Proposed resolution for core issues 411, 1656, and 2333; numeric and universal

character escapes in character and string literals (original) (raw)

Introduction

This paper proposes substantial changes to[lex.phases],[lex.ccon], and[lex.string]intended to address the following core issues as well as several other more minor issues.

This paper follows a priorproposed resolutionthat only attempted to addressCWG 2333. That proposed resolution wasdiscussed on the core mailing listand at theJune 24th, 2019, core issues processing teleconference. The resolution proposed in this paper addresses the feedback provided during those discussions and further review at theJanuary 16th, 2020, core issues processing teleconference, theMarch 23rd, 2020, core issues processing teleconference,discussion on the core mailing list originating on June 28th, 2020,further discussions on the core mailing list originating on July 7th, 2020,July 20th, 2020, core issues processing teleconference, and theOctober 19th, 2020, core paper review teleconference.

The notes for CWG 2333 in the current active issues list (revision 100) state that discussion at theAugust 14th, 2017, core issues processing teleconference 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.

SG16 discussed this issue during itsOctober 17th, 2018, teleconference. 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:

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?

SF F N A SA
8 1 0 0 0

In the polled question, "Continue" refers to existing implementation behavior; to maintain the current implementation status quo exhibited by gcc, Clang and Visual C++.

The proposed resolution reflects the SG16 consensus.

CWG 411 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, '\u0153' is conditionally-supported, has type int and an implementation-defined value, but"\u0153" is a character array of length 3 containing the UTF-8 encoding of U+0153 (LATIN SMALL LIGATURE OE) and a null character (\xC5\x93\x00).

CWG 1656 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.

Additionally, since integer values are now guaranteed to be represented in two's complement following the adoption ofP1236R1in 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.

This paper does not attempt to address the existing issue concerning the order in which adjacent string literals are encoded and concatenated as reported athttps://lists.isocpp.org/core/2020/07/9537.phpand tracked byCWG 2455.

Changes from P2029R3

Proposed resolution overview

The proposed wording changes are intended to resolveCWG 411,CWG 1656, andCWG 2333 by:

Additionally, the wording updates are intended to:

The wording changes introduce the following:

Implementation impact

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.

Semantics of numeric-escape-sequences in UTF-8 literals

Consider the following (C++20) 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

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.

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 char8_t 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 char16_t are rejected rather than being considered a code point value and encoded as a UTF-16 surrogate pair.

Character literal type for characters not representable in the execution character set

Consider the following code assuming that the execution character set does not have representation for the specified Unicode code points.

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

This code should be rejected (both before and after this proposal) because the redeclaration of c1 with type char does not match the first declaration for which c1 should have a deduced type of int. Visual C++ accepts it when compiled with /execution-charset:windows-1252with the following warnings:

<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)

It seems that the Visual C++ compiler translates unrepresentable characters from the Unicode BMP to a single char with value equal to'?', but translates unrepresentable characters from outside the Unicode BMP to int with value equal to the multicharacter literal'??'. This seems unlikely to be intended behavior. It would be conforming if, for the Unicode BMP case, an int with value equal to '?' was produced.

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 c1 as char does not match the deduced int type for its first declaration.

Acknowledgements

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!

Proposed resolution

These changes are relative toN4861.

The changes to[lex.ccon] and[lex.string]are rather pervasive. For ease of review, unchanged paragraphs in these sections are retained in the wording below. These paragraphs are introduced with "No changes to ..." and are highlighted with a blue background.

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[lex.phases]

Change in5.2 [lex.phases] paragraph 1.5:

Each basic source character set memberbasic-c-char,basic-s-char, andr-char in acharacter-literalor astring-literal, as well as eachescape sequenceescape-sequenceanduniversal-character-namein acharacter-literalor a non-raw string literal,is converted to the corresponding member of the execution character set ([lex.ccon],[lex.string]);is encoded in the literal's associated character encoding as specified in[lex.ccon] and[lex.string]. if there is no corresponding member, it is converted to an implementation-defined member other than the null (wide) character.[ Footnote: An implementation need not convert all non-corresponding source characters to the same execution character. ]

Change in5.2 [lex.phases] paragraph 1.6:
Drafting note: This addition duplicates wording in [lex.string], but seems important to include here.

Adjacentstring literal tokensstring-literalsare concatenated and a null character is appended to the result as specified in[lex.string].

[lex.ccon]

Change in5.13.3 [lex.ccon]:

character-literal:
encoding-prefix: one of

u8 u U L

c-char-sequence:
c-char:

any member of the basic source character set except the single-quote

'

, backslash

\

, ornew-linecharacter

basic-c-char

basic-c-char:

any member of the basic source character set except the single-quote

'

, backslash

\

, ornew-linecharacter

escape-sequence:
simple-escape-sequence: one of

\' \" \? \\ \a \b \f \n \r \t \v \ simple-escape-sequence-char

simple-escape-sequence-char: one of

' " ? \
a b f n r t v

numeric-escape-sequence:
octal-escape-sequence:
hexadecimal-escape-sequence:
conditional-escape-sequence:
conditional-escape-sequence-char:

Delete5.13.3 [lex.ccon] paragraph 1:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs.

Acharacter-literalthat does not begin withu8, u,U, or Lis an ordinary character literal. An ordinary character literal that contains a singlec-charrepresentable in the execution character set has type char, with value equal to the numerical value of the encoding of thec-charin the execution character set. An ordinary character literal that contains more than onec-charis a multicharacter literal. A multicharacter literal, or an ordinary character literal containing a singlec-charnot representable in the execution character set, is conditionally-supported, has type int, and has an implementation-defined value.

Delete5.13.3 [lex.ccon] paragraph 2:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs. The note regarding the range of single code unit values was removed.

Acharacter-literalthat begins with u8, such as u8'w', is acharacter-literalof type char8_t, known as a_UTF-8 character literal_. 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. [ Note: That is, provided the code point value is in the range [0,7F] (hexadecimal). —end note ] If the value is not representable with a single UTF-8 code unit, the program is ill-formed. A UTF-8 character literal containing multiplec-chars is ill-formed.

Delete5.13.3 [lex.ccon] paragraph 3:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs. The note regarding the range of singled code unit values was removed.

Acharacter-literalthat begins with the letter u, such asu'x', is acharacter-literalof type char16_t, known as a_UTF-16 character literal_. 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. [ Note: That is, provided the code point value is in the range [0,FFFF] (hexadecimal). — end note ] If the value is not representable with a single 16-bit code unit, the program is ill-formed. A UTF-16 character literal containing multiplec-chars is ill-formed.

Delete5.13.3 [lex.ccon] paragraph 4:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs.

Acharacter-literalthat begins with U, such as U'y', is acharacter-literalof type char32_t, known as a_UTF-32 character literal_. The value of a UTF-32 character literal containing a singlec-charis equal to its ISO/IEC 10646 code point value. A UTF-32 character literal containing multiplec-chars is ill-formed.

Delete5.13.3 [lex.ccon] paragraph 5:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs. The note regarding the ability for wchar_t to store all values of the execution wide-character set is intentionally removed as it conflicts with long standing existing practice for some implementations.

Acharacter-literalthat begins with the letter L, such asL'z', is a wide-character literal. A wide-character literal has type wchar_t.[ Footnote: They are intended for character sets where a character does not fit into a single byte. ]The value of a wide-character literal containing a singlec-charhas value equal to the numerical value of the encoding of thec-charin the execution wide-character set, unless thec-charhas no representation in the execution wide-character set, in which case the value is implementation-defined. [ Note: The type wchar_t is able to represent all members of the execution wide-character set (see[basic.fundamental]). — end note ] The value of a wide-character literal containing multiplec-chars is implementation-defined.

Add a new paragraph (W) before5.13.3 [lex.ccon] paragraph 6:

A non-encodable character literal is acharacter-literalwhosec-char-sequenceconsists of a singlec-charthat is not anumeric-escape-sequenceand 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 multicharacter literal is acharacter-literalwhosec-char-sequenceconsists of more than onec-char. Theencoding-prefixof a non-encodable character literal or a multicharacter literal shall be absent or L. Suchcharacter-literals are conditionally-supported.

Add another new paragraph (X) and table (Y) before5.13.3 [lex.ccon] paragraph 6:

The kind of acharacter-literal, its type, and its associated character encoding are determined by itsencoding-prefixand itsc-char-sequenceas defined by table Y. The special cases for non-encodable character literals and multicharacter literals take precedence over their respective base kinds.

| Table Y: Character literals | | [tab:lex.ccon.literals] | | --------------------------- | | ------------------------- |

Encodingprefix Kind Type Associatedcharacter encoding Example
none ordinary character literal char encoding of the execution character set [ Footnote Y1: see below ] 'v'
— non-encodable ordinary character literal int '\U0001F525'[ Footnote Y2: see below ]
— ordinary multicharacter literal 'abcd'
L wide character literal wchar_t encoding of the execution wide-character set [ Footnote Y3: see below ] L'w'
— non-encodable wide character literal L'\U0001F32A'[ Footnote Y4: see below ]
— wide multicharacter literal L'abcd'
u8 UTF-8 character literal char8_t UTF-8 u8'x'
u UTF-16 character literal char16_t UTF-16 u'y'
U UTF-32 character literal char32_t UTF-32 U'z'

Y1) The associated character encoding for ordinary character literals determines encodability, but does not determine the value of non-encodable ordinary character literals or ordinary multicharacter literals.
Y2) The example assumes that the specified character lacks representation in the execution character set, or that encoding it would require more than one code unit.
Y3) The associated character encoding for wide character literals determines encodability, but does not determine the value of non-encodable wide character literals or wide multicharacter literals.
Y4) The example assumes that the specified character lacks representation in the execution wide-character set, or that encoding it would require more than one code unit.

Add another new paragraph (Z) before5.13.3 [lex.ccon] paragraph 6:

Intranslation phase 4, the value of acharacter-literalis determined using the range of representable values of thecharacter-literal's type intranslation phase 7. A non-encodable character literal or a multicharacter literal has an implementation-defined value. The value of any other kind ofcharacter-literalis determined as follows:

Change in5.13.3 [lex.ccon] paragraph 6:
_Drafting Note:_The deleted text has been removed as redundant since it repeats information implicit in the grammar. The added note is content moved from the deleted footnote.

The character specified by asimple-escape-sequenceis specified in table9. [ Note: Using an escape sequence for a question mark is supported for compatibility with ISO C++ 2014 and ISO C. — end note ] Certain non-graphic characters, the single quote ', the double quote", the question mark?,[ Footnote: Using an escape sequence for a question mark is supported for compatibility with ISO C++ 2014 and ISO C. ]and the backslash \, can be represented according to Table9. The double quote " and the question mark ?, can be represented as themselves or by the escape sequences \" and \?respectively, but the single quote ' and the backslash \ shall be represented by the escape sequences \' and \\ respectively. Escape sequences in which the character following the backslash is not listed in Table 9are conditionally-supported, with implementation-defined semantics. An escape sequence specifies a single character.

| Table 9:ESimple escape sequences | | [tab:lex.ccon.esc] | | ---------------------------------------------------------------------------------------------------------------- | | -------------------- |

new-line NL(LF) \n
horizontal tab HT \t
vertical tab VT \v
backspace BS \b
carriage return CR \r
form feed FF \f
alert BEL \a
backslash \ \\
question mark ? \?
single quote ' \'
double quote " \"
octal number ooo \ooo
hex number hhh \xhhh

Delete5.13.3 [lex.ccon] paragraph 7:
Drafting Note: Wording describing the form of octal and hexadecimal escape sequences has been removed as redundant; the form is implicit in the grammar.

The escape \ooo 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 \xhhh consists of the backslash followed by xfollowed by one or more hexadecimal digits that are taken to specify the value of the desired character. 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. The value of acharacter-literalis implementation-defined if it falls outside of the implementation-defined range defined for char(forcharacter-literals with no prefix) or wchar_t (forcharacter-literals prefixed by L). [ _Note:_ If the value of acharacter-literalprefixed by u, u8, or U is outside the range defined for its type, the program is ill-formed. — end note ]

Delete5.13.3 [lex.ccon] paragraph 8:
Drafting Note: The normative text was combined with wording forbasic-c-char and simple-escape-sequence above. The deleted note duplicates normative text in5.2 [lex.phases] paragraph 1.1.

Auniversal-character-nameis translated to the encoding, in the appropriate execution character set, of the character named. If there is no such encoding, theuniversal-character-nameis translated to an implementation-defined encoding. [ _Note:_ In translation phase 1, auniversal-character-nameis introduced whenever an actual extended character is encountered in the source text. Therefore, all extended characters are described in terms ofuniversal-character-names However, the actual compiler implementation may use its own native character set, so long as the same results are obtained. — end note ]

[lex.string]

Change in5.13.5 [lex.string]:

string-literal:
s-char-sequence:
s-char:

any member of the basic source character set except the double-quote

"

, backslash

\

, ornew-linecharacter

basic-s-char

basic-s-char:

any member of the basic source character set except the double-quote

"

, backslash

\

, ornew-linecharacter

raw-string:
r-char-sequence:
r-char:

any member of the source character set, except a right parenthesis

)

followed by

the initiald-char-sequence(which may be empty) followed by a double quote

"

.

d-char-sequence:
d-char:

any member of the source character set except:

space, the left parenthesis

(

, the right parenthesis

)

, the backslash

\

, and the control characters
representing horizontal tab, vertical tab, form feed, and newline.

Add a new paragraph (X) and table (Y) before5.13.5 [lex.string] paragraph 1:

The kind of astring-literal, its type, and its associated character encoding are determined by its encoding prefix and sequence ofs-chars orr-chars as defined by table Y where n is the number of encoded code units as described below.

| Table Y: String literals | | [tab:lex.string.literals] | | ------------------------ | | --------------------------- |

Encodingprefix Kind Type Associatedcharacter encoding Examples
none ordinary string literal array of n const char encoding of the execution character set "an ordinary string" R"(an ordinary raw string)"
L wide string literal array of n const wchar_t encoding of the execution wide-character set L"a wide string" LR"w(a wide raw string)w"
u8 UTF-8 string literal array of n const char8_t UTF-8 u8"a UTF-8 string" u8R"x(a UTF-8 raw string)x"
u UTF-16 string literal array of n const char16_t UTF-16 u"a UTF-16 string" uR"y(a UTF-16 raw string)y"
U UTF-32 string literal array of n const char32_t UTF-32 U"A UTF-32 string" UR"z(a UTF-32 raw string)z"

No changes to5.13.5 [lex.string] paragraph 1:

Astring-literalthat has an R in the prefix is araw string literal. Thed-char-sequenceserves as a delimiter. The terminatingd-char-sequenceof araw-stringis the same sequence of characters as the initiald-char-sequence. Ad-char-sequenceshall consist of at most 16 characters.

No changes to5.13.5 [lex.string] paragraph 2:

[ _Note:_ The characters '(' and ')' are permitted in araw-string. Thus, R"delimiter((a|b))delimiter" is equivalent to "(a|b)". — end note ]

No changes to5.13.5 [lex.string] paragraph 3:

[ Note: A source-file new-line in a raw string literal results in a new-line in the resulting execution string literal. Assuming no whitespace at the beginning of lines in the following example, the assert will succeed:

const char* p = R"(a\
b
c)";
assert(std::strcmp(p, "a\\\nb\nc") == 0);

— end note ]

No changes to5.13.5 [lex.string] paragraph 4:

[ Example: The raw string is equivalent to "\n)\\\na\"\n". The raw string is equivalent to "x = \"\\\"y\\\"\"". — end example ]

Delete5.13.5 [lex.string] paragraph 5:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

After translation phase 6, astring-literalthat does not begin with anencoding-prefixis an ordinary string literal. An ordinary string literal has type "array of n const char" where n is the size of the string as defined below, has static storage duration ([basic.stc]), and is initialized with the given characters.

Delete5.13.5 [lex.string] paragraph 6:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

Astring-literalthat begins with u8, such as u8"asdf", is a_UTF-8 string literal_. A UTF-8 string literal has type "array of_n_ const char8_t", where n is the size of the string as defined below; each successive element of the object representation ([basic.types]) has the value of the corresponding code unit of the UTF-8 encoding of the string.

No changes to5.13.5 [lex.string] paragraph 7:

Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals.

Delete5.13.5 [lex.string] paragraph 8:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs. The note has been deleted as redundant; the use of surrogate pairs is explicit in the UTF-16 encoding.

Astring-literalthat begins with u, such as u"asdf", is a_UTF-16 string literal_. A UTF-16 string literal has type "array of_n_ const char16_t", where n is the size of the string as defined below; each successive element of the array has the value of the corresponding code unit of the UTF-16 encoding of the string. [ _Note:_ A singlec-charmay produce more than one char16_t character in the form of surrogate pairs. A surrogate pair is a representation for a single code point as a sequence of two 16-bit code units. — end note ]

Delete5.13.5 [lex.string] paragraph 9:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

Astring-literalthat begins with U, such as U"asdf", is a_UTF-32 string literal_. A UTF-32 string literal has type "array of_n_ const char32_t", where n is the size of the string as defined below; each successive element of the array has the value of the corresponding code unit of the UTF-32 encoding of the string.

Delete5.13.5 [lex.string] paragraph 10:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

Astring-literalthat begins with L, such as L"asdf", is a_wide string literal_. A wide string literal has type "array of n const wchar_t", where _n_is the size of the string as defined below; it is initialized with the given characters.

Change in5.13.5 [lex.string] paragraph 11:

Intranslation phase 6([lex.phases]), adjacentstring-literals are concatenated. If bothstring-literals have the sameencoding-prefix, the resulting concatenatedstring-literalhas thatencoding-prefix. If onestring-literalhas noencoding-prefix, it is treated as astring-literalof the sameencoding-prefixas the other operand. If a UTF-8 string literal token is adjacent to a wide string literal token, the program is ill-formed. Any other concatenations are conditionally-supported with implementation-defined behavior. [ _Note:_ This concatenation is an interpretation, not a conversion. Because the interpretation happens in translation phase 6 (after each character from astring-literalhas been translated into a value from the appropriate character setafter the string literal contents have been encoded in thestring-literal's associated character encoding), astring-literal's initial rawness has no effect on the interpretation or well-formedness of the concatenation. — end note ] Table11has some examples of valid concatenations.

| Table 11: String literal concatenations | | [tab:lex.string.concat] | | -------------------------------------------------------------------------------------------------------------------------- | | ------------------------- |

SourceMeans u"a"u"b" u"a""b" "a"u"b" u"ab" u"ab" u"ab" SourceMeans U"a"U"b" U"a""b" "a"U"b" U"ab" U"ab" U"ab" SourceMeans L"a"L"b" L"a""b" "a"L"b" L"ab" L"ab" L"ab"

Characters in concatenated strings are kept distinct.

[ Example:

contains the two characters '\xA' and 'B' after concatenation (and not the single hexadecimal character '\xAB'). — end example ]

Change in5.13.5 [lex.string] paragraph 12:
Drafting note: The literal '\0' was replaced with "a null character" to avoid an otherwise needed correction to qualify the literal to be appended with an encoding prefix matching the kind of the string literal. The rationale regarding why a null character is appended was removed as unnecessary normative text. If desired, it could be restored as a note.

After any necessary concatenation, iIntranslation phase 76([lex.phases]),after adjacentstring-literals are concatenated, '\0'a null character is appended to the resulteverystring-literalso that programs that scan a string can find its end.

Delete5.13.5 [lex.string] paragraph 13:
Drafting note: This wording has been removed as misleading, incomplete, or redundant. String literal contents do not always have the same meaning as in character literals. The wording regarding single and double quotes is redundant with the grammar. The discussion of string length is unnecessary as string length is determined by encoding.

Escape sequences anduniversal-character-names in non-raw string literals have the same meaning as incharacter literals([lex.ccon]), except that the single quote ' is representable either by itself or by the escape sequence \', and the double quote " shall be preceded by a\, and except that auniversal-character-namein a UTF-16 string literal may yield a surrogate pair. In a narrow string literal, auniversal-character-namemay map to more than one char or char8_t element due tomultibyte encoding. The size of a char32_t or wide string literal is the total number of escape sequences,universal-character-names, and other characters, plus one for the terminating U'\0' orL'\0'. The size of a UTF-16 string literal is the total number of escape sequences,universal-character-names, and other characters, plus one for each character requiring a surrogate pair, plus one for the terminating u'\0'. [ Note: The size of achar16_t string literal is the number of code units, not the number of characters. — end note ] [ _Note:_ Anyuniversal-character-names are required to correspond to a code point in the range [0,D800) or [E000,10FFFF] (hexadecimal) ([lex.charset]). — end note ] The size of a narrow string literal is the total number of escape sequences and other characters, plus at least one for the multibyte encoding of eachuniversal-character-name, plus one for the terminating '\0'.

Change in5.13.5 [lex.string] paragraph 14:
Drafting note: Wording for string literal object initialization has been moved to a new paragraph.

Evaluating astring-literalresults in a string literal object with static storage duration([basic.stc]), initialized from the given characters as specified above. Whether allstring-literals are distinct (that is, are stored in nonoverlapping objects) and whether successive evaluations of astring-literalyield the same or a different object is unspecified. [ _Note:_ The effect of attempting to modify astring-literalis undefined. — end note ]

Add a new paragraph (Z) after5.13.5 [lex.string] paragraph 14:

String literal objects are initialized with the sequence of code unit values corresponding to thestring-literal's sequence ofs-chars (for a non-raw string literal) andr-chars (for a raw string literal) in order as follows:

(Z.1) — The sequence of characters denoted by each contiguous sequence ofbasic-s-chars,r-chars,simple-escape-sequences ([lex.ccon]), anduniversal-character-names ([lex.charset]) is encoded to a code unit sequence using thestring-literal's associated character encoding. If a character lacks representation in the associated character encoding, then: When encoding a stateful character encoding, implementations should encode the first such sequence beginning with the initial encoding state and encode subsequent sequences beginning with the final encoding state of the prior sequence. [ Note: The encoded code unit sequence may differ from the sequence of code units that would be obtained by encoding each character independently. — end note ]
(Z.2) — Eachnumeric-escape-sequence ([lex.ccon]) that specifies an integer value V contributes a single code unit with a value as follows: — If V does not exceed the range of representable values of thestring-literal's array element type, then the value is V. — Otherwise, if thestring-literal'sencoding-prefix is absent or L, and V does not exceed the range of representable values of the corresponding unsigned type for the underlying type of thestring-literal's array element type, then the value is the unique value of thestring-literal's array element type T that is congruent to V modulo 2_N_, where N is the width of T. When encoding a stateful character encoding, these sequences should have no effect on encoding state.
(Z.3) — Eachconditional-escape-sequence ([lex.ccon]) contributes an implementation-defined code unit sequence. When encoding a stateful character encoding, it is implementation-defined what effect these sequences have on encoding state.