Regression on mypy when using pydantic.mypy and a SubModel.

[morian]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

I noticed a regression between v2.6.3 and v2.6.4, probably due to merge request #8826.
This occurs in the following situation:

• An optional field is used in a subclass (the optional characteristic is dropped)
• Pydantic's mypy plugin is used through the toml configuration
• Strict optional config item is set to true (default behavior)

Result from the example code bellow is:

$ cat pyproject.toml
[tool.mypy]
plugins = ['pydantic.mypy']

$ mypy xxx.py
xxx.py:10: note: Revealed type is "xxx.Sub"
xxx.py:11: note: Revealed type is "builtins.str"
xxx.py:13: error: Incompatible types in assignment (expression has type "None", variable has type "str")  [assignment]

Everything is fine as long as I don't use the plugin, or pydantic v2.6.3, or if I set no_strict_optional, either from the command line or as a configuration entry in the toml file.

Example Code

from pydantic import BaseModel

class Base(BaseModel):
    field: str | None = None

class Sub(Base):
    pass

def stuff(item: Sub) -> None:
    reveal_type(item)
    reveal_type(item.field)

    item.field = None

Python, Pydantic & OS Version

pydantic version: 2.6.4
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=true
                 install path: /tmp/pydantic/venv/lib/python3.11/site-packages/pydantic
               python version: 3.11.8 (main, Mar 24 2024, 10:14:50) [GCC 13.2.1 20240210]
                     platform: Linux-6.8.1-x86_64-Intel-R-_Core-TM-_i7-6700_CPU_@_3.40GHz-with-glibc2.38
             related packages: typing_extensions-4.10.0 mypy-1.9.0
                       commit: unknown

[sydney-runkle]

Indeed, thanks for reporting this. This should be fixed in 2.7 with @dmontagu's recent mypy fixes. Beta for that release should be out soon!