DocumentSourceUnionWith::optimize() invokes pipeline_optimization::optimizePipeline() on the sub-pipeline without first propagating QuerySettings from the parent ExpressionContext. As a result, optimizations that consult query settings (e.g. index hints, query framework selection, reject) operate on an empty QuerySettings instance for the unioned sub-pipeline at optimization time, even when the user has supplied query settings for the parent query.

The settings are eventually copied at execution time in UnionWithStage::doGetNext() and (for explain) in DocumentSourceUnionWith::serialize(), but by then the sub-pipeline has already been optimized. During optimiztion any components requiring access to query settings fail.

Expected Behavior. Sub-pipeline optimization within $unionWith should see the same QuerySettings as the parent pipeline, consistent with the propagation already performed at execution time in union_with_stage.cpp and at explain time in document_source_union_with.cpp.

This ticket should fix this by initializing query settings on the sub-pipeline's ExpressionContext with the parent's settings before invoking optimizePipeline().