[ruby-core:101484] [Ruby master Bug#17400] Incorrect character downcase for Greek Sigma

Issue #17400 has been updated by shyouhei (Shyouhei Urabe).

Assignee set to duerst (Martin Dürst)

I guess the ultimate reason why this is not implemented in ruby is the corresponding case mapping is commented out in https://unicode.org/Public/UNIDATA/SpecialCasing.txt

So strictly speaking we just follow what Unicode says but well, I agree this is not optimal.  However to implement it "end of a word" has to be determined, which is not that intuitive than it sounds.  @duerst Any idea?

----------------------------------------
Bug #17400: Incorrect character downcase for Greek Sigma
https://bugs.ruby-lang.org/issues/17400#change-89262

* Author: xfalcox (Rafael Silva)
* Status: Open
* Priority: Normal
* Assignee: duerst (Martin Dürst)
* ruby -v: ruby 3.0.0dev (2020-12-16T18:46:44Z master 93ba3ac036) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
An issue caused by this bug was first reported at Discourse support community at https://meta.discourse.org/t/unicode-username-results-in-error-loading-profile-page/173182?u=falco.

The issue is that in Greek, there are two ways to downcase the letter ‘Σ’

 - ‘ς’ when it is used at the end of a word
 - ‘σ’ anywhere else

NodeJS follows this rule:

```
➜  node
Welcome to Node.js v12.11.1.
Type ".help" for more information.
> "ΣΠΥΡΟΣ".toLowerCase()
'σπυρος'
```

Python too:

```
➜ python
Python 3.8.2 (default, Nov 23 2020, 16:33:30) 
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> "ΣΠΥΡΟΣ".lower()
'σπυρος'
```

Ruby (both 2.7 and 3) doesn't:

```
➜  ruby --version           
ruby 3.0.0dev (2020-12-16T18:46:44Z master 93ba3ac036) [x86_64-linux]
➜  irb           
irb(main):001:0> "ΣΠΥΡΟΣ".downcase
=> "σπυροσ"
```

```
➜  ruby --version
ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]
➜  irb
irb(main):001:0> "ΣΠΥΡΟΣ".downcase
=> "σπυροσ"
```




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