local_binding.rb

class C
  attr_reader :read_only

  def assert(assertion, message="assertion failed")
    raise RuntimeError, message unless assertion
  end

  def initialize
    @read_only = 13
    assert read_only == 13
  end

  def m
    assert read_only == 13
    assert !read_only.nil?
    # trying to set a local variable named `read_only`
    read_only = \
      # probably by now Ruby set a ref in the local binding for `read_only`
      # so it is now nil ?
      read_only.dup
  end
end

C.new.m # => raises TypeError: can't dup NilClass