Structural pattern matching and mangling private names

For this code:

class C:
  def f(self, x):
    match x:
      case D(__something=y):
        return y

It appears that the name "__something" isn't mangled. Under most other circumstances I'd expect this to be mangled to "_C__something". Is this:
* intentional,
* accidental, but how that it's done it's the defined behaviour,
* or a defect?

It doesn't seem like it's explicitly tested for in test_patma.py either way.

Thanks
David