[ruby-core:78742] [Ruby trunk Bug#12907] rb_respond_to() return value is incorrect

Issue #12907 has been updated by Eugene Pimenov.

ruby -v changed from 2.3.0, 2.3.1 to 2.3.0-3

We did hit this bug on our system and did a deep dive in it yesterday.

The code to reproduce the issue is:

~~~ c
rb_respond_to(rb_eval_string("BasicObject.new"), rb_intern("anythinghere"))
~~~

It always returns ``1`` (``TRUE``) in ruby 2.3.x. It's caused by [[the following line]](https://github.com/ruby/ruby/blob/c91cb76f8d84b2963f6ede2ef445ad46a6104216/vm_method.c#L1919). Because ``respond_to`` method doesn't exist in ``BasicObject``. The correct code was reverted in [[r55869 ]](https://github.com/ruby/ruby/commit/53369d7e4dab675ede11b584ffc003e51ea62672#diff-6cea7716df08f2b7fdc5fb30c7963884R1919) because it breaks `ActiveSupport` 4.1.

As far as I can tell the issue is fixed in trunk and 2.4.x betas.

----------------------------------------
Bug #12907: rb_respond_to() return value is incorrect
https://bugs.ruby-lang.org/issues/12907#change-62115

* Author: Peter Ohler
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.3.0-3
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
As of Ruby 2.3.0 rb_respond_to incorrectly reports non-zero for methods that are not supported. This occurs with subclasses of BasicObject. This occurs with Oj when but should occur with other C code. A simple sample that demonstrates the failure in below. It works fine on all versions of Ruby until 2.3.0.

    require 'oj'
    
    class A < BasicObject
      def initialize(data)
        @data = data
      end
    end
    
    a = A.new("xyz")
    
    json = Oj.dump(a, mode: :compat, use_to_json: true)
    #=> NoMethodError: undefined method `to_hash' for #<A:0x007fae03de1dc8>

In the C code the rb_respond_to() function is called to determine if an instance of A responds to "to_hash". 1 is returned in Ruby 2.3.x while previous versions correctly returned 0.



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