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/accumulator_js_reduce.h:143: UNINIT_CTOR 114887 Class member declaration for "_pendingCallsMerging".
/src/mongo/db/pipeline/accumulator_js_reduce.h:121: UNINIT_CTOR 114887 Non-static class member "_pendingCallsMerging" is not initialized in this constructor nor in any functions that it calls.