SearchBuilder: contact search for tags with child-tags broken when using '=' operator

Overview

Searching for contact tags using the '=' operator is broken, if they have child tags: you get an SQL error. This problem was probably introduced with #1795 (closed).

Reproduction steps

1. Create tag A
2. Create tag B as a child of tag A
3. Go to the search builder
4. Search for contacts, tag(s), '=', tag A (see screenshot)
5. Got an error "Fatal error: DB error".

Current behaviour

DB Error with a something like

civicrm_entity_tag-60659ba2e61bf`.tag_id IN (  )

in the generated SQL.

Expected behaviour

Search should work :)

Environment information

Could reproduce this on dmaster, version 5.37.alpha1. Also reproduces on 5.35.1.

Comments

I think I know the culprit and am working on a patch.