Bug #15808: Forwardable doesn't allow forwarding the 'print' message

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-darwin17]
• Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN

[kyrylo (Kyrylo Silin)]

The following program forwards two messages to @foo: message 'print' and message 'bar'.
When I forward the 'print' message, I see a warning and the return value of print is always nil
(despite the fact that I specified my own return value).
When I forward the 'bar' message, it works as expected (I see the return value of 'bar').

require 'forwardable'
require 'ostruct'

class A
  extend Forwardable
  def_delegators :@foo, :print, :bar

  def initialize(foo)
    @foo = foo
  end
end

obj = A.new(OpenStruct.new(print: 123, bar: 456))

p "print - #{obj.print}; bar - #{obj.bar}" 

forwardable_bug.rb:15: warning: A#print at /Users/kyrylo/.rubies/ruby-2.6.0/lib/ruby/2.6.0/forwardable.rb:158 forwarding to private method OpenStruct#print
"print - ; bar - 456"

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Rejected

This is not a bug in Forwardable, this is due to the fact that OpenStruct doesn't override print, as Kernel#print is already defined. You must undefine the print method in OpenStruct if you want this to work. Example:

require 'forwardable'
require 'ostruct'

class A
  extend Forwardable
  def_delegators :@foo, :print, :bar

  def initialize(foo)
    @foo = foo
  end
end

os = OpenStruct.new(print: 123, bar: 456)
os.method(:print).owner
# => Kernel
os.singleton_class.send(:undef_method, :print)
os.method(:print).owner
=> #<Class:#<OpenStruct:0x00000038d756dc80>>
obj = A.new(os)

p "print - #{obj.print}; bar - #{obj.bar}" 
# "print - 123; bar - 456"