class T::Types::AttachedClassType

class, this is consistent with the runtime behavior for all type members.
As AttachedClass is modeled statically as a type member on every singleton
tracking, so at runtime we permit all values and rely on the static checker.
Modeling AttachedClass properly at runtime would require additional

def build_type 

def build_type
  nil
end

def initialize(); end 

def initialize(); end

def name

overrides Base 
def name
  "T.attached_class"
end

def subtype_of_single?(other)

overrides Base 
def subtype_of_single?(other)
ther
ttachedClassType

e

def valid?(obj)

overrides Base 
def valid?(obj)
  true
end