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Subject: Re: Conversion of grapheme clusters to (wide) execution encoding
From: Hubert Tong (hubert.reinterpretcast_at_[hidden])
Date: 2020-06-01 09:04:41


On Mon, Jun 1, 2020 at 5:22 AM Corentin via SG16 <sg16_at_[hidden]>
wrote:

>
>
> On Mon, 1 Jun 2020 at 09:48, Peter Brett <pbrett_at_[hidden]> wrote:
>
>> Out of the 5 options laid out, I feel it would be best to make it
>> ill-formed. The source code author's intent is (at best) ambiguous, and
>> the onus should not be placed on the compiler to try to make it work. The
>> status quo allows for programs to be broken in surprising ways.
>>
>
> I think you might be right.
> Especially given making it work add implementation burden for the sake of
> legacy encodings and there is a somewhat easy fix: nfc normalize your
> sources.
>
Wait, does this apply to u, U, and u8 strings? Users can't have
non-NFC-normalized strings?

>
>
>>
>> Best regards,
>>
>> Peter
>>
>> > -----Original Message-----
>> > From: SG16 <sg16-bounces_at_[hidden]> On Behalf Of Jens Maurer via
>> > SG16
>> > Sent: 01 June 2020 07:10
>> > To: sg16_at_[hidden]
>> > Cc: Jens Maurer <Jens.Maurer_at_[hidden]>; Corentin <
>> corentin.jabot_at_[hidden]>
>> > Subject: Re: [SG16] Conversion of grapheme clusters to (wide) execution
>> > encoding
>> >
>> > EXTERNAL MAIL
>> >
>> >
>> > On 01/06/2020 00.21, Corentin via SG16 wrote:
>> > > Hello
>> > >
>> > > Consider a string literal "e\u00B4" (LATIN SMALL LETTER E, ACUTE
>> > ACCENT).
>> > >
>> > > There is some consensus in SG-16 that this should not be normalized in
>> > phase 1, or in phase 5 if the execution encoding of that string literal
>> > encode the Unicode character set.
>> > > However, what should happen if the execution character set is Latin 1,
>> > for example?
>> > >
>> > > ACUTE ACCENT does not have an implementation in latin 1, but the
>> > grapheme cluster LATIN SMALL LETTER E, ACUTE ACCENT does as LATIN SMALL
>> > LETTER E WITH ACUTE has a representation in the latin character set
>> > >
>> > > This is currently implementation defined ("e?" in msvc, ill-formed in
>> > GCC and Clang), but the wording is specific about the conversion
>> happening
>> > independently for each code point.
>> > >
>> > > I think we have several options:
>> > >
>> > > 1. Status quo
>> > > 2. Making the conversion ill formed as per P1854R0 Conversion to
>> > execution encoding should not lead to loss of
>> > meaning
>> https://urldefense.com/v3/__https://wg21.link/p1854r__;!!EHscmS1yg
>> > iU1lA!QIugkkaP5mbGSB4ocs-mkv_CIGbTOKMblzUfGVYDOBbFRaZU6Uu6Bsaa14-Ufg$
>> > > 3. Allowing an implementation to transform each abstract character to
>> > another abstract character represented by more of fewer code points
>> > > 4. Forcing an implementation to transform each abstract character to
>> > another abstract character represented by more of fewer code points.
>> > > 5. Conversion to NFC(K?) before conversion to a non unicode character
>> > set, but that may maybe introduce further issues and adds burden on
>> > implementation
>> > >
>> > >
>> > > Option 4 seems hardly implementable in all cases.
>> > > Option 2 and 5 offer the most consistency across implementations
>> > > Option 3, 4, 5 may be a behavior change
>> > >
>> > > I think i have a preference for 3.
>> > >
>> > > What do you think?
>> >
>> > String literals also have an inherent length. I'm mildly opposed to
>> > normatively
>> > specifying a required alteration of the "source-code-apparent" length
>> for
>> > types
>> > whose encoding are not variable-width to start with (u8, u16). That
>> > leaves
>> > 1 and 2 for me.
>> >
>> > Jens
>> >
>> > --
>> > SG16 mailing list
>> > SG16_at_[hidden]
>> >
>> https://urldefense.com/v3/__https://lists.isocpp.org/mailman/listinfo.cgi/
>> > sg16__;!!EHscmS1ygiU1lA!QIugkkaP5mbGSB4ocs-
>> > mkv_CIGbTOKMblzUfGVYDOBbFRaZU6Uu6BsZdqrBLSw$
>>
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