Clang 19 encounters a compilation failure in bsoncolumnbuilder.h. BSONColumnBuilder defines a unary constructor that takes an instance of a templated allocator type, which defaults to a default-constructed instance. Sometimes BSONColumnBuilder instances instantiate the Allocator template with the TrackingAllocator, which explicitly deletes the nullary constructor. As a result, any such instance that attempts to default-construct will fail in compilation.

It's unclear why Clang 19 uniquely runs into this issue (full compile output will be posted in a comment), but in any case we should tolerate allocator types which are not default-constructible because the C++ standard does not require them to be.