Feature

See PEP 696 – Type defaults for TypeVarLikes for details. The PEP has not yet been accepted though. Example from the PEP:

T = TypeVar("T", default=int)  # This means that if no type is specified T = int

@dataclass
class Box(Generic[T]):
    value: T | None = None

reveal_type(Box())                      # type is Box[int]
reveal_type(Box(value="Hello World!"))  # type is Box[str]

Since I did not find an existing tracking issue for this feature, I decided to open one.

The default= argument is already supported in typing_extensions version 4.4.0+ and typeshed (python/typeshed#8821)

Edit from maintainer: mypy supports basic PEP 696 use cases, but is missing some support for less common use cases. Give it a try!