I think this should be preferred over https://github.com/pydantic/pydantic/pull/6915, mostly because the implementation there involves duplicating the list of types in the union, and generally seems more fragile.

I've opened as draft until docs/tests are added, hoping @sydney-runkle can help get this across the line tomorrow.
Add CallableDiscriminator and Tag. I think this should be preferred over https://github.com/pydantic/pydantic/pull/6915, mostly because the implementation there involves duplicating the list of types in the union, and generally seems more fragile.

I've opened as draft until docs/tests are added, hoping @sydney-runkle can help get this across the line tomorrow.
"""Pydantic-specific warnings."""
from __future__ import annotations as _annotations

from .version import version_short

__all__ = 'PydanticDeprecatedSince20', 'PydanticDeprecationWarning'


class PydanticDeprecationWarning(DeprecationWarning):
    """A Pydantic specific deprecation warning.

    This warning is raised when using deprecated functionality in Pydantic. It provides information on when the
    deprecation was introduced and the expected version in which the corresponding functionality will be removed.

    Attributes:
        message: Description of the warning.
        since: Pydantic version in what the deprecation was introduced.
        expected_removal: Pydantic version in what the corresponding functionality expected to be removed.
    """

    message: str
    since: tuple[int, int]
    expected_removal: tuple[int, int]

    def __init__(
        self, message: str, *args: object, since: tuple[int, int], expected_removal: tuple[int, int] | None = None
    ) -> None:
        super().__init__(message, *args)
        self.message = message.rstrip('.')
        self.since = since
        self.expected_removal = expected_removal if expected_removal is not None else (since[0] + 1, 0)

    def __str__(self) -> str:
        message = (
            f'{self.message}. Deprecated in Pydantic V{self.since[0]}.{self.since[1]}'
            f' to be removed in V{self.expected_removal[0]}.{self.expected_removal[1]}.'
        )
        if self.since == (2, 0):
            message += f' See Pydantic V2 Migration Guide at https://errors.pydantic.dev/{version_short()}/migration/'
        return message


class PydanticDeprecatedSince20(PydanticDeprecationWarning):
    """A specific `PydanticDeprecationWarning` subclass defining functionality deprecated since Pydantic 2.0."""

    def __init__(self, message: str, *args: object) -> None:
        super().__init__(message, *args, since=(2, 0), expected_removal=(3, 0))


class GenericBeforeBaseModelWarning(Warning):
    pass

"""Pydantic-specific warnings."""
from __future__ import annotations as _annotations

from .version import version_short

__all__ = 'PydanticDeprecatedSince20', 'PydanticDeprecationWarning'


class PydanticDeprecationWarning(DeprecationWarning):
    """A Pydantic specific deprecation warning.

    This warning is raised when using deprecated functionality in Pydantic. It provides information on when the
    deprecation was introduced and the expected version in which the corresponding functionality will be removed.

    Attributes:
        message: Description of the warning.
        since: Pydantic version in what the deprecation was introduced.
        expected_removal: Pydantic version in what the corresponding functionality expected to be removed.
    """

    message: str
    since: tuple[int, int]
    expected_removal: tuple[int, int]

    def __init__(
        self, message: str, *args: object, since: tuple[int, int], expected_removal: tuple[int, int] | None = None
    ) -> None:
        super().__init__(message, *args)
        self.message = message.rstrip('.')
        self.since = since
        self.expected_removal = expected_removal if expected_removal is not None else (since[0] + 1, 0)

    def __str__(self) -> str:
        message = (
            f'{self.message}. Deprecated in Pydantic V{self.since[0]}.{self.since[1]}'
            f' to be removed in V{self.expected_removal[0]}.{self.expected_removal[1]}.'
        )
        if self.since == (2, 0):
            message += f' See Pydantic V2 Migration Guide at https://errors.pydantic.dev/{version_short()}/migration/'
        return message


class PydanticDeprecatedSince20(PydanticDeprecationWarning):
    """A specific `PydanticDeprecationWarning` subclass defining functionality deprecated since Pydantic 2.0."""

    def __init__(self, message: str, *args: object) -> None:
        super().__init__(message, *args, since=(2, 0), expected_removal=(3, 0))


class GenericBeforeBaseModelWarning(Warning):
    pass
