Spec: More precision on type parameter order

The spec currently isn't very precise on the order in which type parameters appear in a class. (Given a class C that is generic over type parameters T1 and T2, does C[int, str] mean that T1 is bound to int or that T2 is bound to int?)

I think the rule should be:

• If PEP 695 syntax is used, the order is the order in which the type parameters appear in the type parameter list.
• If the class has Generic[...] as a syntactic base, the order is the order of the type arguments to Generic.
• If the class has Protocol[...] as a syntactic base, the order is the order of the type arguments to Protocol. This applies only if Protocol is subscripted, not if the class inherits from bare Protocol.
• Otherwise, the order is the order in which the type parameters appear syntactically in the base class list.

As far as I know, this is how all type checkers currently behave, but we just found out it's not how the runtime behaves when Protocol is involved: https://github.com/python/cpython/issues/137191#issuecomment-3132748382.

In these examples, all classes are generic over two parameters T1 and T2, and in all cases the type parameter order is T1, T2:

from typing import Generic, Protocol, TypeVar

class C1[T1, T2]: ...  # order is T1, T2

T1 = TypeVar("T1")
T2 = TypeVar("T2")

class C2(C1[T2, T1], Generic[T1, T2]): ...  # order is T1, T2

class GenericProto[T1, T2](Protocol):  ...  # order is T1, T2

class Proto2(GenericProto[T2, T1], Protocol[T1, T2]): ...  # order is T1, T2

class Proto3(GenericProto[T1, T2], Protocol): ...  # order is T1, T2

class C3(C2[T1, T2]): ...  # order is T1, T2

We should change the spec to make this rule explicit. The only current rule appears to be in https://typing.python.org/en/latest/spec/generics.html#arbitrary-generic-types-as-base-classes "Type variables are applied to the defined class in the order in which they first appear in any generic base classes".

Commencez par la section « Arbitrary generic types as base classes » de la spécification des generics, en particulier par sa règle actuelle concernant l’ordre des variables de type. Comparez cette formulation avec les cas proposés dans l’issue pour PEP 695, Generic, Protocol et les classes de base. Le travail est considéré comme terminé lorsque la spécification définit explicitement l’ordre pour chaque cas et correspond aux exemples fournis.