Function invocation pins type of arguments too early

Minimal repro:
```
def foo[T](a: T, b: T) -> T:
    ...

def test(x: int | None, y: int | None) -> int | None:
    if x is None:
        return foo(x, y)
    return None
```
Expected: No error
Actual: Argument `int | None` is not assignable to parameter `b` with type `None` in function `foo` [bad-argument-type]