Issue #9576 has been reported by Shota Fukumori.

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Backport #9576: Backport r44370 to revert Hash#reject behavior
https://bugs.ruby-lang.org/issues/9576

* Author: Shota Fukumori
* Status: Open
* Priority: Normal
* Assignee: Yui NARUSE
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In [Bug #9223], naruse-san rejected incompatible change due to HashWithIndifferentAccess.

r44358 had changed constant name, but there was one `ifdef` condition still use old constant name.
r44370 fixes it to correct one, but it's not backported to 2.1.1.

So HashWithIndifferentAccess in Ruby 2.1.1 is broken because Ruby 2.1.1's Hash#reject doesn't copy extra states.

- Commits around this change: https://gist.github.com/sorah/9265008
- Rails' issue: https://github.com/rails/rails/issues/14188



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