Add new era in Japan called "Reiwa"

[acid-chicken]

【お知らせ】新元号は「令和」に決まりました。
— 首相官邸 on Twitter

FYI: https://www.meti.go.jp/policy/it_policy/kaigen/20190405_kaigen_code.pdf

[krwq]

@acid-chicken we read Eras from the OS but updating fallback data is fine too I believe.

See i.e.:

coreclr/src/System.Private.CoreLib/shared/System/Globalization/JapaneseCalendar.Win32.cs

Line 14 in 261ff1c

private const string JapaneseErasHive = @"System\CurrentControlSet\Control\Nls\Calendars\Japanese\Eras";

cc: @tarekgh

[tarekgh]

@acid-chicken The hard coded table as it is not really used. On Windows we read the Windows registry which is going to be updated by Windows Updates. On Linux, we use ICU which also is going to release a new version with the updated era names.

The hard coded table is used only if we fail getting the needed information from the OS.

@krwq I am not seeing any harm to to take this change even if it is not used. at least the information would reflect the current state. we just want to ensure this change is not going to cause any side effect. I'll double check that quickly before we decide to merge it.

[tarekgh]

@acid-chicken could you please update the table in the following places too?

coreclr/src/System.Private.CoreLib/shared/System/Globalization/JapaneseCalendar.Win32.cs

Line 203 in ff43a80

private static readonly string[] s_japaneseErasEnglishNames = new string[] { "M", "T", "S", "H" };

coreclr/src/System.Private.CoreLib/shared/System/Globalization/JapaneseCalendar.WinRT.cs

Line 50 in ff43a80

private static readonly string[] s_JapaneseErasEnglishNames = new string[] { "M", "T", "S", "H" };

[acid-chicken]

@krwq @tarekgh Thanks for let me know. I just updated those tables too.

[Gnbrkm41]

It appears that some call sites of GetEraInfo() relies on the fact that the first item in the array is the current era. (e.g. GregorianCalendarHelper _helper field in JapaneseCalendar/JapaneseLunisolarCalendar is initialised with the data from GetEraInfo() with the assumption that the first item is the current era, which then is called in various public APIs). This means that some methods may behave weirdly as it will return the value for the new era (Reiwa) instead of the current era (Heisei). It wouldn't matter if this is going to be released after the 1st of May (which then becomes perfectly valid), however until then this would lead to some unexpected behaviour.

Also, s_japaneseErasEnglishNames field and GetJapaneseEnglishEraName() method does not appear to have been referenced in any places (i.e. dead code); Could we just remove them after all?

(By the way, considering that this hard-coded data is only used if every other method of obtaining data fail, which is very unlikely to happen, it might be okay to leave it as is.)

[Gnbrkm41]

Maybe we should change this comment as well, he's still going strong 😀

[tarekgh]

@Gnbrkm41 the change in PR is used only when we fallback when something catastrophic happen. So, your concerns regarding having this update before May 1st are not real and no one will experience this change .

for s_japaneseErasEnglishNames, we still need that.

For the comment, feel free to open a PR with the changes in that comment.