Indeed, sorry, my example was bad. My library was raising at several place, and I've extrapolated about generic substitution.

I've indeed other substitutions (without `TypeVar`), and because they were failing, I've assumed that all of my substitutions were failing; I was wrong about generic one.

For example, if I want to substitute `int | Collection[int]` to `int | list[int]`, I will have to replace `types.UnionType` by `typing.Union` or use `reduce`, while it was not necessary in 3.9 where I could just write `get_origin(tp)[new_args]`.

So I'll have to add some `if` in my code.