[ruby-core:70828] [Ruby trunk - Bug #11532] [Open] private_constant works after module is frozen

Issue #11532 has been reported by Karol Bucek.

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Bug #11532: private_constant works after module is frozen
https://bugs.ruby-lang.org/issues/11532

* Author: Karol Bucek
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
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... and it probably should not allow for changing constant visibility since others such as `const_set` honour `freeze`

```
2.2.2 :001 > module M; C = '1'; D = '2'; private_constant :D; end
 => M 
2.2.2 :002 > M.constants
 => [:C] 
2.2.2 :003 > M.freeze
 => M 
2.2.2 :004 > M.private_constant :D
 => M 
2.2.2 :005 > M.private_constant :C
 => M 
2.2.2 :006 > M.constants
 => [] 
```



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