The primary effort to implement the schema propagation for $lookup will be done in SERVER-96872. 

The focus of this ticket is to ensure the schema propagation when dealing with FLE2 schemas correctly blocks (i.e returns errors), when unsupported workflows are requested. 

For reference, the test cases that need to be covered are:

In addition, the following constraints will need to be enforced as part of this change:

• Test the following test cases with both CSFLE and QE schemas
  • Join fields are both encrypted -> Fail
  • Local join field encrypted/foreign join field unencrypted -> Fail
  • Local join field unencrypted/Foreign join field encrypted -> fail
  • No encrypted fields used as join field -> Succeed
• Test that mixing CSFLE & QE schemas is not supported.