defined?(super) on Ruby 2.0.0-p0 is different from 1.9.3-p392

broken-super.txt

± aa+rm |ruby2 ✗| → bundle exec ruby reduced.rb
This is not nil, it is a PG::Result
#<PG::Result:0x007fb632e778a0 @connection=#<PG::Connection:0x007fb632e7fa28>>

This is a PG::Result on 1.9.3, but it is nil on Ruby 2.0 because defined?(super) returned nil
the value of defined?(super) is: nil
nil

|ruby-2.0.0-p0| Lyra-Heartstrings-5 in ~/workspace/makara
± aa+rm |ruby2 ✗| → rvm use 1.9.3
Using /Users/wanelo/.rvm/gems/ruby-1.9.3-p392

|ruby-1.9.3-p392| Lyra-Heartstrings-5 in ~/workspace/makara
± aa+rm |ruby2 ✗| → bundle exec ruby reduced.rb
This is not nil, it is a PG::Result
#<PG::Result:0x007fa343165ec0>

This is a PG::Result on 1.9.3, but it is nil on Ruby 2.0 because defined?(super) returned nil
the value of defined?(super) is: "super"
#<PG::Result:0x007fa343165448>

reduced.rb

#!/usr/bin/env ruby

require 'active_record'

module Makara
  
  def self.connection
    nil
  end
  
  module Connection
    # module which gets extended on adapter instances
    # overrides execute so it will delegate to the makara adapter once
    module Decorator

      # if we have a makara adapter and we're not alrady hijacked,
      # allow the adapter to handle the execute
      def with_makara
        adapter = Makara.connection
        if adapter && !adapter.hijacking?
          yield adapter
        else
          yield nil
        end
      end

      def execute(*args)
        with_makara do |acceptor|
          puts "the value of defined?(super) is: #{defined?(super).inspect}"
          return (defined?(super) ? super : nil) if acceptor.nil?
          acceptor.execute(*args)
        end
      end

    end
  end
end

ActiveRecord::Base.establish_connection(:adapter => "postgresql", :database => "makara_test")
con = ActiveRecord::Base.connection

puts "This is not nil, it is a PG::Result"
p con.execute("select 1")
puts

con.extend ::Makara::Connection::Decorator
puts "This is a PG::Result on 1.9.3, but it is nil on Ruby 2.0 because defined?(super) returned nil"
p con.execute("select 1")

This code, on the other hand, works correctly on both 1.9.3 and 2.0.0:

module Makara  
  module Connection
    module Decorator

      def execute(*args)
        acceptor = nil
        puts "the value of defined?(super) is: #{defined?(super).inspect}"
        return (defined?(super) ? super : nil) if acceptor.nil?
        acceptor.execute(*args)
      end

    end
  end
end

To make things even more awesome, this breaks Rspec as well for the same code:

# Works on 1.9.3 but not 2.0.0
master.should_receive(:execute).with(select, nil).and_call_original

# Works on both 1.9.3 and 2.0.0
result = master.execute(select)
master.should_receive(:execute).with(select, nil){ result }

Extremely minimal example courtesy of @fxn:

class C
  def x
  end
end

module M
  def b
    yield
  end

  def x
    b do
      # On 1.9.3, this is "super", but on 2.0.0 it is nil
      p defined?(super)
    end
  end
end

c = C.new
c.extend M
c.x