cockroachdb crdb - enums not working with prepared statements

• asyncpg version: 0.21.0
• CockroachDB version: v20.2.1
• PostgreSQL version: 12.4
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?:
• Python version: 3.9.0
• Platform: linux-x86_64
• Do you use pgbouncer?: no
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?:
• Can the issue be reproduced under both asyncio and
  uvloop?: didn't test uvloop

enums have just been added to crdb. testing asyncpg shows an issue with prepared statements.

running this:

await conn.execute("INSERT INTO accounts2 (id, balance, mode) VALUES ($1, $2, $3)", 9, 2000, "active", )
throws

    statement = await self._protocol.prepare(stmt_name, query, timeout)
  File "asyncpg/protocol/protocol.pyx", line 163, in prepare
asyncpg.exceptions.FeatureNotSupportedError: CTEs may not be correlated

the prepared statement works fine in pgsql. enums work fine without prepared statements.

I have a silly bug exerciser, but needs some cleanup, you can see the relevant test2 functions if you need working code. i've tested psycopg2 and asyncpg against crdb and pgsql and out of the four runs, only asyncpg+crdb shows an issue.

crdbissues.py.txt

Start with the attached crdbissues.py.txt reproducer and the prepare path cited at asyncpg/protocol/protocol.pyx:163. Compare the prepared enum insert behavior on CockroachDB and PostgreSQL, then trace the resulting protocol handling. Done means a regression test covers the prepared enum statement and it succeeds against the affected database without the CTE error.