JRuby block bug

broken.rb

JRUBY_VERSION #=> "1.7.0.preview2" # also present in 1.6.7.2 in 1.9.2 mode
RUBY_VERSION #=> "1.9.3"

data = {"x"=>[{"y"=>42}]} #=> {"x"=>[{"y"=>42}]}

class Hash
  def map_as_hash
    Hash[map { |k, v| [k, yield(v)] }]
  end
end #=> nil

data.map_as_hash { |x| x } #=> {"x"=>[{"y"=>42}]}
data #=> {"x"=>[{"y"=>42}]}

data.map_as_hash { |x| x.first } #=> {"x"=>{"y"=>42}}
data #=> {"x"=>[{"y"=>42}]}

data.map_as_hash(&:first) #=> {"x"=>["y", 42]}
data #=> {"x"=>[]}

works.rb

JRUBY_VERSION #=> "1.7.0.preview2" # also present in 1.6.7.2 in 1.9.2 mode
RUBY_VERSION #=> "1.9.3"

data = {"x"=>[{"y"=>42}]} #=> {"x"=>[{"y"=>42}]}

class Hash
  def map_as_hash(&block)
    Hash[map { |k, v| [k, block.call(v)] }]
  end
end #=> nil

data.map_as_hash { |x| x } #=> {"x"=>[{"y"=>42}]}
data #=> {"x"=>[{"y"=>42}]}

data.map_as_hash { |x| x.first } #=> {"x"=>{"y"=>42}}
data #=> {"x"=>[{"y"=>42}]}

data.map_as_hash(&:first) #=> {"x"=>{"y"=>42}} 
data #=> {"x"=>[{"y"=>42}]} 

You're a waste of a jruby bug