myronmarston/crazy_ruby_bug.md

## crazy_ruby_bug.md

      
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              crazy_ruby_bug.md
            
          
    I've discovered a crazy bug that's really confusing me.  I'm curious to hear if anyone can explain it.
Here's some code in foo.rb:
class Superclass
  unless ENV['NORMAL_METHOD_DEF']
    define_method :regex do
      /^(\d)$/
    end
  else
    def regex
      /^(\d)$/
    end
  end
end

class Subclass < Superclass
  unless ENV['FORCE_NIL_BLOCK']
    def self.override(name)
      define_method(name) { super() }
    end
 else
    def self.override(name)
      define_method(name) { super(&nil) }
    end
 end

  unless ENV['DONT_PASS_BLOCK']
    override(:regex) { }
  else
    override(:regex)
  end
end

puts "Subclass.new.regex returns a regular expression object:"
puts Subclass.new.regex.inspect
puts
puts "String#match(regex) returns a MatchData object:"
puts "8".match(/^(\d)$/).inspect
puts
puts "But somehow, when I combine these, I can get nil:"
puts "8".match(Subclass.new.regex).inspect
puts
puts "Unless I add a tap block that does nothing:"
puts "8".match(Subclass.new.regex.tap { |s| }).inspect
And the results of running it:
➜ ruby --version
ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-darwin11.4.0]
➜ ruby foo.rb 
Subclass.new.regex returns a regular expression object:
/^(\d)$/

String#match(regex) returns a MatchData object:
#<MatchData "8" 1:"8">

But somehow, when I combine these, I can get nil:
nil

Unless I add a tap block that does nothing:
#<MatchData "8" 1:"8">

For some odd reason, String#match(regex) is returning nil when it should be returning a MatchData object. Adding a no-op tap block fixes this, but I have no idea why.
It seems to be related to the fact that Superclass#regex was defined using define_method rather than def; note what happens when I run it with NORMAL_METHOD_DEF=1:
➜ NORMAL_METHOD_DEF=1 ruby foo.rb 
Subclass.new.regex returns a regular expression object:
/^(\d)$/

String#match(regex) returns a MatchData object:
#<MatchData "8" 1:"8">

But somehow, when I combine these, I can get nil:
#<MatchData "8" 1:"8">

Unless I add a tap block that does nothing:
#<MatchData "8" 1:"8">

Also, the fact that I'm passing a block to override(:regex) seems to matter, too; if I run it with DONT_PASS_BLOCK=1 to avoid that, it fixes the problem again:
➜ DONT_PASS_BLOCK=1 ruby foo.rb
Subclass.new.regex returns a regular expression object:
/^(\d)$/

String#match(regex) returns a MatchData object:
#<MatchData "8" 1:"8">

But somehow, when I combine these, I can get nil:
#<MatchData "8" 1:"8">

Unless I add a tap block that does nothing:
#<MatchData "8" 1:"8">

It appears that I can work around the bug by explicitly passing no block in the call to super using &nil:
➜ FORCE_NIL_BLOCK=1 ruby foo.rb
Subclass.new.regex returns a regular expression object:
/^(\d)$/

String#match(regex) returns a MatchData object:
#<MatchData "8" 1:"8">

But somehow, when I combine these, I can get nil:
#<MatchData "8" 1:"8">

Unless I add a tap block that does nothing:
#<MatchData "8" 1:"8">

I don't understand this.  At all.  I'm not passing a block to Subclass#regex.
Can anyone explain this?