Cannot migrate query that contains a function over a join column

Description

When testing readyset with benchbase's tpc-h benchmark, several queries contain a join condition, expressed in the where clause that causes a peculiar error when at least one of the columns is wrapped with a function.

Here is the query

create cache from
          SELECT c_id,
              c_last,
              sum(ol_amount) AS revenue,
              c_city,
              c_phone,
              n_name
              FROM customer,
              oorder,
              order_line,
              nation
              WHERE c_id = o_c_id
              AND c_w_id = o_w_id
              AND c_d_id = o_d_id
              AND ol_w_id = o_w_id
              AND ol_d_id = o_d_id
              AND ol_o_id = o_id
              AND o_entry_d >= '2007-01-02 00:00:00.000000'
              AND o_entry_d <= ol_delivery_d
              AND n_nationkey = ascii(substring(c_state from  1  for  1))
              GROUP BY c_id,
              c_last,
              c_city,
              c_phone,
              n_name
              ORDER BY revenue DESC;

Here is the error, as seen in the mysql client (carriage returns added by me):

ERROR 1105 (HY000): Error during RPC (extend_recipe (in readyset-client/src/controller/rpc.rs:49:26)): 
   SQL SELECT query 'q_6565248d6c10a8b' couldn't be added: MIR node '18' couldn't be lowered to dataflow: 
   Internal error: in readyset-server/src/controller/mir_to_flow.rs:45:5: 
    invariant failed: columns.len() == names.len();

The problem occurs because of this where predicate (which is a join condition):

AND n_nationkey = ascii(substring(c_state from  1  for  1))

Additional data points:

I tried several different functions (ascii, length, …) and they all exhibited the same error.
I changed the join clause to a naive literal (AND ascii(substring(c_state from 1 for 1)) ='f'), and it successfully migrated.
I created a variant where the two functions are over the same table's columns (where ascii(first_name) = ascii(last_name)), and this successfully migrated as well.
I moved the join condition, with the function, into the from clause, and it failed to migrate.

Thus, i think it is because at least one of the join condition's columns is wrapped in a function.

Steps to reproduce

create table user (id int, dept_name varchar(32));
create table dept (dept_id int, name varchar(32));
create cache from select user.id, dept.dept_id 
    from user, dept 
    where ascii(user.dept_name) = dept.name;

ReadySet version

git sha b647d97e3f1a0dfbe47168d0814a67f003066bb6

cli command

cargo --locked run --bin readyset -- \
   --upstream-db-url=mysql://root:noria@127.0.0.1:3306/benchbase --feature-post-lookup

readysettech / readyset