@liudangyi I am not sure but may it has something to do with this

@veekram Yes, I understand why it happened. But I think it's still a bug of activesupport, since to_json should be expected to never fail.

since to_json should be expected to never fail.

Where are you getting that idea from?

I was trying to serialize request.env in sinatra and found it'a OK when only requiring "json" but bugs when requiring "active_support".

So introducing "active_support" just breaks the existing code, right?

This issue has been automatically marked as stale because it has not been commented on for at least three months.
The resources of the Rails team are limited, and so we are asking for your help.
If you can still reproduce this error on the 5-1-stable branch or on master, please reply with all of the information you have about it in order to keep the issue open.
Thank you for all your contributions.

This can still be reproduced on master. Adding this to activesupport/test/json/encoding_test.rb demonstrates it:

class Loggable
  def initialize
    @io = STDOUT
  end
end

def test_to_json_works_on_a_logger
  assert_nothing_raised { Loggable.new.to_json }
end

This issue has been automatically marked as stale because it has not been commented on for at least three months.
The resources of the Rails team are limited, and so we are asking for your help.
If you can still reproduce this error on the 5-1-stable branch or on master, please reply with all of the information you have about it in order to keep the issue open.
Thank you for all your contributions.

This still fails on both 5-1-stable (198bc1f) and master (61cc630)

This issue has been automatically marked as stale because it has not been commented on for at least three months.
The resources of the Rails team are limited, and so we are asking for your help.
If you can still reproduce this error on the 5-1-stable branch or on master, please reply with all of the information you have about it in order to keep the issue open.
Thank you for all your contributions.

This still fails on both 5-1-stable (581fd57) and master (70656c4)

You should fix this also for rails 5.0

I just observed this in activesupport-5.1.6. I have a script that does not require activesupport, but it's in my Gemfile for other scripts. Temporarily removing activesupport from my Gemfile fixes the issue. Problem appears to have been fixed in activesupport-5.2.0.