Issue #12855 has been reported by Benoit Daloze.

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Bug #12855: Inconsistent keys identity in compare_by_identity Hash when using literals
https://bugs.ruby-lang.org/issues/12855

* Author: Benoit Daloze
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux]
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
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This seems a regression since 2.2.
I would guess it's due to some optimization for having a string literal between []=.
That optimization should not trigger for compare_by_identity hashes, so both cases below are consistent.

~~~ruby
h = {}.compare_by_identity
h['pear'] = 1
h['pear'] = 2

p h.size # => 1
p h


h = {}.compare_by_identity
k1 = 'pear'
h[k1] = 1
k2 = 'pear'
h[k2] = 2

p h.size # => 2
p h
~~~



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