mypy thinks a config variable can never be None when nested in inherited BaseModel class, again

[Greesb]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

Hello,

mypy thinks that some variable can never be None when the typing and the default value clearly indicate otherwise.

Was already declared for 2.3 #7399, fixed until 2.6.3 and it seems to be back again for version 2.6.4.

Example Code

import pydantic


class ModelNested(pydantic.BaseModel):
    my_var: str | None = pydantic.Field(default=None)


class Model(ModelNested):
    pass


m = Model()
if m.my_var is None:
    print("hello there")  # test.py:14: error: Statement is unreachable  [unreachable]

Python, Pydantic & OS Version

pydantic version: 2.6.4
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=true
                 install path: ...
               python version: 3.12.2 (main, Feb 14 2024, 11:13:34) [Clang 15.0.0 (clang-1500.1.0.2.5)]
                     platform: macOS-14.4-arm64-arm-64bit
             related packages: fastapi-0.110.0 mypy-1.9.0 typing_extensions-4.10.0 pydantic-settings-2.2.1
                       commit: unknown

mypy settings

[tool.mypy]
strict = true
warn_unreachable = true
exclude = [".venv"]
show_error_codes = true
plugins = ["pydantic.mypy"]

[[tool.mypy.overrides]]
module = [
       "deepdiff.*",
       "vcr.*",
       "voluptuous.*",
       "msgpack.*",
       "bs4.*",
       "graphql_utils",
       "markdownify",
       "asgi_lifespan",
       "authlib.*",
       "imia.*",
       "starsessions.*",
       "sqlalchemy_utils.*",
       "alembic_utils.*",
       "google.*",
]
ignore_missing_imports = true

[sydney-runkle]

Should be fixed in 2.7, coming out soon! Fixed by @dmontagu here: #9008