to urlencode.

I would like to see:

• A stronger type annotation for MatrixFederationRequest.params
• Some unification of QueryArgs and QueryParams
• Some kind of audit of how we pass query parameters to MatrixFederationRequest
  • at the very least something that would have caught the regression fixed by Fix fetching public rooms over federation #12410.