Not sure if this is a bug or feature request: require 'minitest/autorun' require 'set' module ClassToProc refine Class do def to_proc lambda{ |*args| self.new(*args) } end end end using ClassToProc describe 'Class#to_proc' do it 'works when called directly' do Set.to_proc[[1,2]].must_equal Set[1,2] end it 'fails when called via ampersand' do [[1,2]].map(&Set).must_equal [Set[1,2]] end end The seco