@AlexWaygood discovered that even though we've been propagating
specializations to _parent_ base classes correctly, we haven't been
passing them on to _grandparent_ base classes:
https://github.com/astral-sh/ruff/pull/17832#issuecomment-2854360969
```py
class Bar[T]:
x: T
class Baz[T](Bar[T]): ...
class Spam[T](Baz[T]): ...
reveal_type(Spam[int]().x) # revealed: `T`, but should be `int`
```
This PR updates the MRO machinery to apply the current specialization
when starting to iterate the MRO of each base class.
If a typevar is declared as having a default, we shouldn't require a
type to be specified for that typevar when explicitly specializing a
generic class:
```py
class WithDefault[T, U = int]: ...
reveal_type(WithDefault[str]()) # revealed: WithDefault[str, int]
```
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Fixes
https://github.com/astral-sh/ruff/pull/17832#issuecomment-2851224968. We
had a comment that we did not need to apply specializations to generic
aliases, or to the bound `self` of a bound method, because they were
already specialized. But they might be specialized with a type variable,
which _does_ need to be specialized, in the case of a "multi-step"
specialization, such as:
```py
class LinkedList[T]: ...
class C[U]:
def method(self) -> LinkedList[U]:
return LinkedList[U]()
```
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
