Problem

Mongodump/mongorestore currently builds unversioned 2dsphere indexes on the destination based on the default index version. This means that unversioned 2dsphere indexes are built with version 3 on the destination, although server treats them as v1 on the source.

According to the server release notes, v3 added a performance improvement, but v2 included a big behavior change.

Solution & Acceptance Criteria

Mongodump/mongorestore should stop automatically building unversioned 2dsphere indexes with the default version (3). See the linked IA writing ticket for the comprehensive solution.

Likewise, we should also add an invariant to prevent unversioned text indexes.

Impact

This bug can cause a destination 2dsphere index to behave differently than an unversioned 2dsphere index on the source.