On 6/14/20 6:53 PM, Corentin Jabot wrote:

On Mon, 15 Jun 2020 at 00:36, Tom Honermann <tom@honermann.net> wrote:
On 6/14/20 6:21 PM, Hubert Tong wrote:
On Sun, Jun 14, 2020 at 6:03 PM Tom Honermann via SG16 <sg16@lists.isocpp.org> wrote:
On 6/14/20 4:57 PM, Corentin Jabot wrote:

On Sun, 14 Jun 2020 at 22:45, Jens Maurer <Jens.Maurer@gmx.net> wrote:
On 14/06/2020 22.19, Corentin Jabot wrote:
> On Sun, 14 Jun 2020 at 21:55, Jens Maurer <Jens.Maurer@gmx.net <mailto:Jens.Maurer@gmx.net>> wrote:

>     No, each code point in a sequence (given Unicode input) is a separate abstract character
>     in my view (after combining surrogate pairs, of course).
> For example diatrics, when preceded by a letter are not considered abstract characters of their own.

"Abstract character" is defined in https://www.unicode.org/glossary/ as follows:

"A unit of information used for the organization, control, or representation of textual data."
(ISO 10646 does not appear to have a definition in its clause 3.)

I'm not seeing a conflict between that definition and my view that a diacritic,
preceded by a letter, can be viewed as two different abstract characters.
I agree that the alternate viewpoint "single abstract character" is not
in conflict with the definition, either.

What is your statement "are not considered abstract characters of their own"
(which seems to leave little room for alternatives) based on?

Right the glossary, is very much incomplete

The definition is given in Unicode 13. 3.4 ( http://www.unicode.org/versions/Unicode13.0.0/ch03.pdf ) 

Abstract character: A unit of information used for the organization, control, or representation of textual data.
• When representing data, the nature of that data is generally symbolic as
opposed to some other kind of data (for example, aural or visual). Examples of
such symbolic data include letters, ideographs, digits, punctuation, technical
symbols, and dingbats.
• An abstract character has no concrete form and should not be confused with a
• An abstract character does not necessarily correspond to what a user thinks of
as a “character” and should not be confused with a grapheme.
• The abstract characters encoded by the Unicode Standard are known as Unicode abstract characters.
• Abstract characters not directly encoded by the Unicode Standard can often be represented by the use of combining character sequences.

My reading of that aligns with Jens' interpretation.  An abstract character can be composed from abstract characters.  The emphasized statement above appears to reaffirm that.

The definition of encoded character is also informative

 Encoded character: An association (or mapping) between an abstract character and a code point.
• An encoded character is also referred to as a coded character.
• While an encoded character is formally defined in terms of the mapping
between an abstract character and a code point, informally it can be thought of
as an abstract character taken together with its assigned code point.
Occasionally, for compatibility with other standards, a single abstract character
may correspond to more than one code point—for example, “Å” corresponds
both to U+00C5 Å latin capital letter a with ring above and to U+212B
Å angstrom sign.
• A single abstract character may also be represented by a sequence of code
points—for example, latin capital letter g with acute may be represented by the
sequence <U+0047 latin capital letter g, U+0301 combining acute
accent>, rather than being mapped to a single code point.

Likewise here, these examples indicate that an abstract character may have multiple encoded representations, but I don't read this as precluding the indicated code points reflecting abstract characters on their own.

It seems that, because we are not looking at a model where we retain coded characters in their original form for as long as possible, we're dealing with certain issues in larger scopes than may be strictly necessary. Are we sure that the same text processing should occur for the entirety of the source? In other words, should we consider more context-dependent (e.g., specific to raw strings, specific to identifiers, etc.) text processing?

I've been thinking along those lines as well.  I've been considering a model in which an extended-source-character is introduced in phase 1 and then, in phase 3, all extended-source-characters outside of raw string literals are converted to universal-character-names.

I would really love it if someone could explain to me the value of introducing universal-character-names (or extended-source-character) etc in the internal representation,
instead of unicode codepoints, knowing that these things represent unicode codepoints.

I find the UCN mechanism to be quite elegant.  It simultaneously accomplishes several things:

  1. It allows the source language, as seen by all phases of translation after phase 1 (except for the magical revert for raw string literals) to be completely and abstractly defined by characters defined in the standard.  An implementation's internal representation only needs to differentiate 96 characters.  I don't need to know how to type, write, read, or pronounce ሴ; I just need to know what \u1234 represents (abstractly; from a language perspective, I probably don't care what character it denotes).
  2. It allows explicit encoding of Unicode code points regardless of the encoding of the source input.
  3. It enables source input to use extended characters with no special support beyond phase 1 (well, beyond phase 3 because of raw string literals).
  4. It enables an escape from Unicode should such an escape prove necessary (e.g., to support those EBCDIC control characters, or to encode whether a UCN was explicit in the source or the result of character conversion, or to encode which of the possible Shift-JIS code points a character was written in).  Yes, such an escape could always be introduced anyway.  And yes, these are edge cases, some of which are probably not deserving of support.

It would be so much simpler to map the source physical characters (using existing terminology so nobody gets confused) to unicode code points in phase 1 and then replace universal character-names that appear as escape sequence in non-raw string and character literals, as well as header names, pp-identifier and pp-number later on when parsing pp-tokens in phase 3 (which would allow filtering out raw literals at that point)
It seems like we're converging towards do-something-in-phase-1 and then do-something-further-in-phase-3.  Good.  But mapping to code points assumes a bijective mapping between source input characters and Unicode and we know we don't have that in all cases today.  The idea of an extended-source-character is that it could carry additional implementation-defined information (e.g., the actual code unit value(s) for a source input character).

This still doesn't help with the reversion of line splicing in raw string literals, but a similar approach should work for that.

I think reversing that is fine - ish (see CWG1655, which i hope is one of the issue we can close as a result of all of that)

See also CWG1709.