Generic BaseModel parsing from within a generic class

[hwong557]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

I have a generic class MyParser[T] which parses a python object into a generic type Payload[T]. When specifying a concrete type (SubPayload in this case), the generic field fails to be parsed into a SubPayload.

I am aware Payload[SubPayload].model_validate works, but for my purposes, I would like the MyParser generic wrapper because in reality, it contains more logic.

Thank you.

Example Code

from typing import Generic, TypeVar

from pydantic import BaseModel

T = TypeVar("T")


class SubPayload(BaseModel):
    result: str
    conf: float


class Payload(BaseModel, Generic[T]):
    job_id: int
    response: T


class MyParser(Generic[T]):
    def parse(self, obj) -> Payload[T]:
        return Payload[T].model_validate(obj)


parser = MyParser[SubPayload]()

out = {"job_id": 999, "response": {"result": "other", "conf": 0.99}}
parsed = parser.parse(out)

assert isinstance(parsed.response, SubPayload)  # this fails!

Python, Pydantic & OS Version

pydantic version: 2.7.0
        pydantic-core version: 2.18.1
          pydantic-core build: profile=release pgo=true
                 install path: /.venv/lib/python3.11/site-packages/pydantic
               python version: 3.11.8 (main, Feb 12 2024, 14:50:05) [GCC 13.2.1 20230801]
                     platform: Linux-6.6.28-1-lts-x86_64-with-glibc2.39
             related packages: typing_extensions-4.10.0 pydantic-settings-2.2.1 fastapi-0.110.0 email-validator-2.0.0.post2 typing_extensions-4.10.0 pydantic-settings-2.2.1 fastapi-0.110.1 mypy-1.9.0 pydantic-extra-types-2.6.0
                       commit: unknown

[rdeaton-freenome]

Probably same root cause as #8410

[sydney-runkle]

@hwong557,

Thanks for reporting this. Definitely doesn't seem like the right behavior. PRs welcome with a fix! Could be a great issue for someone looking to learn more about our internal generic processing.