From classes, you can reference toplevel constants even with a namespace! It'll just issue warnings. From modules, you get a NameError

wtf.rb

C = 1
CA = 1
CB = 1

class A
  CA = 2
  class B
    CB = 3

    def c
      A::B::C
    end

    def self.c
      A::B::C
    end

    def ca
      A::B::CA
    end

    def cb
      A::B::CB
    end
  end
end

b = A::B.new
b.c
# warning: toplevel constant C referenced by A::B::C
# => 1 

b.ca
# warning: toplevel constant CA referenced by A::B::CA
# => 1 
#    ^ Not 2!

b.cb
# => 3

A::B.c
# warning: toplevel constant C referenced by A::B::C
# => 1

module D
  module E
    def self.c
      D::E::C
    end
  end
end

D::E.c
# NameError: uninitialized constant D::E::C