So I get the same behavior with my fix as MRI does but I had to make a guess. My guess was that since we are assuming that branch was dealing with an IO we should make sure it is in fact an IO before calling open?.

MRI implements all this in C and we implement ours in Ruby so a side-by-side comparison is difficult. I definitely made sure we will only safely call open? and the comment next to this like implies something about digging in internals of IO.

This is still a problem on JRuby 9.0.4.0:

$ ruby -v
jruby 9.0.4.0 (2.2.2) 2015-11-12 b9fb7aa Java HotSpot(TM) 64-Bit Server VM 24.71-b01 on 1.7.0_71-b14 +jit [darwin-x86_64]
$ irb
>> require "io/console"
false
>> IO.console
#<File:/dev/tty>
crw-rw-rw-  1 root  wheel    2,   0 Feb  2 07:28 /dev/tty
>> IO.console.winsize
NoMethodError: undefined method `open?' for #<File:/dev/tty>
    from /Users/mbrictson/.rbenv/versions/jruby-9.0.4.0/lib/ruby/stdlib/io/console.rb:159:in `console'
    from (irb):3:in `<eval>'
    from org/jruby/RubyKernel.java:978:in `eval'
    from org/jruby/RubyKernel.java:1291:in `loop'
    from org/jruby/RubyKernel.java:1098:in `catch'
    from org/jruby/RubyKernel.java:1098:in `catch'
    from /Users/mbrictson/.rbenv/versions/jruby-9.0.4.0/bin/irb:13:in `<top>'

I am unable to reproduce on JRuby 9.1, but perhaps my io/console is loading and yours is not? You do appear to get the proper File opened on /dev/tty, but for some reason it doesn't have the methods it should.

Ah-ha, I believe this is fixed by @kares in 343f2a9, where he realizes there's no File#open? and replaces it with a proper call to File#closed?.

This is fixed as of JRuby 9.0.5.0.