Why does the second 'p arg' report the Foo instance?

Question

class Foo
  def with_yield
    yield(self)
  end

  def with_instance_eval(&block)
    instance_eval(&block)
  end
end

f = Foo.new

f.with_yield do |arg|
  p self
  # => main
  p arg
  # => #<Foo:0x100124b10>
end

f.with_instance_eval do |arg|
  p self
  # => #<Foo:0x100124b10>
  p arg
  # => #<Foo:0x100124b10>
end

Why does the second 'p arg' report the Foo instance? Shouldn't it report nil since with_instance_eval does not yield self to the block?

Answer 1

Apparently in ruby 1.8 instance_eval does not only change the value of self inside the block to its receiver, it also yields that value. In 1.9 this is no longer the case (arg will be nil there), so that behavior should not be relied upon (I'm also pretty sure it's undocumented).