results.txt

$ mypy t.py 
t.py:16: error: Argument 1 of "foo" is incompatible with supertype "BaseTest"; supertype defines the argument type as "TBase"  [override]
t.py:16: note: This violates the Liskov substitution principle
t.py:16: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
Found 1 error in 1 file (checked 1 source file)

t.py

#!/usr/bin/env python3

from pydantic import BaseModel

class TBase:
    pass

class TDerived(TBase):
    pass

class BaseTest(BaseModel):
    def foo(self, v: TBase) -> TBase:
        return v

class DerivedTest(BaseTest):
    def foo(self, v: TDerived) -> TDerived:
        return v