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Re: [SG16] Is it an error to encounter a character without a valid UCN?

From: Corentin Jabot <corentinjabot_at_[hidden]>
Date: Tue, 2 Jun 2020 13:57:05 +0200
On Tue, Jun 2, 2020, 13:34 Alisdair Meredith via SG16 <sg16_at_[hidden]>

> 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
<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

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
defined manner to a sequence of Unicode codepoint representing the same
character. (introducing new-line characters for end-of-line indicators if
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.*
An implementation supports source files representing a sequence of UTF-8
code units.
Any additional physical source file character sets accepted are
How the the character set of a source file is determined is

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
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


> AlisdairM
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Received on 2020-06-02 07:00:24