Any chance you can make a shareable repro? E.g. try removing stuff from the file until you get something small enough to redact that mypy still crashes on. Also worth checking if the crash reproduces without the sqlalchemy plugin.

I spent a couple hours trying to reproduce this but was struggling. I think it does have something to do with the sqlalchemy plugin. If I remove @declared_mixin from any of the classes I'm mixing in it no longer seems to error. If I include the library that has the mixin in it directly in the project it also no longer seems to error which makes it tough to create a minimal example. I don't think I can spend too much more time on this, so feel free to close if it's not actionable.

What if you keep @declared_mixin, but remove plugins = "sqlalchemy.ext.mypy.plugin" from your mypy configuration?

Adding another config example, as we're running into this too while trying to upgrade to 3.11. We're trying 1.8 and also 1.7 to see if it works, but no dice.

/Users/me/Library/Caches/pypoetry/virtualenvs/backend-ZZ6YV39x-py3.11/lib/python3.11/site-packages/fastapi_users_db_sqlalchemy/__init__.py:-1: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.8.0
Traceback (most recent call last):
  File "mypy/checkexpr.py", line 5727, in accept
  File "mypy/nodes.py", line 2213, in accept
  File "mypy/checkexpr.py", line 5202, in visit_lambda_expr
  File "mypy/nodes.py", line 2206, in expr
IndexError: list index out of range

[mypy]
# We cannot use the pydantic.mypy plugin yet as it is not compatible with mypy 0.910
plugins = loguru_mypy, sqlalchemy.ext.mypy.plugin
exclude = alembic/versions/
follow_imports = normal
check_untyped_defs = True
disallow_incomplete_defs = True
disallow_untyped_defs = True
namespace_packages = True
warn_unused_ignores = True
show_error_codes = True
local_partial_types = True

It seems like this might be specific to sqlalchemy.ext.mypy.plugin, in which case you might be better off reporting to that project. They're probably also going to ask for a repro though

this might be already filed with sqlalchemy here sqlalchemy/sqlalchemy#10282

(yes, that one is newer but comes with a reproducer!)