Alembic 1.10.4 incorrectly reflects nulls not distinct

[CaselIT]

          Hi, I'm not sure this is exactly related but might help.

We have updated to alembic 1.10.4 and we see some changes are always detected and not expected, everytime we generate a new migration.

-- requirements.txt

alembic==1.10.4
SQLAlchemy==2.0.19

-- models.py

from datetime import datetime

from sqlalchemy import UniqueConstraint
from sqlalchemy.orm import Mapped, mapped_column, DeclarativeBase

class Base(DeclarativeBase):
    pass

class Exemple(Base):
    __tablename__ = "dummy"
    id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
    a: Mapped[int]
    b: Mapped[int]
    tomb_date: Mapped[datetime] = mapped_column(nullable=True)
    __table_args__ = (
        UniqueConstraint(
            "a",
            "b",
            "tomb_date",
            postgresql_nulls_not_distinct=True,
            name="dummy_a_b_tomb_date_key"
        ),
    )

Initial migration is looking great, and postgresql nulls not distinct works.
But when I rerun autogenerate it always detect the same change.

alembic revision --autogenerate -m "test"
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.ddl.postgresql] Detected sequence named 'dummy_id_seq' as owned by integer column 'dummy(id)', assuming SERIAL and omitting
INFO  [alembic.autogenerate.compare] Detected changed unique constraint 'dummy_a_b_tomb_date_key' on 'dummy': expression ('UNIQUE_CONSTRAINT', 'a', 'b', 'tomb_date') to ('UNIQUE_CONSTRAINT', 'a', 'b', 'tomb_date', ('nulls_not_distinct', True))
  Generating ... /alembic/versions/733f639ec673_test.py ...  done

It generates a new migration file every time with the same content

"""test

Revision ID: 733f639ec673
Revises: a96502e5e7d3
Create Date: 2023-08-08 17:29:31.612196

"""
from typing import Sequence, Union

from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision: str = '733f639ec673'
down_revision: Union[str, None] = 'a96502e5e7d3'
branch_labels: Union[str, Sequence[str], None] = None
depends_on: Union[str, Sequence[str], None] = None


def upgrade() -> None:
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_constraint('dummy_a_b_tomb_date_key', 'dummy', type_='unique')
    op.create_unique_constraint('dummy_a_b_tomb_date_key', 'dummy', ['a', 'b', 'tomb_date'], postgresql_nulls_not_distinct=True)
    # ### end Alembic commands ###


def downgrade() -> None:
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_constraint('dummy_a_b_tomb_date_key', 'dummy', type_='unique')
    op.create_unique_constraint('dummy_a_b_tomb_date_key', 'dummy', ['a', 'b', 'tomb_date'])
    # ### end Alembic commands ###

Originally posted by @cedric-roussel in #1288 (comment)

[CaselIT]

complete example from @cedric-roussel at https://github.com/cedric-roussel/alembic-1288

[CaselIT]

from the log message it seems that the constraint is not detected with null not distinct

[CaselIT]

this is completely broken, also the indexes. it seems that fd59164 added tests but they were not very effective ...

[sqla-tester]

Federico Caselli has proposed a fix for this issue in the main branch:

Fix autogenerate issue in nulls not distinct https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/4797