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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>P2029R0</td>
</tr>
<tr>
<th>Date:</th>
<td>2020-01-11</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>
<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 attempts to address the feedback
provided during those discussions.
</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>
<h1>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 applicable 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 applicable execution character set.
(CWG 1656)</li>
<li>specify values that may result in string literals that are ill-formed
according to their applicable 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 applicable
character encoding may be specified in string literals. (CWG 411)</li>
</ul>
</p>
<p>
The concept of an "associated character encoding" is introduced for character
and string literals so as to enable wording to be independent of the particular
kind of literal (ordinary, wide or Unicode).
</p>
<p>
New <tt>basic-c-char</tt>, <tt>basic-s-char</tt>, and
<tt>numeric-escape-sequence</tt> grammar productions are proposed in order
to simplify wording. The <tt>c-char</tt>, <tt>s-char</tt>, and
<tt>escape-sequence</tt> grammar productions are updated to define them in
terms of the new grammar productions.
</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 applicable character encoding.
(Wording was previously missing)</li>
<li>Specify behavior for simple-escape-sequences that denote characters that
lack representation in the applicable character encoding.
(Wording was previously missing)</li>
<li>Remove wording that states that <tt>wchar_t</tt> is able to represent all
members of the execution wide-character set as this contradicts wide
spread existing practice (SG16 has an issue tracking this at
<a href="https://github.com/sg16-unicode/sg16/issues/9">https://github.com/sg16-unicode/sg16/issues/9</a>).
<li>Modernize the wording with current style preferences and terminology.</li>
</ul>
</p>
<h1>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.
</p>
<p>
However, there is implementation impact to the Microsoft Visual C++ compiler.
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>
<h1>Acknowledgements</h1>
<p>Thank you to Jens Maurer and Steve Downey for providing feedback on initial
drafts of this paper!
</p>
<h1>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/2019/n4835.pdf">
N4835</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>
<p>These changes do not reflect recent editorial changes made in
<a href="https://github.com/cplusplus/draft/pull/2768">
https://github.com/cplusplus/draft/pull/2768</a>;
merge conflict resolution will be required.
</p>
<input type="checkbox" id="hideins">Hide inserted text</input><br/>
<input type="checkbox" id="hidedel">Hide deleted text</input>
<p>Change in
<a href="http://eel.is/c++draft/lex.phases#1.5">
5.2 [lex.phases] paragraph 5</a>:<br/>
<em>Drafting Note:</em> The change of "escape sequence" to
"<tt>simple-escape-sequence</tt>" addresses
<a href="http://wg21.link/cwg1656">CWG 1656</a>.
<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 character literal or a string literal, as well as each
<del>escape sequence</del><ins><a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence"><em>simple-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 character literal or a non-raw string literal, is converted to the
<del>corresponding member of the execution character set</del><ins>literal's associated character encoding</ins>
(<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>;</del><ins>.</ins>
<del>if there is no corresponding member, it is converted to an implementation
defined member other than the null (wide) character</del><ins>If a character
lacks representation in the associated character encoding, then the character is
converted to an implementation-defined character or the literal is ill-formed 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>.
<sup><a href="http://eel.is/c++draft/lex.phases#footnote-8">8</a></sup>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.phases#1.7">
5.2 [lex.phases] paragraph 7</a>:<br/>
<em>Drafting note:</em> This addition duplicates wording in [lex.string], but
seems important to include here.
<blockquote>
<ins>A null character is appended to every
<a href="http://eel.is/c++draft/lex.string#nt:string-literal"><em>string-literal</em></a>.
</ins>White-space
characters separating tokens are no longer significant. Each
preprocessing token is converted into a token
(<a href="http://eel.is/c++draft/lex.token">[lex.token]</a>). The resulting
tokens are syntactically and semantically analyzed and translated as a
translation unit. [ <em>Note:</em> The process of analyzing and translating
the tokens may occasionally result in one token being replaced by a sequence
of other tokens
(<a href="http://eel.is/c++draft/temp.names">[temp.names]</a>).
— <em>end note</em> ] It is implementation-defined whether the sources
for module units and header units on which the current translation unit has an
interface dependency
(<a href="http://eel.is/c++draft/module.unit">[module.unit]</a>,
<a href="http://eel.is/c++draft/module.import">[module.import]</a>)
are required to be available. [ <em>Note:</em> Source files, translation units
and translated translation units need not necessarily be stored as files, nor
need there be any one-to-one correspondence between these entities and any
external representation. The description is conceptual only, and does not
specify any particular implementation. — <em>end note</em> ]
</blockquote>
</p>
<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>numeric-escape-sequence</ins>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence">simple-escape-sequence:</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>
numeric-escape-sequence:
<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>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon#1">
5.13.3 [lex.ccon] paragraph 1</a>:
<blockquote>
A <del>character literal</del><ins><em>character literal</em></ins> is
<del>one or more characters</del><ins>a
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char-sequence"><em>c-char-sequence</em></a></ins>
enclosed in single quotes, as in
<del><tt>'x'</tt></del><ins><tt>'v'</tt></ins>, optionally preceded by
<del><tt>u8</tt>, <tt>u</tt>, <tt>U</tt>, or <tt>L</tt></del><ins>an
<a href="http://eel.is/c++draft/lex.ccon#nt:encoding-prefix"><em>encoding-prefix</em></a></ins>,
as in <tt>u8'w'</tt>, <tt>u'x'</tt>, <tt>U'y'</tt>, or <tt>L'z'</tt><del>,
respectively</del>.
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon#2">
5.13.3 [lex.ccon] paragraph 2</a>:<br/>
<em>Drafting Note:</em> Wording for multicharacter literals, characters that
lack representation in the execution character set, and the value of a character
literal has been moved to new paragraphs.
<blockquote>
A character literal that does not begin with
<del>
<tt>u8</tt>, <tt>u</tt>,
<tt>U</tt>, or <tt>L</tt>
</del><ins>an
<a href="http://eel.is/c++draft/lex.ccon#nt:encoding-prefix"><em>encoding-prefix</em></a>,
such as <tt>'v'</tt>,</ins>
is an <em>ordinary character literal</em><ins>,
has type <tt>char</tt> unless otherwise specified to have type <tt>int</tt> as
described below, and has the encoding of the execution character set
(<a href="http://eel.is/c++draft/lex.charset">[lex.charset]</a>)
as its associated character encoding</ins>.<del>
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.</del>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon#3">
5.13.3 [lex.ccon] paragraph 3</a>:<br/>
<em>Drafting Note:</em> Wording for multicharacter UTF-8 literals, characters
that cannot be represented in a single UTF-8 code unit, and the value of a
UTF-8 character literal has been moved to new paragraphs.
<blockquote>
A character literal that begins with <tt>u8</tt>, such as <tt>u8'w'</tt>,
is a
<del>
character literal of type <tt>char8_t</tt>, known as a
</del>
<em>UTF-8 character literal</em><ins>, has type <tt>char8_t</tt>, and has UTF-8
as its associated character encoding</ins>.<del>
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 0x0-0x7F (inclusive). —
<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.
</del>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon#4">
5.13.3 [lex.ccon] paragraph 4</a>:<br/>
<em>Drafting Note:</em> Wording for multicharacter UTF-16 literals, characters
that cannot be represented in a single UTF-16 code unit, and the value of a
UTF-16 character literal has been moved to new paragraphs.
<blockquote>
A character literal that begins with <del>the letter </del><tt>u</tt>, such as
<tt>u'x'</tt>, is a
<del>character literal of type <tt>char16_t</tt>, known as a</del>
<em>UTF-16 character literal</em><ins>, has type <tt>char16_t</tt>, and has
UTF-16 as its associated character encoding</ins>.<del>
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 0x0-0xFFFF (inclusive).
— <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.
</del>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon#5">
5.13.3 [lex.ccon] paragraph 5</a>:<br/>
<em>Drafting Note:</em> Wording for multicharacter UTF-32 literals, and the
value of a UTF-32 character literal has been moved to new paragraphs.
<blockquote>
A character literal that begins with <del>the letter </del><tt>U</tt>, such as
<tt>U'y'</tt>, is a
<del>character literal of type <tt>char32_t</tt>, known as a</del>
<em>UTF-32 character literal</em><ins>, has type <tt>char32_t</tt>, and has
UTF-32 as its associated character encoding</ins>.<del>
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.
</del>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon#6">
5.13.3 [lex.ccon] paragraph 6</a>:<br/>
<em>Drafting Note:</em> Wording for multicharacter wide literals, and the value
of a wide character literal has been moved to 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
(<a href="https://github.com/sg16-unicode/sg16/issues/9">https://github.com/sg16-unicode/sg16/issues/9</a>).
<blockquote>
A character literal that begins with <del>the letter </del><tt>L</tt>, such as
<tt>L'z'</tt>, is a <em>wide-character literal</em><ins>, has type
<tt>wchar_t</tt><sup><a href="http://eel.is/c++draft/lex.ccon#footnote-18">18</a></sup>,
and has the encoding of the execution wide-character set
(<a href="http://eel.is/c++draft/lex.charset">[lex.charset]</a>)
as its associated character encoding</ins>.<del>
A wide-character literal
has type <tt>wchar_t</tt>.<sup><a href="http://eel.is/c++draft/lex.ccon#footnote-18">18</a></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.
</del>
</blockquote>
</p>
<p>Add a new paragraph after
<a href="http://eel.is/c++draft/lex.ccon#6">
5.13.3 [lex.ccon] paragraph 6</a>:
<blockquote class="stdins">
A character literal with a
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char-sequence"><em>c-char-sequence</em></a>
containing more than one
<a href="http://eel.is/c++draft/lex.ccon#nt:c-char"><em>c-char</em></a>,
as in <tt>'abcd'</tt>, is a <em>multicharacter literal</em>.
An ordinary character literal that is a multicharacter literal is conditionally
supported, has type <tt>int</tt>, and has an implementation-defined value.
A wide-character literal that is a multicharacter literal has an
implementation-defined value. A UTF-8 character literal, UTF-16 character
literal, or UTF-32 character literal that is a multicharacter literal is
ill-formed.
</blockquote>
</p>
<p>Add another new paragraph (X):
<blockquote class="stdins">
The value of a character literal consisting of a single
<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.ccon#nt:simple-escape-sequence"><em>simple-escape-sequence</em></a>,
or
<a href="http://eel.is/c++draft/lex.charset#nt:universal-character-name"><em>universal-character-name</em></a>
is the code point value of the specified character encoded according to the
character literal's associated character encoding. If the character lacks
representation in the associated character encoding, then:
<table>
<tr>
<td>(X.1) —</td>
<td>for an ordinary character literal, the literal is conditionally
supported, has type <tt>int</tt>, and has an implementation-defined
value that is not the null character value.</td>
</tr>
<tr>
<td>(X.2) —</td>
<td>for a wide-character literal, the literal has an implementation-defined
value that is not the null wide character value.</td>
</tr>
<tr>
<td>(X.3) —</td>
<td>for a UTF-8, UTF-16, or UTF-32 character literal, the literal is
ill-formed.</td>
</tr>
</table>
If the character requires multiple code units to be encoded in the associated
character encoding, then:
<table>
<tr>
<td>(X.4) —</td>
<td>for an ordinary character literal, the literal is conditionally
supported, has type <tt>int</tt>, and has an implementation-defined
value that is not the null character value.</td>
</tr>
<tr>
<td>(X.5) —</td>
<td>for a wide-character literal, the literal has an implementation-defined
value that is not the null wide character value.</td>
</tr>
<tr>
<td>(X.6) —</td>
<td>for a UTF-8 or UTF-16 character literal, the literal is
ill-formed. <em>[ Note:</em> UTF-32 never requires multiple code units
for an encoded character. <em>— end note ]</em>
</td>
</tr>
</table>
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon#7">
5.13.3 [lex.ccon] paragraph 7</a>:<br/>
<em>Drafting Note:</em> Wording for additional conditionally-supported simple
escape sequences has been moved to a new paragraph.
<blockquote>
<ins>The character specified by a
<a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence"><em>simple-escape-sequence</em></a>
is specified in table
<a href="http://eel.is/c++draft/lex.ccon#tab:lex.ccon.esc">8</a>.
<em>[ Note:</em> The execution character set and the execution wide-character
set do not require representation for the
<tt>new-line (NL)</tt>,
<tt>horizontal tab (HT)</tt>,
<tt>vertical tab (VT)</tt>, or
<tt>form feed (FF)</tt> characters
(<a href="http://eel.is/c++draft/lex.charset">[lex.charset]</a>); the UTF-8,
UTF-16, and UTF-32 encodings are able to represent all of the
<a href="http://eel.is/c++draft/lex.ccon#nt:simple-escape-sequence"><em>simple-escape-sequence</em></a>
characters using a single code unit.
<em>— end note ]</em>
</ins>
<del>Certain non-graphic characters, the single quote <tt>'</tt>, the double quote
<tt>"</tt>, the question mark
<tt>?</tt>,<sup><a href="http://eel.is/c++draft/lex.ccon#footnote-19">19</a></sup>
and the backslash <tt>\</tt>, can be represented according to Table
<a href="http://eel.is/c++draft/lex.ccon#tab:lex.ccon.esc">8</a>.
The double quote <tt>"</tt> and the question mark <tt>?</tt>, can be
represented as themselves or by the escape sequences <tt>\"</tt> and <tt>\?</tt>
respectively, but the single quote <tt>'</tt> and the backslash <tt>\</tt> shall
be represented by the escape sequences <tt>\'</tt> and <tt>\\</tt> respectively.
Escape sequences in which the character following the backslash is not listed in
Table <a href="http://eel.is/c++draft/lex.ccon#tab:lex.ccon.esc">8</a>
are conditionally-supported, with implementation-defined semantics. An escape
sequence specifies a single character.
</del>
<br/>
<br/>
<div align="center">
<table border="0">
<tr>
<td align="left">Table <a href="http://eel.is/c++draft/lex.ccon#tab:lex.ccon.esc">8</a>:
<del>E</del><ins>Simple e</ins>scape sequences</td>
<td/>
<td align="right">[tab:lex.ccon.esc]</td>
</tr>
</table>
<br/>
<table border="1">
<tr><td>new-line</td> <td>NL(LF)</td><td><tt>\n</tt></td></tr>
<tr><td>horizontal tab</td> <td>HT</td> <td><tt>\t</tt></td></tr>
<tr><td>vertical tab</td> <td>VT</td> <td><tt>\v</tt></td></tr>
<tr><td>backspace</td> <td>BS</td> <td><tt>\b</tt></td></tr>
<tr><td>carriage return</td><td>CR</td> <td><tt>\r</tt></td></tr>
<tr><td>form feed</td> <td>FF</td> <td><tt>\f</tt></td></tr>
<tr><td>alert</td> <td>BEL</td> <td><tt>\a</tt></td></tr>
<tr><td>backslash</td> <td>\</td> <td><tt>\\</tt></td></tr>
<tr><td>question mark
<ins><sup><a href="http://eel.is/c++draft/lex.ccon#footnote-19">19</a></sup></ins></td> <td>?</td> <td><tt>\?</tt></td></tr>
<tr><td>single quote</td> <td>'</td> <td><tt>\'</tt></td></tr>
<tr><td>double quote</td> <td>"</td> <td><tt>\"</tt></td></tr>
<tr><td><del>octal number</del></td> <td><del>ooo</del></td> <td><tt><del>\ooo</del></tt></td></tr>
<tr><td><del>hex number</del></td> <td><del>hhh</del></td> <td><tt><del>\xhhh</del></tt></td></tr>
</table>
</div>
</blockquote>
</p>
<p>Add a new paragraph after
<a href="http://eel.is/c++draft/lex.ccon#7">
5.13.3 [lex.ccon] paragraph 7</a>:
<blockquote class="stdins">
Additional simple escape sequences in which the character following the
backslash is not listed in Table
<a href="http://eel.is/c++draft/lex.ccon#tab:lex.ccon.esc">8</a>
are conditionally-supported, with implementation-defined semantics.
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.ccon#8">
5.13.3 [lex.ccon] paragraph 8</a>:<br/>
<em>Drafting Note:</em> Wording describing the form of octal and hexadecimal
escape sequences has been removed as redundant; the form is implicit in the
grammar.
<blockquote>
<ins>The value of a character literal consisting of a single
<a href="http://eel.is/c++draft/lex.ccon#nt:numeric-escape-sequence"><em>numeric-escape-sequence</em></a>
is the numeric value of the octal or hexadecimal number.</ins><del> The
escape <tt>\ooo</tt> consists of the backslash followed by one, two, or
three octal digits that are taken to specify the value of the desired character.
The escape <tt>\xhhh</tt> consists of the backslash followed by <tt>x</tt>
followed by one or more hexadecimal digits that are taken to specify the value
of the desired character. </del>There is no limit to the number of digits in a
hexadecimal sequence. A sequence of octal or hexadecimal digits is terminated
by the first character that is not an octal digit or a hexadecimal digit,
respectively.
<del> The value of a character literal is implementation-defined if it
falls outside of the implementation-defined range defined for <tt>char</tt>
(for character literals with no prefix) or <tt>wchar_t</tt> (for character
literals prefixed by <tt>L</tt>). <em>[ Note:</em> If the value of
a character literal prefixed by <tt>u</tt>, <tt>u8</tt>, or <tt>U</tt> is
outside the range defined for its type, the program is ill-formed.
<em>— end note ]</em></del><ins>If the numeric value exceeds the range
of the character literal type, then:
<table>
<tr>
<td>(8.1) —</td>
<td>for an ordinary character literal, the value is
implementation-defined.</td>
</tr>
<tr>
<td>(8.2) —</td>
<td>for a wide character literal, the value is
implementation-defined.</td>
</tr>
<tr>
<td>(8.3) —</td>
<td>for a UTF-8, UTF-16, or UTF-32 character literal, the literal is
ill-formed.</td>
</tr>
</table>
</ins>
</blockquote>
</p>
<p>Delete
<a href="http://eel.is/c++draft/lex.ccon#9">
5.13.3 [lex.ccon] paragraph 9</a>:<br/>
<em>Drafting Note:</em> The normative text was combined with wording for
<tt>basic-c-char</tt> and <tt>simple-escape-sequence</tt> above. The deleted
note duplicates normative text in
<a href="http://eel.is/c++draft/lex.phases#1.1">
5.2 [lex.phases] paragraph 1</a>.
<blockquote class="stddel">
A
<a href="http://eel.is/c++draft/lex.charset#nt:universal-character-name"><em>universal-character-name</em></a>
is translated to the encoding, in the appropriate execution character set, of
the character named. If there is no such encoding, the
<a href="http://eel.is/c++draft/lex.charset#nt:universal-character-name"><em>universal-character-name</em></a>
is translated to an implementation-defined encoding.
[ <em>Note:</em> In translation phase 1, a
<a href="http://eel.is/c++draft/lex.charset#nt:universal-character-name"><em>universal-character-name</em></a>
is introduced whenever an actual extended character is encountered in the
source text. Therefore, all extended characters are described in terms of
<a href="http://eel.is/c++draft/lex.charset#nt:universal-character-name"><em>universal-character-name</em></a>s
However, the actual compiler implementation may use its own native character
set, so long as the same results are obtained. — <em>end note</em> ]
</blockquote>
</p>
<p>Change in
<a href="http://eel.is/c++draft/lex.string">
5.13.5 [lex.string]</a>:
<blockquote>
<a href="http://eel.is/c++draft/lex.string#nt:string-literal">string-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.string#nt:s-char-sequence">s-char-sequence</a><sub>opt</sub>
<tt>"</tt>
</div>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.ccon#nt:encoding-prefix">encoding-prefix</a><sub>opt</sub>
<tt>R</tt>
<a href="http://eel.is/c++draft/lex.string#nt:raw-string">raw-string</a>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.string#nt:s-char-sequence">s-char-sequence:</a>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.string#nt:s-char">s-char</a>
</div>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.string#nt:s-char-sequence">s-char-sequence</a>
<a href="http://eel.is/c++draft/lex.string#nt:s-char">s-char</a>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.string#nt:s-char">s-char:</a>
<del><div style="margin-left: 1em;">
any member of the basic source character set except the double-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-s-char
</div></ins>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.string#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-s-char:
<div style="margin-left: 1em;">
any member of the basic source character set except the double-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.string#nt:raw-string">raw-string:</a>
<div style="margin-left: 1em;">
<tt>"</tt>
<a href="http://eel.is/c++draft/lex.string#nt:d-char-sequence">d-char-sequence</a><sub>opt</sub>
<tt>(</tt>
<a href="http://eel.is/c++draft/lex.string#nt:r-char-sequence">r-char-sequence</a><sub>opt</sub>
<tt>)</tt>
<a href="http://eel.is/c++draft/lex.string#nt:d-char-sequence">d-char-sequence</a><sub>opt</sub>
<tt>"</tt>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.string#nt:r-char-sequence">r-char-sequence:</a>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.string#nt:r-char">r-char</a>
</div>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.string#nt:r-char-sequence">r-char-sequence</a>
<a href="http://eel.is/c++draft/lex.string#nt:r-char">r-char</a>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.string#nt:r-char">r-char:</a>
<div style="margin-left: 1em;">
any member of the source character set, except a right parenthesis <tt>)</tt> followed by
<div style="margin-left: 1em;">
the initial
<a href="http://eel.is/c++draft/lex.string#nt:d-char-sequence">d-char-sequence</a>
(which may be empty) followed by a double quote <tt>"</tt>.
</div>
</div>
<br/>
<a href="http://eel.is/c++draft/lex.string#nt:d-char-sequence">d-char-sequence:</a>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.string#nt:d-char">d-char</a>
</div>
<div style="margin-left: 1em;">
<a href="http://eel.is/c++draft/lex.string#nt:d-char-sequence">d-char-sequence</a>