superclass=

klass.rb

require "fiddle"

require_relative "settable_superclass"

class Klass
  using SettableSuperclass

  HEAP_TYPE = ->(obj) { Fiddle::Pointer.new(Fiddle.dlwrap(obj))[0, Fiddle::SIZEOF_VOIDP].unpack1("J") & 0x1f }
  private_constant :HEAP_TYPE

  def initialize(superclass = Object)
    class << self
      attr_reader :superclass

      def superclass=(new_super)
        raise FrozenError, "can't modify frozen #{self.class}" if frozen?
        if new_super.instance_of?(Class) && HEAP_TYPE.call(new_super.allocate) != HEAP_TYPE.call(self)
          raise TypeError, "#{new_super} instances have an incompatible heap layout"
        end
        singleton_class.superclass = new_super
        @superclass = new_super
      end

      def with_superclass(new_super, &block)
        original = superclass
        self.superclass = new_super
        begin
          block.call(self)
        ensure
          self.superclass = original
        end
      end

      def deconstruct_keys(keys) = {superclass: @superclass}

      def <(other)
        raise TypeError, "compared with non class/module" unless other.is_a?(Module)
        singleton_class.ancestors.include?(other) || nil
      end

      def to_s = "#<Klass superclass: #{@superclass}>"
      alias_method :inspect, :to_s
    end

    self.superclass = superclass
  end
end

settable_superclass.rb

require "fiddle"

module SettableSuperclass
  parent = Class.new
  child = Class.new(parent)
  child_pointer = Fiddle::Pointer.new(Fiddle.dlwrap(child))
  SUPER_OFFSET = 0.step(by: Fiddle::SIZEOF_VOIDP, to: 160).find do
    child_pointer[it, Fiddle::SIZEOF_VOIDP].unpack1("J") == Fiddle.dlwrap(parent)
  end

  refine Kernel do
    def superclass=(new_super)
      raise TypeError, "new_super must be a Class, got: #{new_super.class}" unless new_super.instance_of?(Class)
      Fiddle::Pointer.write(Fiddle.dlwrap(self) + SUPER_OFFSET, [Fiddle.dlwrap(new_super)].pack("J"))
    end
  end
end

shapeshifter.rb

require_relative "klass"

class Duck def quack = "Quack." end
class Goose def honk = "Honk." end

shapeshifter = Klass.new(Duck)
shapeshifter.quack #=> "Quack."
shapeshifter.superclass = Goose
shapeshifter.honk #=> "Honk."