[ruby-core:107735] [Ruby master Bug#18590] String#downcase and CAPITAL LETTER I WITH DOT ABOVE

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

Status changed from Closed to Open

@duerst Let me confirm. The rdoc of 3.1 and master refers to https://www.unicode.org/charts/case/.

> Default Case Mapping
> By default, all of these methods use full Unicode case mapping, which is suitable for most languages. See [Unicode Latin Case Chart](https://www.unicode.org/charts/case/).

It is not clear to me that the document says "0069 0307 for 'İ'.downcase". Is it okay? Should it be replaced with https://www.unicode.org/Public/UCD/latest/ucd/SpecialCasing.txt ?

----------------------------------------
Bug #18590: String#downcase and CAPITAL LETTER I WITH DOT ABOVE
https://bugs.ruby-lang.org/issues/18590#change-96661

* 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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