This is caused by from __future__ import annotations in main.py added by #1560

I tracked down why this is happening but I don't know the best solution.
sqlmodel main.py imports Undefined and UndefinedType from _.compat and then again from pydantic_core in the if TYPE_CHECKING case. For some reason the type checker (when using future annotations) considers both types in the type check, so any argument that uses UndefinedType becomes Unknown | UndefinedType, which leads to the error.

I don't know how it would affect older versions and tests, but adding an else clause to the if TYPE_CHECKING statement and importing UndefinedType and Undefined from ._compat in that block, while removing them from the ._compat import above fixes the issue pyright is having. It seems like pyright is evaluating the if-else block in ._compat to be a union of the possible types in the if IS_PYDANTIC_V2: ... else: ... clause, not the ones that are actually present based on the version installed.

The relevant part of the import block now looks like this:

from ._compat import (  # type: ignore[attr-defined]
    IS_PYDANTIC_V2,
    PYDANTIC_MINOR_VERSION,
    BaseConfig,
    ModelField,
    ModelMetaclass,
    Representation,
    SQLModelConfig,
    _calculate_keys,
    finish_init,
    get_annotations,
    get_config_value,
    get_field_metadata,
    get_model_fields,
    get_relationship_to,
    get_sa_type_from_field,
    init_pydantic_private_attrs,
    is_field_noneable,
    is_table_model_class,
    post_init_field_info,
    set_config_value,
    sqlmodel_init,
    sqlmodel_validate,
)
from .sql.sqltypes import AutoString

if TYPE_CHECKING:
    from pydantic._internal._model_construction import ModelMetaclass as ModelMetaclass
    from pydantic._internal._repr import Representation as Representation
    from pydantic_core import PydanticUndefined as Undefined
    from pydantic_core import PydanticUndefinedType as UndefinedType
else:
    from ._compat import Undefined, UndefinedType

Thoughts? I can submit a PR to see if it still passes tests, if you want; though I feel this is just a band-aid to a larger issue... As far as I understand pydantic_core didn't exist until pydantic v2, so the type checker will get mad at the if TYPE_CHECKING block in main.py if pydantic v1 is in use anyway. Is pydantic v1 support a priority moving forward with this project?

Any updates on the error?

If someone is wondering about the complete error message -

Type of "Field" is partially unknown Type of "Field" is "Overload[(default: Any = Undefined, *, default_factory: (() -> Any) | None = None, alias: str | None = None, title: str | None = None, description: str | None = None, exclude: AbstractSet[int | str] | Mapping[int | str, Any] | Any = None, include: AbstractSet[int | str] | Mapping[int | str, Any] | Any = None, const: bool | None = None, gt: float | None = None, ge: float | None = None, lt: float | None = None, le: float | None = None, multiple_of: float | None = None, max_digits: int | None = None, decimal_places: int | None = None, min_items: int | None = None, max_items: int | None = None, unique_items: bool | None = None, min_length: int | None = None, max_length: int | None = None, allow_mutation: bool = True, regex: str | None = None, discriminator: str | None = None, repr: bool = True, primary_key: bool | Unknown | PydanticUndefinedType = Undefined, foreign_key: Any = Undefined, unique: bool | Unknown | PydanticUndefinedType = Undefined, nullable: bool | Unknown | PydanticUndefinedType = Undefined, index: bool | Unknown | PydanticUndefinedType = Undefined, sa_type: Type[Any] | Unknown | PydanticUndefinedType = Undefined, sa_column_args: Sequence[Any] | Unknown | PydanticUndefinedType = Undefined, sa_column_kwargs: Mapping[str, Any] | Unknown | PydanticUndefinedType = Undefined, schema_extra: Dict[str, Any] | None = None) -> Any, (default: Any = Undefined, *, default_factory: (() -> Any) | None = None, alias: str | None = None, title: str | None = None, description: str | None = None, exclude: AbstractSet[int | str] | Mapping[int | str, Any] | Any = None, include: AbstractSet[int | str] | Mapping[int | str, Any] | Any = None, const: bool | None = None, gt: float | None = None, ge: float | None = None, lt: float | None = None, le: float | None = None, multiple_of: float | None = None, max_digits: int | None = None, decimal_places: int | None = None, min_items: int | None = None, max_items: int | None = None, unique_items: bool | None = None, min_length: int | None = None, max_length: int | None = None, allow_mutation: bool = True, regex: str | None = None, discriminator: str | None = None, repr: bool = True, primary_key: bool | Unknown | PydanticUndefinedType = Undefined, foreign_key: str, ondelete: Unknown | PydanticUndefinedType | Literal['CASCADE', 'SET NULL', 'RESTRICT'] = Undefined, unique: bool | Unknown | PydanticUndefinedType = Undefined, nullable: bool | Unknown | PydanticUndefinedType = Undefined, index: bool | Unknown | PydanticUndefinedType = Undefined, sa_type: Type[Any] | Unknown | PydanticUndefinedType = Undefined, sa_column_args: Sequence[Any] | Unknown | PydanticUndefinedType = Undefined, sa_column_kwargs: Mapping[str, Any] | Unknown | PydanticUndefinedType = Undefined, schema_extra: Dict[str, Any] | None = None) -> Any, (default: Any = Undefined, *, default_factory: (() -> Any) | None = None, alias: str | None = None, title: str | None = None, description: str | None = None, exclude: AbstractSet[int | str] | Mapping[int | str, Any] | Any = None, include: AbstractSet[int | str] | Mapping[int | str, Any] | Any = None, const: bool | None = None, gt: float | None = None, ge: float | None = None, lt: float | None = None, le: float | None = None, multiple_of: float | None = None, max_digits: int | None = None, decimal_places: int | None = None, min_items: int | None = None, max_items: int | None = None, unique_items: bool | None = None, min_length: int | None = None, max_length: int | None = None, allow_mutation: bool = True, regex: str | None = None, discriminator: str | None = None, repr: bool = True, sa_column: Column[Any] | Unknown | PydanticUndefinedType = Undefined, schema_extra: Dict[str, Any] | None = None) -> Any]"
Pylance reportUnknownVariableType

Submitted #1644

This has been resolved, there is no errors in SQLModel 0.0.31