This PR fixes how mypy handles generic classes with default type parameters when they refer to themselves inside the class body. ItFixes#20336

Before this change, if you wrote something like:

classFoo[X,Y,Z=object]:deftest(self,swapped:Foo[Y,X])->None:reveal_type(swapped)

mypy would show:

Revealed type is "Foo[X, Y, builtins.object]"

It ignored the fact thatFoo[Y, X] swaps the type variables.

With this change, mypy now keeps the order that the user wrote:

Revealed type is "Foo[Y, X, builtins.object]"

The fix is infix_instance inmypy/typeanal.py.
It checks for cases where all the type arguments are the class’s own type variables (likeFoo[Y, X]) and skips the extra expansion step that was reordering them.