Currently aggregation commands are parsed after the CommandInvocation has been created, during run() or explain(). This means that supportsReadConcern() (which gets called before run()/explain()) has to be very broad, accepting the RC for any possible aggregation (with further validation later).

It would be much better if the AggregationRequest::parseFromBSON() construction was lifted out of PipelineCommand::Invocation::run()/explain() and into PipelineCommand::parse(), because this would allow supportsReadConcern() to consult the AggregationRequest to more accurately indicate if the RC is supported, and more particularly, if the default RC should be applied.