std.utf: Add isValidCodepoint.

[berni44]

For improving formatting characters with std.format I need this function (e.g. fixing issue 21815, but can also be used for issue 6125). I could add it as a private function to std.format, but I think, it fits better in here and might be useful for users too. Especially, because the dchar version already exists here as isValidDchar, while isValidWchar and isValidChar are missing (and it's not D like to use these functions instead of a single template).

I did not manage to find out, why isValidDchar was designed like this. It was added in 2007 to git, but probably existed prior to that somewhere else. Eventually, isValidCharacter might replace isValidDchar. (Should I do so immediately?)

cc @atilaneves

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[dukc]

What's the problem with isValidDChar? char and wchar convert implicitly to dchar so the old function can be used with all character types.

I suggest a different name. This function is named as if it was testing the validity of a code unit. isCodePoint would be much better.

[aG0aep6G]

What's the problem with isValidDChar? char and wchar convert implicitly to dchar so the old function can be used with all character types.

dchar(0x80) represents the code point U+0080. char(0x80) does not represent a code point. Having char and wchar implicitly convert to dchar is a mistake at the language level. isValidDchar does not give sensible results for all character types.

[dukc]

dchar(0x80) represents the code point U+0080. char(0x80) does not represent a code point. Having char and wchar implicitly convert to dchar is a mistake at the language level.

Or at least they should convert to dchar.init when the source has only a partial code point. But anyway, this explains why we might want a new function.

[dukc]

@berni44 You got tired of waiting for review?

@atilaneves is there anything anymore blocking merging this right away?

[thewilsonator]

Sorry for the late review. Looks fine.

[MoonlightSentinel]

The changelog was not updated to the new name!

https://issues.dlang.org/show_bug.cgi?id=22503