existing_type option for alter_column in batch does not work with Variant

[THS-on]

Describe the bug
Batch altering a column with the existing_type option set to a Variant like sa.Text().with_variant(sa.Text(10000), "mysql") fails with an error in alembic newer alembic versions starting with version 1.7.0.

Expected behavior
Migrations still work with a Variant set as existing type. In this case sa.Text().with_variant(sa.Text(10000), "mysql").

To Reproduce

1. create a new project with: alembic init alembic
2. create alembic/versions/f9828e772e26_init.py with the following content

"""init

Revision ID: f9828e772e26
Revises: 
Create Date: 2022-01-31 18:08:27.382366

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = 'f9828e772e26'
down_revision = None
branch_labels = None
depends_on = None


def upgrade():
    op.create_table("example", sa.Column('text', sa.Text().with_variant(sa.Text(10000), "mysql")))
    # This is done because sqlite does not support renaming a column
    with op.batch_alter_table('example') as batch_op:
        batch_op.alter_column('text', new_column_name='text_new',
                              existing_type=sa.Text().with_variant(sa.Text(10000), "mysql"), existing_nullable=True)

def downgrade():
    pass

4. change sqlalchemy.url in alembic.ini to sqlite:///test.sqlite
5. run migrations with: alembic upgrade head

Error

INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade  -> f9828e772e26, init
Traceback (most recent call last):
  File "<user_dir>venv/bin/alembic", line 8, in <module>
    sys.exit(main())
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/config.py", line 588, in main
    CommandLine(prog=prog).main(argv=argv)
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/config.py", line 582, in main
    self.run_cmd(cfg, options)
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/config.py", line 559, in run_cmd
    fn(
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/command.py", line 320, in upgrade
    script.run_env()
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/script/base.py", line 563, in run_env
    util.load_python_file(self.dir, "env.py")
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/util/pyfiles.py", line 92, in load_python_file
    module = load_module_py(module_id, path)
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/util/pyfiles.py", line 108, in load_module_py
    spec.loader.exec_module(module)  # type: ignore
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "<user_dir>alembic/env.py", line 77, in <module>
    run_migrations_online()
  File "<user_dir>alembic/env.py", line 71, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/runtime/environment.py", line 851, in run_migrations
    self.get_context().run_migrations(**kw)
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/runtime/migration.py", line 620, in run_migrations
    step.migration_fn(**kw)
  File "<user_dir>alembic/versions/f9828e772e26_init.py", line 22, in upgrade
    with op.batch_alter_table('example') as batch_op:
  File "/usr/lib/python3.10/contextlib.py", line 142, in __exit__
    next(self.gen)
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/operations/base.py", line 374, in batch_alter_table
    impl.flush()
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/operations/batch.py", line 138, in flush
    fn(*arg, **kw)
  File "<user_dir>venv/lib/python3.10/site-packages/alembic/operations/batch.py", line 469, in alter_column
    and kw["existing_type"].name  # type:ignore[attr-defined]
  File "<user_dir>venv/lib/python3.10/site-packages/sqlalchemy/sql/type_api.py", line 1462, in __getattr__
    return getattr(self.impl, key)
AttributeError: 'Text' object has no attribute 'name'

Versions.

• OS: Archlinux
• Python: 3.10.2
• Alembic: 1.7.5
• SQLAlchemy: 1.4.31
• Database: sqlite
• DBAPI: SQLite

Additional context
This is a regression introduced in alembic 1.7.0 with the following commit: e34f6c1

[zzzeek]

thanks for reporting will assume this is specific to batch mode

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the main branch:

resolve for variant before testing for schema type https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/3570

[zzzeek]

we're overdue for a release so will do that now.

[THS-on]

Can confirm that this fixes the issue. Thank you for providing a fix this fast.