class RSpec::Mocks::VerifyingMessageExpectation

def ensure_arity!(actual)
  return if method_reference.nil?
  method_reference.when_defined do |method|
    calculator = ArityCalculator.new(method)
    unless calculator.within_range?(actual)
      # Fail fast is required, otherwise the message expecation will fail
      # as well ("expected method not called") and clobber this one.
      @failed_fast = true
      @error_generator.raise_arity_error(calculator, actual)
    end
  end
end