Uninitialized scalar field
The field will contain an arbitrary value left over from earlier computations. A scalar field is not initialized by the constructor
/src/mongo/db/pipeline/document_source_unwind.cpp:92: UNINIT_CTOR 120391 Class member declaration for "_index".
/src/mongo/db/pipeline/document_source_unwind.cpp:102: UNINIT_CTOR 120391 Non-static class member "_index" is not initialized in this constructor nor in any functions that it calls.