caused by SERVER-57334
Here are two lines of document_source_densify.cpp:

508:            _current = addValues(stdx::get<Value>(*_current), _range.getStep());
509:            _current = stdx::get<Value>(*_current), _range.getStep();

I'm pretty sure 509 is a corrupted copy of 508.
This compiles but throws a fatal warning on my virtual workstation having to do with memcpy of a string onto itself. Details unimportant but the good news is that it's caught.

(Some more detail here in this comment: https://jira.mongodb.org/browse/SERVER-57334?focusedCommentId=4038170&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-4038170
)