Mock#to_ary and Array#flatten problem

I can reproduce this problem in a slightly simpler way :-

$ ruby -v
ruby 1.9.2p14 (2010-10-02 revision 29393) [x86_64-darwin10.4.0]
$ irb
ruby-1.9.2-head > require "rubygems"; require "mocha"
 => true
ruby-1.9.2-head > [[Object.new]].flatten
 => [#<Object:0x000001009ca3a0>]
ruby-1.9.2-head > [[Mocha::Mock.new]].flatten
 => Mocha::ExpectationError: unexpected invocation: #<Mock:0x1009f2990>.to_ary()
ruby-1.9.2-head > [[Mocha::Mock.new.quacks_like(Object.new)]].flatten
 => [#<Mock:0x1009e1f00>] 
ruby-1.9.2-head > Object.new.to_ary
 => NoMethodError: undefined method `to_ary' for #<Object:0x000001009bbf58>
ruby-1.9.2-head > Mocha::Mock.new.to_ary
 => Mocha::ExpectationError: unexpected invocation: #<Mock:0x1009c2498>.to_ary()
ruby-1.9.2-head > Mocha::Mock.new.quacks_like(Object.new).to_ary
 => NoMethodError: undefined method `to_ary' for #<Mock:0x1009a23a0> which responds like #<Object:0x1009a22d8>

It seems as if the problem is that Array#flatten is relying on an object to raise a NoMethodError. It doesn't seem to be enough to return false to respond_to?. Explicityly telling the mock object to respond like a ruby object seems to solve the problem, because then it raises a NoMethodError instead of a Mocha::ExpectationError.

I think this is not a problem in 1.8.7, because, although it generates a warning, Object#to_a returns a single element array of itself. I think flatten will try calling Object#to_a as well as Object#to_ary, and since Mocha::Mock inherits from Object, Mocha::Mock#to_a will return a single element array of itself too.

$ ruby -v 
ruby 1.8.7 (2010-08-16 patchlevel 302) [i686-darwin10.4.0]
$ irb
ruby-1.8.7-p302 > require "rubygems"; require "mocha"
 => true
ruby-1.8.7-p302 > [[Object.new]].flatten
 => [#<Object:0x101276a08>]
ruby-1.8.7-p302 > [[Mocha::Mock.new]].flatten
 => [#<Mock:0x101272908>]
ruby-1.8.7-p302 > Object.new.to_ary
 => NoMethodError: undefined method `to_ary' for #<Object:0x101269998>
ruby-1.8.7-p302 > Mocha::Mock.new.to_ary
 => Mocha::ExpectationError: unexpected invocation: #<Mock:0x101266338>.to_ary()
ruby-1.8.7-p302 > Object.new.to_a
 warning: default `to_a' will be obsolete
 => [#<Object:0x10125d418>] 
ruby-1.8.7-p302 > Mocha::Mock.new.to_a
 warning: default `to_a' will be obsolete
 => [#<Mock:0x10125a290>]
ruby-1.8.7-p302 > Mocha::Mock.new.quacks_like(Object.new).to_a
 warning: default `to_a' will be obsolete
 => [#<Mock:0x101254c50>]

I'm going to have to have a bit of a think about whether there's a better solution to this. One option, would be to set Object.new as the responder by default, but I'm not sure how backwardly compatible that change would be. Another possibility would be to raise NoMethodError's instead of Mocha::ExpectationError, at least in the case where no method has been stubbed, but again I need to think about the implications.

In the end it may turn out that it's better to bump the major version and create a new version of Mocha that supports Ruby 1.9, but not Ruby 1.8. And keep the old version supporting Ruby 1.8.