annotations.rb

module Annotations
  def override
    @check_method = true
  end

  def singleton_method_added method_name
    if @check_method
      clazz = self.ancestors.reject{|e| e == self}.first
      unless clazz.methods.include? method_name
        raise "None of #{self}'s ancestors defines ##{method_name}"
      end
      super_method = clazz.method method_name
      my_method = self.method method_name
      unless super_method.arity == my_method.arity
        raise "Arity Mismatch: #{clazz}>>##{method_name}(#{super_method.arity}) vs. #{self}>>##{method_name}(#{my_method.arity})"
      end
    end
    @check_method = false
  end

  def method_added method_name
    if @check_method
      clazz = self.ancestors.reject{|e| e == self}.first
      unless clazz.instance_methods.include? method_name
        raise "None of #{self}'s ancestors defines ##{method_name}"
      end
      super_method = clazz.instance_method method_name
      my_method = self.instance_method method_name
      unless super_method.arity == my_method.arity
        raise "Arity Mismatch: #{clazz}##{method_name}(#{super_method.arity}) vs. #{self}##{method_name}(#{my_method.arity})"
      end
    end
    @check_method = nil
  end
end

output.txt

! ruby try-annotations.rb 
Example B:
Arity Mismatch: A>>#with(1) vs. B>>#with(0)

Example C:
None of C's ancestors defines #not

Example D:
None of D's ancestors defines #two

Example E:
Arity Mismatch: A#number(0) vs. E#number(1)

try-annotations.rb

$:.unshift '.'
require 'annotate'

class A
  def initialize num
    @num = num
  end
  
  def number
    @num
  end

  def self.with num
    self.new num
  end
end

begin
class B < A
  extend Annotations

  override
  def self.with
    self.new [1,2]
  end
end
rescue
  puts "Example B:\n#{$!.message}\n\n"
end

begin
class C < A
  extend Annotations
  override
  def self.not num
    self.new num
  end
end
rescue
  puts "Example C:\n#{$!.message}\n\n"
end

begin
class D < A
  extend Annotations
  def initialize
    super 2
  end

  def one
    1
  end

  override
  def two
    2
  end
end
rescue
  puts "Example D:\n#{$!.message}\n\n"
end

begin
class E < A
  extend Annotations
  override
  def number num
    num
  end
end
rescue
  puts "Example E:\n#{$!.message}\n\n"
end