类型提示 具有后备类型的通用 Class

Question

考虑以下示例

from typing import Generic, Optional, TypeVar

T = TypeVar("T", int, tuple[int, ...])
S = TypeVar("S", int, tuple[int, ...])


class Foo(Generic[S, T]):
    fallback_int: S
    fallback_tuple: T

    def __init__(self, x: Optional[S] = None, y: Optional[T] = None):
        if x is None:
            # Incompatible types in assignment (expression has type "int", variable has type "Tuple[int, ...]")
            self.fallback_int = 0  
        else:
            self.fallback_int = x

        if y is None:
            # Incompatible types in assignment (expression has type "Tuple[]", variable has type "int")
            self.fallback_tuple = () 
        else:
            self.fallback_tuple = y

我理解为什么mypy会引发错误，因为在两个代码路径中，要么T是int ，要么当T是tuple[int, ...]时， Foo.x的类型必须等于T 。

如何正确键入提示Foo以避免此错误？