Uncaught Error Error: Debug Failure. undefined is not expected
at onMessage (/home/rich/source/pyright/packages/pyright-internal/src/backgroundAnalysisBase.ts:222:23)
at (/home/rich/source/pyright/packages/pyright-internal/src/backgroundAnalysisBase.ts:197:62)
at emit (events:513:28)
at (internal/worker:233:53)
at [nodejs.internal.kHybridDispatch] (internal/event_target:735:20)
at exports.emitMessage (internal/per_context/messageport:23:28)
at callbackTrampoline (internal/async_hooks:130:17)

I didn't test with the background thread on. Pylance's perf tests caught this.