[Sqlalchemy-tickets] Issue #3454: Can't use a CAST in a PostgreSQL ExcludeConstraint (zzzeek/sqlalchemy)

[vr2262 <iss...@bi...>]

New issue 3454: Can't use a CAST in a PostgreSQL ExcludeConstraint
https://bitbucket.org/zzzeek/sqlalchemy/issue/3454/cant-use-a-cast-in-a-postgresql

vr2262:

In PostgreSQL I can create a table with an exclusion constraint involving a `CAST` (the `CAST` is necessary because the `UUID` type doesn't have a default operator class for `gist`):

```
#!sql
CREATE EXTENSION btree_gist;
CREATE TABLE example (
  id         UUID,
  some_range INT4RANGE,
  EXCLUDE USING gist (CAST("id" AS TEXT) WITH =, some_range WITH &&)
);
```

When I try to do the same in SQLAlchemy:

```
#!python
from sqlalchemy import *
from sqlalchemy.dialects.postgresql import (
    UUID, INT4RANGE, ExcludeConstraint, TEXT
)

class Example(Base):
    __tablename__ = 'example'
    id = Column(UUID)
    some_range = Column(INT4RANGE)
    __table_args__ = (
        ExcludeConstraint(
            (cast('id', TEXT), '='), ('some_range', '&&')
        ),
    )
```

I get the `error sqlalchemy.exc.ArgumentError: Can't add unnamed column to column collection`.

`('id::TEXT', '=')` doesn't work because SQLAlchemy doesn't recognize `'id::TEXT'` as a column.