In OwnedValueAccessor, _val will be left uninitialized if the default constructor is used. It should probably have a default value of zero, as other classes in this file do so.

/**
 * Accessor for a slot which can own the value held by that slot.
 */
class OwnedValueAccessor final : public SlotAccessor {
public:
    OwnedValueAccessor() = default;

// omitted

private:
    bool _owned{false};
    TypeTags _tag{TypeTags::Nothing};
    Value _val;  // <--------------
};

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/exec/sbe/values/slot.h:199: UNINIT_CTOR 119880 The compiler-generated constructor for this class does not initialize "_val".