Generic type aliases with TypeVar defaults infer to an incorrect type

[alexdrydew]

Bug Report

When using type alias of a generic type which uses type variables with defaults unbound type variables may infer to an incorrect default type.

To Reproduce

from typing import Generic, TypeVar, TypeAlias

T1 = TypeVar('T1', default=int | None)
T2 = TypeVar('T2', default=int | None)

class A(Generic[T1, T2]):
    def __init__(self, a: T1, b: T2) -> None:
        self.a = a
        self.b = b


MyA: TypeAlias = A[T1, int]
a: MyA = A(None, 10)

https://mypy-play.net/?mypy=latest&python=3.12&gist=abece06adfb80e28c6d23bd99997964e

Expected Behavior

MyA should preserve T1 default type, which is int | None

Actual Behavior

T1 is inferred to int default in MyA, so assignment to a fails with:

Argument 1 to "A" has incompatible type "None"; expected "int" [arg-type]

Your Environment

• Mypy version used: 1.13
• Python version used: 3.12

[alexdrydew]

After a closer look it seems that only None type disappears from type variable default type. It seems that the bug is simply an overlook in #16859 and can be easily fixed