The dependency graph can be constructed partially. This is used during Rule-based rewrites.

The graph APIs allow the user to pass nullptr to reference fields at the end of a pipeline.

/**
 * Returns false if the path as seen at the input of 'stage' can be proven to not be an array.
 * Returns true otherwise. If 'stage' is nullptr, the path is evaluated as it appears at the end
 * of the pipeline.
 */
bool canPathBeArray(const DocumentSource* stage, PathRef path) const;

With a partially-built graph, instead of referencing the end of the pipeline, the nullptr value incorrectly resolves to the end of the covered range.

I don't believe there are currently instances where nullptr is passed for a partially built graph, but this is an issue which will be hit eventually as the graph is used in more places in the codebase and should be resolved.