[Proposal] Make toLowerCase and toUpperCase based on Locale.ROOT by default

[Remi Forax]

> From: "Glavo" <zjx001202 at gmail.com>
> To: "core-libs-dev" <core-libs-dev at openjdk.org>
> Sent: Thursday, January 26, 2023 1:35:06 PM
> Subject: [Proposal] Make toLowerCase and toUpperCase based on Locale.ROOT by
> default

> At present, the no-parameter toLowerCase and toUpperCase methods of String are
> based on the default locale.

> I checked all the uses of this method in OpenJDK, and found that most of the use
> cases are suspicious, and even there are some hidden bugs.
> For example, I just found that jdk.incubator.vector.LaneType will trigger
> assertion failure on Turkish locale, and opened a PR for this problem:

>> [ https://github.com/openjdk/panama-vector/pull/210 |
>> https://github.com/openjdk/panama-vector/pull/210 ]

> In addition to such obvious problems, some use cases behave suspiciously after
> calling Locale.setDefault.
> I am investigating these problems and preparing to open a PR to solve these
> hidden bugs.

> In all the third-party libraries I have used, I have never seen the correct use
> of these two methods.
> Although the behavior of modifying API methods is destructive, I think it is
> worthwhile to consider whether to modify its behavior for such a suspicious
> method.
> If users need locale-sensitive case conversion, it may be better to explicitly
> use Locale.getDefault().
> Using Locale.ROOT as the default value also helps to keep the behavior of these
> two methods consistent with equalsIgnoreCase, Character.toLowerCase/toUpperCase
> and other methods.

> This is my rough idea. I hope to get your suggestions.

yes, it's a very very common bug, IntelliJ already warns when toLowerCase()/toUpperCase() is used without a Locale. 
I think the logical nest step is more to deprecate toLowerCase()/toUpperCase() than to change their implementation. 

regards, 
Rémi 
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