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<!doctype html public "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<head>
<title>
Proposed resolution for core issues 411, 1656, and 2333; numeric and universal
character escapes in character and string literals
</title>
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<body>
<table id="header">
<tr>
<th>Document Number:</th>
<td>P2029R3</td>
</tr>
<tr>
<th>Date:</th>
<td>2020-09-11</td>
</tr>
<tr>
<th>Audience:</th>
<td>CWG</td>
</tr>
<tr>
<th>Reply-to:</th>
<td>Tom Honermann <[email protected]></td>
</tr>
</table>
<h1>
Proposed resolution for core issues 411, 1656, and 2333; numeric and universal
character escapes in character and string literals
</h1>
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#changes">Changes from P2029R2</a></li>
<li><a href="#proposed">Proposed resolution overview</a></li>
<li>
<a href="#impact">Implementation impact</>
<ul>
<li><a href="#impact-escapes">Semantics of numeric-escape-sequences in UTF-8 literals</a></li>
<li><a href="#impact-badchar">Character literal type for characters not representable in the execution character set</a></li>
</ul>
</li>
<li><a href="#ack">Acknowledgements</a></li>
<li>
<a href="#pr">Proposed resolution</a>
<ul>
<li><a href="#lex.phases">[lex.phases]</a></li>
<li><a href="#lex.ccon">[lex.ccon]</a></li>
<li><a href="#lex.string">[lex.string]</a></li>
</ul>
</li>
</ul>
<h1 id="introduction">Introduction</h1>
<p>
This paper proposes substantial changes to
<a href="http://eel.is/c++draft/lex.phases">[lex.phases]</a>,
<a href="http://eel.is/c++draft/lex.ccon">[lex.ccon]</a>, and
<a href="http://eel.is/c++draft/lex.string">[lex.string]</a>
intended to address the following core issues as well as several other
more minor issues.
</p>
<ul>
<li>
<a href="http://wg21.link/cwg411">Core Issue 411: Use of universal-character-name in character versus string literals</a><br/>
</li>
<li>
<a href="http://wg21.link/cwg1656">Core Issue 1656: Encoding of numerically-escaped characters</a><br/>
</li>
<li>
<a href="http://wg21.link/cwg2333">Core Issue 2333: Escape sequences in UTF-8 character literals</a>
</li>
</ul>
<p>
This paper follows a prior
<a href="http://wiki.edg.com/pub/Wg21kona2019/CoreWorkingGroup/cwg2333.html">proposed resolution</a>
that only attempted to address
<a href="http://wg21.link/cwg2333">CWG 2333</a>.
That proposed resolution was
<a href="https://lists.isocpp.org/core/2019/01/5395.php">discussed on the core mailing list</a>
and at the
<a href="http://wiki.edg.com/bin/view/Wg21cologne2019/CoreIssuesProcessingTeleconference2019-06-24">June 24th, 2019, core issues processing teleconference</a>.
The resolution proposed in this paper addresses the feedback
provided during those discussions and further review at the
<a href="http://wiki.edg.com/bin/view/Wg21prague/IssuesProcessingTeleconference2020-01-16">January 16th, 2020, core issues processing teleconference</a>,
the
<a href="http://wiki.edg.com/bin/view/Wg21summer2020/IssuesProcessingTeleconference2020-03-23">March 23rd, 2020, core issues processing teleconference</a>,
<a href="https://lists.isocpp.org/core/2020/06/9455.php">discussion on the core mailing list originating on June 28th, 2020</a>,
<a href="https://lists.isocpp.org/core/2020/07/9545.php">further discussions on the core mailing list originating on July 7th, 2020</a>,
and the
<a href="http://wiki.edg.com/bin/view/Wg21summer2020/IssuesProcessingTeleconference2020-07-20">July 20th, 2020, core issues processing teleconference</a>.
</p>
<p>
The notes for <a href="http://wg21.link/cwg2333">CWG 2333</a> in the current
active issues list (revision 100) state that discussion at the
<a href="http://wiki.edg.com/bin/view/Wg21albuquerque/IssuesTeleconference2017-08-14">
August 14th, 2017, core issues processing teleconference</a> resulted in a
determination that numeric escape sequences in UTF-8 character literals
should be ill-formed. The issue has remained in drafting status since then.
</p>
<p>
SG16 discussed this issue during its
<a href="https://github.com/sg16-unicode/sg16-meetings#october-17th-2018">October 17th, 2018, teleconference</a>.
The SG16 consensus was for a different resolution than is currently described
in the active issues list. The SG16 consensus was based on the following
observations:
<ul>
<li>The current resolution in the active issues list contradicts existing
practice. gcc, Clang, and Visual C++ all allow octal and hexadecimal
escape sequences in UTF-8 literals.</li>
<li>Octal and hexadecimal escape sequences in UTF-8 literals are useful for
a number of purposes:
<ul>
<li>Embedding null characters: <tt>u8"\0"</tt></li>
<li>Creating ill-formed code unit sequences for testing purposes.</li>
<li>Creating Modified UTF-8, CESU-8, and WTF-8 string literals. This
entails two abilities:
<ul>
<li>Embedding <tt>U+0000</tt> as an overlong UTF-8 sequence:
<tt>u8"\xC0\x80"</tt></li>
<li>Embedding lone surrogate code points as individual UTF-8 code
unit sequences. For example, encoding <tt>U+D800</tt> as
<tt>u8"\xED\xA0\x80"</tt>. (Note that use of <tt>\u</tt> escapes
specifying surrogate code points is ill-formed).</li>
</ul>
</li>
<li>Compatibility with existing log/debug systems that output literals
with non-printable characters represented with escapes so as to
facilitate copy/paste of such output into code.</li>
</ul>
</li>
</ul>
SG16 conducted the following poll:
<div style="margin-left: 1em;">
Continue to allow hex and octal escapes that indicate code unit values,
requiring only that they fit into the range of the code unit type?
<table border="1">
<tr>
<th>SF</th>
<th>F</th>
<th>N</th>
<th>A</th>
<th>SA</th>
</tr>
<tr>
<td>8</td>
<td>1</td>
<td>0</td>
<td>0</td>
<td>0</td>
</tr>
</table>
</div>
In the polled question, "Continue" refers to existing implementation behavior;
to maintain the current implementation status quo exhibited by gcc, Clang and
Visual C++.
</p>
<p>
The proposed resolution reflects the SG16 consensus.
</p>
<p>
<a href="http://wg21.link/cwg411">CWG 411</a> is addressed by specifying
different behavior for character literals vs string literals for characters
that are not representable by a single code unit. For example, when the
execution character set is UTF-8, <tt>'\u0153'</tt> is conditionally-supported,
has type <tt>int</tt> and an implementation-defined value, but
<tt>"\u0153"</tt> is a character array of length 3 containing the
UTF-8 encoding of U+0153 (LATIN SMALL LIGATURE OE) and a null character
(<tt>\xC5\x93\x00</tt>).
</p>
<p>
<a href="http://wg21.link/cwg1656">CWG 1656</a> is addressed by clarifying
that numeric escape sequences denote code unit values in the execution
character set; that the values are not subject to conversion from the
encoding of the source file to the execution character set.
</p>
<p>
Additionally, since integer values are now guaranteed to be represented in
two's complement following the adoption of
<a href="https://wg21.link/p1236r1">P1236R1</a>
in C++20, numeric escape sequences in ordinary and wide character and string
literals that specify an integer value outside the representable range of the
code unit type will now initialize the code unit similarly to an integral
conversion if the integer value is within the range of representable values of
the corresponding unsigned type of the code unit type, and are ill-formed
otherwise.
This matches the observed behavior in the gcc 10.1, Clang 10, Microsoft
Visual C++ 19.24, and Intel 19 compilers.
Previously such sequences resulted in implementation-defined behavior.
</p>
<p>
This paper does not attempt to address the existing issue concerning the
order in which adjacent string literals are encoded and concatenated as
reported at
<a href="https://lists.isocpp.org/core/2020/07/9537.php">https://lists.isocpp.org/core/2020/07/9537.php</a>
and tracked by
<a href="http://wg21.link/cwg2455">CWG 2455</a>.
</p>
<h1 id="changes">Changes from <a href="https://wg21.link/p2029r2">P2029R2</a></h1>
<p>
<ul>
<li>Addressed changes requested during
<a href="https://lists.isocpp.org/core/2020/07/9545.php">discussion on the core mailing list originating on July 7th, 2020</a>:
<ul>
<li>[lex.ccon]pZ, [lex.string]pZ: Updated the wording to clarify that the
"range of a [code unit] type" refers to the range of that type's
representable values (and is not restricted only to code unit values
that might appear in well-formed encoded text).</li>
<li>[lex.ccon]pZ, [lex.string]pZ: Updated wording for ordinary and wide
character and string literals to specify that a numeric escape
sequence that specifies an integer value outside the representable
range of the code unit type will now initialize the code unit
similarly to an integral conversion if the integer value is within
the range of representable values of the corresponding unsigned type
of the code unit type, and is ill-formed otherwise.
Updated the introduction accordingly.</li>
</ul>
</li>
<li>[lex.ccon]pZ: Modified the order of Z.2-Z.4 to match the order of the
equivalent presentation in [lex.string]pZ. This order presents the more
common cases first.</li>
<li>[lex.ccon]p6: Corrected the table number reference to
[tab:lex.ccon.esc].</li>
<li>Adressed changes requested during the
<a href="http://wiki.edg.com/bin/view/Wg21summer2020/IssuesProcessingTeleconference2020-07-20">July 20th, 2020, core issues processing teleconference</a>:
<ul>
<li>Changed the introductory and proposed resolution overview to remove
incorrect uses of "narrowing integer conversion" in prose intended
to describe something more like an "integral conversion".</li>
<li>[lex.ccon]pY: Added footnotes explaining that, for nonencodable
character literals and multicharacter literals, the associated
character encoding may be used solely to determine encodability and
not to actually encode values.</li>
<li>[lex.ccon]pZ: Merged Z.1 into the introductory text to make the short
circuiting behavior clear and renumbered the remaining
subparagraphs.</li>
<li>[lex.ccon]pZ: Specified that the determination of a character literal
value during translation phase 4 uses the range of representable
values of the character literal's type in translation phase 7.</li>
<li>[lex.string]pX: Added normative wording stating that the <em>n</em>
that appears in the string literal array type corresponds to the
number of encoded code units.</li>
<li>[lex.string]pZ: Elevated notes related to stateful character encodings
to normative encouragement.</li>
</ul>
</li>
</ul>
</p>
<h1 id="proposed">Proposed resolution overview</h1>
<p>
The proposed wording changes are intended to resolve
<a href="http://wg21.link/cwg411">CWG 411</a>,
<a href="http://wg21.link/cwg1656">CWG 1656</a>, and
<a href="http://wg21.link/cwg2333">CWG 2333</a> by:
<ul>
<li>Clarifying that hexadecimal and octal escape sequences:
<ul>
<li>are valid in <tt>u8</tt>, <tt>u</tt>, and <tt>U</tt> character
literals. (CWG 2333)</li>
<li>specify values that need not correspond to valid code unit
values for the literal's associated character encoding.
(CWG 2333)</li>
<li>specify the execution-time value of character literals or individual
code unit values of string literals; that the value expressed is not
subject to conversion to the literal's associated character encoding.
(CWG 1656)</li>
<li>specify values that may result in string literals that are ill-formed
according to the literal's associated character encoding.
(CWG 2333)</li>
</ul>
<li>Clarifying that characters that cannot be specified in a character
literal because they require more than one code unit in the literal's
associated character encoding may be specified in string literals.
(CWG 411)</li>
</ul>
</p>
<p>
Additionally, the wording updates are intended to:
<ul>
<li>Specify behavior for non-ordinary character and string literals when a
character lacks representation in the literal's associated character
encoding (wording was previously missing).</li>
<li>Broaden use of the term multicharacter literal to allow it to be
applicable to both ordinary and wide character literals.</li>
<li>Specify that wide multicharacter literals are conditionally
supported (consistent with multicharacter literals).</li>
<li>Specify that ordinary and wide string literals that specify characters
that are not representable in the literal's associated character encoding
are conditionally supported (gcc 10 rejects).</li>
<li>Specify that wide character literals that have a <tt>c-char-sequence</tt>
that specifies a character that is not representable in the execution
wide-character set or that cannot be encoded in a single code unit are
conditionally supported (consistent with ordinary character
literals).</li>
<li>Remove non-normative wording that states that <tt>wchar_t</tt> is able to
represent all members of the execution wide-character set as this
contradicts long standing existing practice for some implementations
(Visual C++ has a 16-bit <tt>wchar_t</tt> type, but encodes wide strings
as UTF-16; some characters require multiple code units).</li>
<li>Modernize the wording with current style preferences and terminology.</li>
<li>Move appending of a null character to string literals from translation
phase 7 to translation phase 6.</li>
<li>Specify that a numeric escape sequence in ordinary and wide character and
string literals that specifies an integer value outside the representable
range of the code unit type will now initialize the code unit similarly
to an integral conversion if the integer value is within the range of
representable values of the corresponding unsigned type of the code unit
type, and is ill-formed otherwise.</li>
</ul>
</p>
<p>
The wording changes introduce the following:
<ul>
<li>The concept of an "associated character encoding" for character and string
literals; this enables wording to be independent of the particular kind of
literal (ordinary, wide or Unicode).</li>
<li>New <tt>basic-c-char</tt>,
<tt>basic-s-char</tt>,
<tt>numeric-escape-sequence</tt>, and
<tt>simple-escape-sequence-char</tt> grammar non-terminals; these
factor the existing <tt>c-char</tt>, <tt>s-char</tt>,
<tt>escape-sequence</tt>, and <tt>simple-escape-sequence</tt> non-terminals
to give names to their productions for reference purposes.</li>
<li>New <tt>conditional-escape-sequence</tt> and
<tt>conditional-escape-sequence-char</tt> grammar non-terminals; these
address previously missing grammar support for conditionally-supported
implementation-defined escape sequences.</li>
<li>A new <tt>non-encodable character literal</tt> kind; this provides a name
for conditionally-supported implementation-defined ordinary and wide
character literals that have <tt>c-char-sequence</tt>s that specify
characters that either lack representation in the literal's associated
character encoding or that cannot be encoded as a single code unit.</li>
</ul>
</p>
<h1 id="impact">Implementation impact</h1>
<p>
The author intends the proposed wording to reflect existing practice for the
gcc and Clang compilers. If this proposal is adopted, neither of those
compilers are expected to require updates.
However, there is implementation impact to the Microsoft Visual C++ compiler.
</p>
<h2 id="impact-escapes">Semantics of numeric-escape-sequences in UTF-8 literals</h2>
<p>
Consider the following (C++20) code:
<div style="margin-left: 1em;">
<pre><code>constexpr const char8_t c[] = u8"\xc3\x80"; // UTF-8 encoding of U+00C0 {LATIN CAPITAL LETTER A WITH GRAVE}
#if defined(_MSC_VER)
// Microsoft Visual C++:
static_assert(c[0] == 0xC3); // UTF-8 encoding of U+00C3 {LATIN CAPITAL LETTER A WITH TILDE}
static_assert(c[1] == 0x83);
static_assert(c[2] == 0xC2); // UTF-8 encoding of U+0080 {<control>}
static_assert(c[3] == 0x80);
static_assert(c[4] == 0x00); // null
#else
// Gcc and Clang:
static_assert(c[0] == 0xC3); // UTF-8 encoding of U+00C0 {LATIN CAPITAL LETTER A WITH GRAVE}
static_assert(c[1] == 0x80);
static_assert(c[2] == 0x00); // null
#endif
</code></pre>
</div>
Gcc and Clang encode the hexadecimal escapes as code units in the target (UTF-8)
encoding and perform no conversions (consistent with the behavior intended by
this proposal). However, Visual C++ considers each hexadecimal escape to
specify a code point in the source encoding and encodes each as UTF-8.
</p>
<p>
The author does not know if the Visual C++ behavior exhibited for UTF-8 literals
is intentional or reflective of a defect. The behavior is inconsistent for
UTF-8 and UTF-16 literals. For UTF-8 literals, numeric escape sequences that
specify values outside the range of <tt>char8_t</tt> are accepted as code
point values and encoded as UTF-8. However, for UTF-16 literals, numeric escape
sequences that specify values outside the range of <tt>char16_t</tt> are
rejected rather than being considered a code point value and encoded as a UTF-16
surrogate pair.
</p>
<h2 id="impact-badchar">Character literal type for characters not representable in the execution character set</h2>
<p>
Consider the following code assuming that the execution character set does not
have representation for the specified Unicode code points.
<div style="margin-left: 1em;">
<pre><code>auto c1 = '\uFF10';
extern char c1;
#ifdef _MSC_VER
static_assert('\uFF10' == '?');
#endif
auto c2 = '\U0001F235';
extern int c2;
#ifdef _MSC_VER
static_assert('\U0001F235' == '??');
#endif
</code></pre>
</div>
This code should be rejected (both before and after this proposal) because the
redeclaration of <tt>c1</tt> with type <tt>char</tt> does not match the first
declaration for which <tt>c1</tt> should have a deduced type of <tt>int</tt>.
Visual C++ accepts it when compiled with <tt>/execution-charset:windows-1252</tt>
with the following warnings:
<div style="margin-left: 1em;">
<pre><code><source>(1): warning C4566: character represented by universal-character-name '\uFF10' cannot be represented in the current code page (1252)
<source>(4): warning C4566: character represented by universal-character-name '\uFF10' cannot be represented in the current code page (1252)
<source>(6): warning C4566: character represented by universal-character-name '\U0001F235' cannot be represented in the current code page (1252)
<source>(9): warning C4566: character represented by universal-character-name '\U0001F235' cannot be represented in the current code page (1252)
</code></pre>
</div>
It seems that the Visual C++ compiler translates unrepresentable characters
from the Unicode BMP to a single <tt>char</tt> with value equal to
<tt>'?'</tt>, but translates unrepresentable characters from outside the
Unicode BMP to <tt>int</tt> with value equal to the multicharacter literal
<tt>'??'</tt>. This seems unlikely to be intended behavior. It would be
conforming if, for the Unicode BMP case, an <tt>int</tt> with value equal
to <tt>'?'</tt> was produced.
</p>
<p>
Gcc and Clang both reject the above code regardless of whether those Unicode
characters have representation in the execution character set. If they are
representable, then the code is rejected (as permitted) because the characters
cannot be encoded in a single code unit. If they are not representable (which
only happens for gcc since Clang always targets UTF-8), then the code is
rejected because the redeclaration of <tt>c1</tt> as <tt>char</tt> does not
match the deduced <tt>int</tt> type for its first declaration.
</p>
<h1 id="ack">Acknowledgements</h1>
<p>Thank you to
Steve Downey,
Corentin Jabot,
Jens Maurer,
Alisdair Meredith,
Richard Smith, and
Hubert Tong
for their tireless feedback on numerous drafts of this paper!
</p>
<h1 id="pr">Proposed resolution</h1>
<p>These changes are relative to
<a title="Working Draft, Standard for Programming Language C++"
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf">
N4861</a>.
</p>
<p>The changes to
<a href="http://eel.is/c++draft/lex.ccon">[lex.ccon]</a> and
<a href="http://eel.is/c++draft/lex.string">[lex.string]</a>
are rather pervasive. For ease of review, unchanged paragraphs in these
sections are retained in the wording below. These paragraphs are
introduced with "<em>No</em> changes to ..." and are <nop>highlighted with
a blue background</nop>.
</p>
<input type="checkbox" id="hideins">Hide inserted text</input><br/>
<input type="checkbox" id="hidedel">Hide deleted text</input>
<h2 id="lex.phases">[lex.phases]</h2>
<p>Change in
<a href="http://eel.is/c++draft/lex.phases#1.5">
5.2 [lex.phases] paragraph 1.5</a>:<br/>
<blockquote>
Each <del>basic source character set member</del><ins><a href="http://eel.is/c++draft/lex.ccon#nt:basic-c-char"><em>basic-c-char</em></a>,
<a href="http://eel.is/c++draft/lex.string#nt:basic-s-char"><em>basic-s-char</em></a>, and
<a href="http://eel.is/c++draft/lex.string#nt:r-char"><em>r-char</em></a>
</ins>
in a
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
or a
<a href="http://eel.is/c++draft/lex.string#nt:string-literal"><em>string-literal</em></a>,
as well as each
<del>escape sequence</del><ins><a href="http://eel.is/c++draft/lex.ccon#nt:escape-sequence"><em>escape-sequence</em></a></ins>
and
<a href="http://eel.is/c++draft/lex.charset#nt:universal-character-name"><em>universal-character-name</em></a>
in a
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
or a non-raw string literal,
<del>is converted to the
corresponding member of the execution character set
(<a href="http://eel.is/c++draft/lex.ccon">[lex.ccon]</a>,
<a href="http://eel.is/c++draft/lex.string">[lex.string]</a>);
</del><ins>is encoded in the literal's
associated character encoding as specified in
<a href="http://eel.is/c++draft/lex.ccon">[lex.ccon]</a> and
<a href="http://eel.is/c++draft/lex.string">[lex.string]</a>.</ins>
<del>if there is no corresponding member, it is converted to an
implementation-defined member other than the null (wide) character.
<sup>
[ Footnote: An implementation need not convert all non-corresponding source
characters to the same execution character. ]
</sup></del>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.phases#1.6">
5.2 [lex.phases] paragraph 1.6</a>:<br/>
<em>Drafting note:</em> This addition duplicates wording in [lex.string], but
seems important to include here.
<blockquote>
Adjacent
<del>string literal tokens</del><ins><a href="http://eel.is/c++draft/lex.string#nt:string-literal"><em>string-literal</em></a>s</ins>
are concatenated<ins> and a null character is
appended to each
<a href="http://eel.is/c++draft/lex.string#nt:string-literal"><em>string-literal</em></a>
as specified in
<a href="http://eel.is/c++draft/lex.string">[lex.string]</a></ins>.
</blockquote>
</p>
<h2 id="lex.ccon">[lex.ccon]</h2>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon">
5.13.3 [lex.ccon]</a>:
<blockquote>
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal">character-literal:</a>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:encoding-prefix">encoding-prefix</a><sub>opt</sub>
<tt>'</tt>
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char-sequence">c-char-sequence</a>
<tt>'</tt>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.ccon#nt:encoding-prefix">encoding-prefix:</a> one of
<div style="margin-left: 1em;">
<tt>u8</tt>  <tt>u</tt>  <tt>U</tt>  <tt>L</tt>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char-sequence">c-char-sequence:</a>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char">c-char</a>
</div>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char-sequence">c-char-sequence</a>
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char">c-char</a>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char">c-char:</a>
<del><div style="margin-left: 1em;">
any member of the basic source character set except the single-quote <tt>'</tt>,
backslash <tt>\</tt>, or
<a href="http://eel.is/c++draft/cpp.pre#nt:new-line">new-line</a>
character
</div></del>
<ins><div style="margin-left: 1em;">
basic-c-char
</div></ins>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:escape-sequence">escape-sequence</a>
</div>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.charset#nt:universal-character-name">universal-character-name</a>
</div>
<br/>
<ins>basic-c-char:
<div style="margin-left: 1em;">
any member of the basic source character set except the single-quote <tt>'</tt>,
backslash <tt>\</tt>, or
<a href="http://eel.is/c++draft/cpp.pre#nt:new-line">new-line</a>
character
</div>
<br/></ins>
<a href="http://eel.is/c++draft/lex.ccon#nt:escape-sequence">escape-sequence:</a>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence">simple-escape-sequence</a>
</div>
<del><div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:octal-escape-sequence">octal-escape-sequence</a>
</div></del>
<del><div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:hexadecimal-escape-sequence">hexadecimal-escape-sequence</a>
</div></del>
<div style="margin-left: 1em;">
<ins><a href="http://eel.is/c++draft/lex.ccon#nt:numeric-escape-sequence">numeric-escape-sequence</a></ins>
</div>
<div style="margin-left: 1em;">
<ins><a href="http://eel.is/c++draft/lex.ccon#nt:conditional-escape-sequence">conditional-escape-sequence</a></ins>
</div>
<br/>
</ins>
<a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence">simple-escape-sequence:</a><del> one of</del>
<div style="margin-left: 1em;">
<del>
<tt>\'</tt>  <tt>\"</tt>  <tt>\?</tt>  <tt>\\</tt><br/>
<tt>\a</tt>  <tt>\b</tt>  <tt>\f</tt>  <tt>\n</tt>  <tt>\r</tt>  <tt>\t</tt>  <tt>\v</tt> <br/>
</del>
<ins>
<tt>\</tt> <a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence-char">simple-escape-sequence-char</a>
</ins>
</div>
<br/>
<ins>
<a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence-char">simple-escape-sequence-char</a>: one of
<div style="margin-left: 1em;">
<tt>'</tt> <tt>"</tt> <tt>?</tt> <tt>\</tt><br/>
<tt>a</tt> <tt>b</tt> <tt>f</tt> <tt>n</tt> <tt>r</tt> <tt>t</tt> <tt>v</tt>
</div>
<br/>
</ins>
<ins>
<a href="http://eel.is/c++draft/lex.ccon#nt:numeric-escape-sequence">numeric-escape-sequence</a>:
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:octal-escape-sequence">octal-escape-sequence</a>
</div>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:hexadecimal-escape-sequence">hexadecimal-escape-sequence</a>
</div>
<br/>
</ins>
<a href="http://eel.is/c++draft/lex.ccon#nt:octal-escape-sequence">octal-escape-sequence:</a>
<div style="margin-left: 1em;">
<tt>\</tt> <a href="http://eel.is/c++draft/lex.icon#nt:octal-digit">octal-digit</a>
</div>
<div style="margin-left: 1em;">
<tt>\</tt> <a href="http://eel.is/c++draft/lex.icon#nt:octal-digit">octal-digit</a>
<a href="http://eel.is/c++draft/lex.icon#nt:octal-digit">octal-digit</a>
</div>
<div style="margin-left: 1em;">
<tt>\</tt> <a href="http://eel.is/c++draft/lex.icon#nt:octal-digit">octal-digit</a>
<a href="http://eel.is/c++draft/lex.icon#nt:octal-digit">octal-digit</a>
<a href="http://eel.is/c++draft/lex.icon#nt:octal-digit">octal-digit</a>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.ccon#nt:hexadecimal-escape-sequence">hexadecimal-escape-sequence:</a>
<div style="margin-left: 1em;">
<tt>\x</tt> <a href="http://eel.is/c++draft/lex.icon#nt:hexadecimal-digit">hexadecimal-digit</a>
</div>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:hexadecimal-escape-sequence">hexadecimal-escape-sequence</a>
<a href="http://eel.is/c++draft/lex.icon#nt:hexadecimal-digit">hexadecimal-digit</a>
</div>
<ins>
<br/>
<a href="http://eel.is/c++draft/lex.ccon#nt:conditional-escape-sequence">conditional-escape-sequence</a>:
<div style="margin-left: 1em;">
<tt>\</tt> <a href="http://eel.is/c++draft/lex.ccon#nt:conditional-escape-sequence-char">conditional-escape-sequence-char</a>
</div>
<br/>
</ins>
<ins>
<a href="http://eel.is/c++draft/lex.ccon#nt:conditional-escape-sequence-char">conditional-escape-sequence-char</a>:
<div style="margin-left: 1em;">
any member of the basic source character set that is not an
<a href="http://eel.is/c++draft/lex.icon#nt:octal-digit">octal-digit</a>,
a
<a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence-char">simple-escape-sequence-char</a>,
or the characters
<tt>u</tt>,
<tt>U</tt>, or
<tt>x</tt>
</div>
</ins>
<br/>
</blockquote>
</p>
<p>Delete
<a href="http://eel.is/c++draft/lex.ccon#1">
5.13.3 [lex.ccon] paragraph 1</a>:<br/>
<em>Drafting Note:</em> The contents of paragraphs 1-5 were incorporated into
new paragraphs.
<blockquote class="stddel">
A
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
that does not begin with
<tt>u8</tt>, <tt>u</tt>,
<tt>U</tt>, or <tt>L</tt>
is an <em>ordinary character literal</em>.
An ordinary character literal that contains a single
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
representable in the execution character set
has type <tt>char</tt>, with value equal to the numerical value of the encoding
of the
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
in the execution character set. An ordinary character literal that
contains more than one
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
is a <em>multicharacter literal</em>. A multicharacter literal, or an
ordinary character literal containing a single
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
not representable in the execution character set, is conditionally-supported,
has type <tt>int</tt>, and has an implementation-defined value.
</blockquote>
</p>
<p>Delete
<a href="http://eel.is/c++draft/lex.ccon#2">
5.13.3 [lex.ccon] paragraph 2</a>:<br/>
<em>Drafting Note:</em> The contents of paragraphs 1-5 were incorporated into
new paragraphs. The note regarding the range of single code unit values was
removed.
<blockquote class="stddel">
A
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
that begins with <tt>u8</tt>, such as <tt>u8'w'</tt>, is a
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
of type <tt>char8_t</tt>, known as a
<em>UTF-8 character literal</em>.
The value of a UTF-8 character literal is
equal to its ISO/IEC 10646 code point value, provided that the code point value
can be encoded as a single UTF-8 code unit. [ <em>Note:</em> That is, provided
the code point value is in the range [0,7F] (hexadecimal). —
<em>end note</em> ] If the value is not representable with a single UTF-8 code
unit, the program is ill-formed. A UTF-8 character literal containing multiple
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>s is
ill-formed.
</blockquote>
</p>
<p>Delete
<a href="http://eel.is/c++draft/lex.ccon#3">
5.13.3 [lex.ccon] paragraph 3</a>:<br/>
<em>Drafting Note:</em> The contents of paragraphs 1-5 were incorporated into
new paragraphs. The note regarding the range of singled code unit values was
removed.
<blockquote class="stddel">
A
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
that begins with the letter <tt>u</tt>, such as
<tt>u'x'</tt>, is a
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
of type <tt>char16_t</tt>, known as a
<em>UTF-16 character literal</em>. The value of a UTF-16 character literal
is equal to its ISO/IEC 10646 code point value, provided that the code point
value is representable with a single 16-bit code unit. [ <em>Note:</em> That
is, provided the code point value is in the range [0,FFFF] (hexadecimal).
— <em>end note</em> ] If the value is not representable with a single
16-bit code unit, the program is ill-formed. A UTF-16 character literal
containing multiple
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>s is
ill-formed.
</blockquote>
</p>
<p>Delete
<a href="http://eel.is/c++draft/lex.ccon#4">
5.13.3 [lex.ccon] paragraph 4</a>:<br/>
<em>Drafting Note:</em> The contents of paragraphs 1-5 were incorporated into
new paragraphs.
<blockquote class="stddel">
A
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
that begins with <tt>U</tt>, such as <tt>U'y'</tt>, is a
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
of type <tt>char32_t</tt>, known as a
<em>UTF-32 character literal</em>.
The value of a UTF-32 character literal containing a single
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
is equal to its ISO/IEC 10646 code point value. A UTF-32 character literal
containing multiple
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>s is
ill-formed.
</blockquote>
</p>
<p>Delete
<a href="http://eel.is/c++draft/lex.ccon#5">
5.13.3 [lex.ccon] paragraph 5</a>:<br/>
<em>Drafting Note:</em> The contents of paragraphs 1-5 were incorporated into
new paragraphs.
The note regarding the ability for <tt>wchar_t</tt> to store all
values of the execution wide-character set is intentionally removed as it
conflicts with long standing existing practice for some implementations.
<blockquote class="stddel">
A
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
that begins with the letter <tt>L</tt>, such as
<tt>L'z'</tt>, is a <em>wide-character literal</em>. A wide-character literal
has type <tt>wchar_t</tt>.
<sup>
[ Footnote: They are intended for character sets where a character does not
fit into a single byte. ]
</sup>
The value of a wide-character literal containing a single
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
has value equal to the numerical value of the encoding of the
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
in the execution wide-character set, unless the
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
has no representation in the execution wide-character set, in which case the
value is implementation-defined.
[ <em>Note:</em> The type <tt>wchar_t</tt> is able to represent all members of
the execution wide-character set (see
<a href="http://eel.is/c++draft/basic.fundamental">[basic.fundamental]</a>).
— <em>end note</em> ]
The value of a wide-character literal containing multiple
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>s
is implementation-defined.
</blockquote>
</p>
<p>Add a new paragraph (W) before
<a href="http://eel.is/c++draft/lex.ccon#6">
5.13.3 [lex.ccon] paragraph 6</a>:
<blockquote class="stdins">
A <em>non-encodable character literal</em> is a
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
whose
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char-sequence"><em>c-char-sequence</em></a>
consists of a single
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>
that is not a
<a href="http://eel.is/c++draft/lex.ccon#nt:numeric-escape-sequence"><em>numeric-escape-sequence</em></a>
and that specifies a character that either
lacks representation in the literal's associated character encoding or that
cannot be encoded as a single code unit.
A <em>multicharacter literal</em> is a
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>
whose
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char-sequence"><em>c-char-sequence</em></a>
consists of more than one
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>.
The
<a href="http://eel.is/c++draft/lex.ccon#nt:encoding-prefix"><em>encoding-prefix</em></a>
of a non-encodable character literal or a multicharacter literal shall be
absent or <tt>L</tt>.
Such
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>s
are conditionally-supported.
</blockquote>
</p>
<p>Add another new paragraph (X) and table (Y) before
<a href="http://eel.is/c++draft/lex.ccon#6">
5.13.3 [lex.ccon] paragraph 6</a>:
<blockquote class="stdins">
The kind of a
<a href="http://eel.is/c++draft/lex.ccon#nt:character-literal"><em>character-literal</em></a>,
its type, and its associated character encoding are determined by its
<a href="http://eel.is/c++draft/lex.ccon#nt:encoding-prefix"><em>encoding-prefix</em></a>
and its
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char-sequence"><em>c-char-sequence</em></a>
as defined by table Y.
The special cases for non-encodable character literals and multicharacter
literals take precedence over their respective base kinds.
</blockquote>
</p>
<blockquote class="stdins">
<div align="center">
<table border="0">
<tr>
<td align="left">Table Y: Character literals
<td/>
<td align="right">[tab:lex.ccon.literals]</td>
</tr>
</table>
<br/>
<table border="1">
<tr>
<th>Encoding<br/>prefix</th>
<th>Kind</th>
<th>Type</th>
<th>Associated<br/>character encoding</th>
<th>Example</th>
</tr>
<tr>
<td rowspan="3">none</td>
<td><em>ordinary character literal</em></td>
<td><tt>char</tt></td>
<td rowspan="3">encoding of the execution character set <sup>[ Footnote Y1: <em>see below</em> ]</sup></td>
<td><tt>'v'</tt></td>
</tr>
<tr>
<td>— non-encodable ordinary character literal</td>
<td rowspan="2"><tt>int</tt></td>
<td>
<tt>'\U0001F525'</tt><sup>[ Footnote Y2: <em>see below</em> ]</sup>
</td>
</tr>
<tr>
<td>— ordinary multicharacter literal</td>
<td><tt>'abcd'</tt></td>
</tr>
<tr>
<td rowspan="3"><tt>L</tt></td>
<td><em>wide character literal</em></td>
<td rowspan="3"><tt>wchar_t</tt></td>
<td rowspan="3">encoding of the execution wide-character set <sup>[ Footnote Y3: <em>see below</em> ]</sup></td>
<td><tt>L'w'</tt></td>
</tr>
<tr>
<td>— non-encodable wide character literal</td>
<td>
<tt>L'\U0001F32A'</tt><sup>[ Footnote Y4: <em>see below</em> ]</sup>
</td>
</tr>
<tr>
<td>— wide multicharacter literal</td>
<td><tt>L'abcd'</tt></td>
</tr>
<tr>
<td><tt>u8</tt></td>
<td><em>UTF-8 character literal</em></td>
<td><tt>char8_t</tt></td>
<td>UTF-8</td>
<td><tt>u8'x'</tt></td>
</tr>