[ruby-core:107736] [Ruby master Bug#18590] String#downcase and CAPITAL LETTER I WITH DOT ABOVE

Issue #18590 has been updated by mame (Yusuke Endoh).


BTW, the rdoc of String#downcase in 3.1 and master is very less informative, and has a broken link (which is maybe the same issue as #18468). It was changed at commit:f7e266e6d2ccad63e4245a106a80c82ef2b38cbf between 3.0 and 3.1. Personally I strongly prefer [the 3.0 style](https://ruby-doc.org/core-3.0.0/String.html#method-i-downcase).

----------------------------------------
Bug #18590: String#downcase and CAPITAL LETTER I WITH DOT ABOVE
https://bugs.ruby-lang.org/issues/18590#change-96662

* Author: andrykonchin (Andrew Konchin)
* Status: Open
* Priority: Normal
* Assignee: duerst (Martin Dürst)
* ruby -v: 3.1.0p0
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
Downcasing for "İ" character works in an unexpected way:

```ruby
'İ'.downcase
=> "i̇"
```

Expected result - downcasing should return "i". Instead, it returns small "i" and additional "dot" character:

```ruby
'İ'.downcase.chars
=> ["i", "̇"]
```

According to the standard Unicode case mapping character 'İ'(0130) maps to lowercased 'i' (0069).

```
0130;LATIN CAPITAL LETTER I WITH DOT ABOVE;Lu;0;L;0049 0307;;;;N;LATIN CAPITAL LETTER I DOT;;;0069;
```

https://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt




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