class NestedPathsDict(BaseModel):
    """Nested dict with paths."""

    __root__: Dict[  # See https://github.com/samuelcolvin/pydantic/discussions/4220
        Union[AbsolutePath, RelativePath], Optional[NestedPathsDict]
    ]

class NestedPathsDict(BaseModel):
    __root__: dict[
        Union[AbsolutePath, RelativePath], Optional[NestedPathsDict]
    ]

TypeError: To define root models, use `pydantic.RootModel` rather than a field called '__root__'

NestedPathsDict: RootModel[dict[
    Union[AbsolutePath, RelativePath], Optional[NestedPathsDict]
]]

pydantic.errors.PydanticUndefinedAnnotation: name 'NestedPathsDict' is not defined