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Subject: Re: Is it an error to encounter a character without a valid UCN?
From: Hubert Tong (hubert.reinterpretcast_at_[hidden])
Date: 2020-06-03 18:25:26


On Wed, Jun 3, 2020 at 3:19 AM Corentin Jabot <corentinjabot_at_[hidden]>
wrote:

>
>
> On Wed, Jun 3, 2020, 08:52 Corentin Jabot <corentinjabot_at_[hidden]> wrote:
>
>>
>>
>> On Wed, Jun 3, 2020, 03:38 Hubert Tong <hubert.reinterpretcast_at_[hidden]>
>> wrote:
>>
>>> On Tue, Jun 2, 2020 at 7:57 AM Corentin Jabot via SG16 <
>>> sg16_at_[hidden]> wrote:
>>>
>>>>
>>>>
>>>>
>>>> On Tue, Jun 2, 2020, 13:34 Alisdair Meredith via SG16 <
>>>> sg16_at_[hidden]> wrote:
>>>>
>>>>> Translation phase 1 maps source code to either a member of the
>>>>> basic character set, or a UCN corresponding to that character.
>>>>> What if there is no such UCN? Is that undefined behavior, or is
>>>>> the program ill-formed? I can find nothing on this in [lex.phases]
>>>>> where we describe processing the source through an implemetation
>>>>> defined character mapping.
>>>>>
>>>>> When we get to [lex.charset] we can see it is clearly ill-formed if
>>>>> the produced UCN is invalid - is that supposed to be the resolution
>>>>> here? Source must always map to a UCN, but the UCN need not
>>>>> be valid, so we get an error parsing the (implied) UCN in a later
>>>>> phase?
>>>>>
>>>>
>>>> One more reason i want to rewrite phase 1.
>>>>
>>>> 2 things should be specified here:
>>>>
>>>> > Any source file character not in the basic source character set
>>>> <http://eel.is/c++draft/lex#def:basic_source_character_set> is
>>>> replaced by the
>>>> universal-character-name
>>>> <http://eel.is/c++draft/lex#nt:universal-character-name> that
>>>> designates that character.
>>>> <http://eel.is/c++draft/lex#phases-1.1.sentence-3>
>>>>
>>>>
>>>> This is wrong, characters may map to ucn sequences, not single ucns.
>>>>
>>>> Characters that do not have representation in Unicode should
>>>> be ill-formed - with the caveat that implementers can do _anything_ in
>>>> phase 0
>>>>
>>>> Note that the existence of a mapping is different from the validity of
>>>> a UCN
>>>> It is an implementation strategy to map characters without
>>>> representation to nothing.
>>>> Other valid strategies would be to use the PUA to represent these
>>>> characters
>>>>
>>> Both of these are harmful to the integrity of user strings unless if
>>> (for the second) there is a reserved space of codepoints for the
>>> implementation to avoid confusion between the implementation's use of a
>>> codepoint and the user's use of the same codepoint.
>>>
>>
>
> The standard doesn't make any such guarantee at the moment.
> And can not make such guarantee for non unicode source files as some
> transcoding are not reversible
>
Well, just because it currently doesn't does not mean it can't.

>
>>>
>>>>
>>>>
>>>> To give you an idea of where i want to be, here is a very early draft
>>>> of what I think phase 1 and 2 should do, pending
>>>> a couple of design changes that EWG would have to look at
>>>>
>>>> 1. If the physical source character is the Unicode character set, each
>>>> code point in the source
>>>> file is converted to the internal representation of that same code
>>>> point. Codepoints that
>>>> are surrogate codepoints or invalid codepoints are ill-formed.
>>>> Otherwise, each abstract character in the source file is mapped in an
>>>> implementation-
>>>> defined manner to a sequence of Unicode codepoint representing the same
>>>> abstract
>>>> character. (introducing new-line characters for end-of-line indicators
>>>> if necessary).
>>>> An implementation may use any internal encoding able to represent
>>>> uniquely any Uni-
>>>> code codepoint.
>>>> *If an abstract character in the source file is not representable in
>>>> theUnicode character set, the program is ill-formed.*
>>>>
>>> I'm not sure where we are expecting this diagnostic to come into play.
>>> If a vendor is dealing with an encoding that has such characters and it is
>>> both the source and assumed execution character set, then I doubt they are
>>> interested in telling their users that their strings have been outlawed by
>>> the committee.
>>>
>>
>> The scenario here would be a file encoded in big 5, with the execution
>> also in big 5 for the few characters that do not have representation in
>> Unicode.
>>
>> An even less realistic scenario would be a piece of paper with a Klingon
>> symbol.
>>
> I am fairly certain that EBCDIC control characters do appear in strings in
some user programs. Some of these control characters have no semantic
equivalent in Unicode. The existence of mappings does not mean that they
retain the semantic value of the abstract characters. For a pair of file
encodings, there may be different mappings for different purposes.

I'm willing to buy that phase 1 translation can, as a fiction, contextually
replace such characters with numeric escape sequences or stronger
transformations to retain the best mapping of source character to the
presumed execution (narrow or wide) character set.

>
>
> Note that this doesn't change existing behavior:
>
> Any source file character not in the basic source character set
> <http://eel.is/c++draft/lex#def:basic_source_character_set> is replaced
> by the universal-character-name
> <http://eel.is/c++draft/lex#nt:universal-character-name> that designates
> that character
>
> This wording assumes a mapping always exist - that is not the case in
> limited scenario
>
>>
>>>
>>>> An implementation supports source files representing a sequence of
>>>> UTF-8 code units.
>>>> Any additional physical source file character sets accepted are
>>>> implementation-defined.
>>>> How the the character set of a source file is determined is
>>>> implementation-defined.
>>>>
>>>> 2. Each implementation-defined line termination sequence of characters
>>>> is replaced by a
>>>> LINE FED character (U+000A). Each instance of a BACKSLASH (\)
>>>> immediately
>>>> followed by a LINE FEED or at the end of a file is deleted, splicing
>>>> physical source
>>>> lines to form logical source lines. Only the last backslash on any
>>>> physical source line shall
>>>> be eligible for being part of such a splice. Except for splices
>>>> reverted in a raw string literal,
>>>> if a splice results in a codepoint sequence that matches the syntax of
>>>> a universal-character-
>>>> name, the behavior is implementation-defined. A source file that is not
>>>> empty and that does not end
>>>> in a *LINE FEED*, or that ends in a LINE FEED immediately preceded by
>>>> a BACKSLASH before any such splicing takes place, shall be processed
>>>> as if an
>>>> additional LINE FEED were appended to the file.
>>>> Sequences of whitespace codepoints at the end of each line are removed.
>>>> Each universal-character-name is replaced by the Unicode codepoint it
>>>> designates.
>>>>
>>>> Corentin
>>>>
>>>>
>>>>
>>>>>
>>>>> AlisdairM
>>>>> --
>>>>> SG16 mailing list
>>>>> SG16_at_[hidden]
>>>>> https://lists.isocpp.org/mailman/listinfo.cgi/sg16
>>>>>
>>>> --
>>>> SG16 mailing list
>>>> SG16_at_[hidden]
>>>> https://lists.isocpp.org/mailman/listinfo.cgi/sg16
>>>>
>>>



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