Issue #13107 has been reported by Thomas Walpole.

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Bug #13107: def_delegators causes random errors in MRI 2.4.0
https://bugs.ruby-lang.org/issues/13107

* Author: Thomas Walpole
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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In the Capybara project we use the rack_test gem which uses def_delegators to forward last_reponse to @rack_mock_session - https://github.com/brynary/rack-test/blob/master/lib/rack/test.rb#L29

When running the Capybara test test suite calling last_reponse on a rack_test Session object will sporadically result in a TypeError from forwardable.rb

TypeError:
       wrong argument type Integer (expected Proc)

Patching around the delegator with

class Rack::Test::Session
  def last_response
    @rack_mock_session.last_response
  end
 end

stops the error from occurring, so it appears that something def_delegators is doing is causing problems.  Unfortunately I have not yet been able to establish exactly what causes the issues to produce a simple test case.



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