Proposed resolution for core issues 411, 1656, and 2333; numeric and universal character escapes in character and string literals

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 prior proposed resolution that only attempted to address CWG 2333. That proposed resolution was discussed on the core mailing list and at the June 24th, 2019, core issues processing teleconference. The resolution proposed in this paper addresses the feedback provided during those discussions and further review at the January 16th, 2020, core issues processing teleconference, the March 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 the October 19th, 2020, core paper review teleconference.

The notes for CWG 2333 in the current active issues list (revision 100) state that discussion at the August 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 its October 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:

SG16 conducted the following poll:
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 of P1236R1 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.

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 https://lists.isocpp.org/core/2020/07/9537.php and tracked by CWG 2455.

Changes from P2029R3

Proposed resolution overview

The proposed wording changes are intended to resolve CWG 411, CWG 1656, and CWG 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-1252 with 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 to N4861.

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 in 5.2 [lex.phases] paragraph 1.5:

Each basic source character set memberbasic-c-char, basic-s-char, and r-char in a character-literal or a string-literal, as well as each escape sequenceescape-sequence and universal-character-name in a character-literal or 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 in 5.2 [lex.phases] paragraph 1.6:
Drafting note: This addition duplicates wording in [lex.string], but seems important to include here.

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

[lex.ccon]

Change in 5.13.3 [lex.ccon]:

character-literal:
encoding-prefixopt ' c-char-sequence '

encoding-prefix: one of
u8uUL

c-char-sequence:
c-char
c-char-sequence c-char

c-char:
any member of the basic source character set except the single-quote ', backslash \, or new-line character
basic-c-char
escape-sequence
universal-character-name

basic-c-char:
any member of the basic source character set except the single-quote ', backslash \, or new-line character

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

octal-escape-sequence:
\ octal-digit
\ octal-digit octal-digit
\ octal-digit octal-digit octal-digit

hexadecimal-escape-sequence:
\x hexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit

conditional-escape-sequence:
\ conditional-escape-sequence-char

conditional-escape-sequence-char:
any member of the basic source character set that is not an octal-digit, a simple-escape-sequence-char, or the characters u, U, or x

Delete 5.13.3 [lex.ccon] paragraph 1:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs.

A character-literal that does not begin with u8, u, U, or L is an ordinary character literal. An ordinary character literal that contains a single c-char representable in the execution character set has type char, with value equal to the numerical value of the encoding of the c-char in the execution character set. An ordinary character literal that contains more than one c-char is a multicharacter literal. A multicharacter literal, or an ordinary character literal containing a single c-char not representable in the execution character set, is conditionally-supported, has type int, and has an implementation-defined value.

Delete 5.13.3 [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.

A character-literal that begins with u8, such as u8'w', is a character-literal of 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 multiple c-chars is ill-formed.

Delete 5.13.3 [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.

A character-literal that begins with the letter u, such as u'x', is a character-literal of 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 multiple c-chars is ill-formed.

Delete 5.13.3 [lex.ccon] paragraph 4:
Drafting Note: The contents of paragraphs 1-5 were incorporated into new paragraphs.

A character-literal that begins with U, such as U'y', is a character-literal of type char32_t, known as a UTF-32 character literal. The value of a UTF-32 character literal containing a single c-char is equal to its ISO/IEC 10646 code point value. A UTF-32 character literal containing multiple c-chars is ill-formed.

Delete 5.13.3 [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.

A character-literal that begins with the letter L, such as L'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 single c-char has value equal to the numerical value of the encoding of the c-char in the execution wide-character set, unless the c-char has 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 multiple c-chars is implementation-defined.

Add a new paragraph (W) before 5.13.3 [lex.ccon] paragraph 6:

A non-encodable character literal is a character-literal whose c-char-sequence consists of a single c-char that is not a numeric-escape-sequence 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 multicharacter literal is a character-literal whose c-char-sequence consists of more than one c-char. The encoding-prefix of a non-encodable character literal or a multicharacter literal shall be absent or L. Such character-literals are conditionally-supported.

Add another new paragraph (X) and table (Y) before 5.13.3 [lex.ccon] paragraph 6:

The kind of a character-literal, its type, and its associated character encoding are determined by its encoding-prefix and its c-char-sequence as 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]

Encoding
prefix
Kind Type Associated
character 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) before 5.13.3 [lex.ccon] paragraph 6:

In translation phase 4, the value of a character-literal is determined using the range of representable values of the character-literal's type in translation phase 7. A non-encodable character literal or a multicharacter literal has an implementation-defined value. The value of any other kind of character-literal is determined as follows:
(Z.1) — A character-literal with a c-char-sequence consisting of a single basic-c-char, simple-escape-sequence, or universal-character-name is the code unit value of the specified character as encoded in the literal's associated character encoding. [ Note: If the specified character lacks representation in the literal's associated character encoding or if it cannot be encoded as a single code unit, then the literal is a non-encodable character literal. — end note ]
(Z.2) — A character-literal with a c-char-sequence consisting of a single numeric-escape-sequence that specifies an integer value V has a value as follows:
— If V does not exceed the range of representable values of the character-literal's type, then the value is V.
— Otherwise, if the character-literal's encoding-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 the character-literal's type, then the value is the unique value of the character-literal's type T that is congruent to V modulo 2N, where N is the width of T.
— Otherwise, the character-literal is ill-formed.
(Z.3) — A character-literal with a c-char-sequence consisting of a single conditional-escape-sequence is conditionally-supported and has an implementation-defined value.

Change in 5.13.3 [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 a simple-escape-sequence is specified in table 9. [ 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 Table 9. 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 9 are 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 returnCR \r
form feed FF \f
alert BEL \a
backslash \ \\
question mark ? \?
single quote ' \'
double quote " \"
octal number ooo \ooo
hex number hhh \xhhh

Delete 5.13.3 [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 x followed 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 a character-literal is implementation-defined if it falls outside of the implementation-defined range defined for char (for character-literals with no prefix) or wchar_t (for character-literals prefixed by L). [ Note: If the value of a character-literal prefixed by u, u8, or U is outside the range defined for its type, the program is ill-formed. — end note ]

Delete 5.13.3 [lex.ccon] paragraph 8:
Drafting Note: The normative text was combined with wording for basic-c-char and simple-escape-sequence above. The deleted note duplicates normative text in 5.2 [lex.phases] paragraph 1.1.

A universal-character-name is translated to the encoding, in the appropriate execution character set, of the character named. If there is no such encoding, the universal-character-name is translated to an implementation-defined encoding. [ Note: In translation phase 1, a universal-character-name is introduced whenever an actual extended character is encountered in the source text. Therefore, all extended characters are described in terms of universal-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 in 5.13.5 [lex.string]:

string-literal:
encoding-prefixopt " s-char-sequenceopt "
encoding-prefixopt R raw-string

s-char-sequence:
s-char
s-char-sequence s-char

s-char:
any member of the basic source character set except the double-quote ", backslash \, or new-line character
basic-s-char
escape-sequence
universal-character-name

basic-s-char:
any member of the basic source character set except the double-quote ", backslash \, or new-line character

raw-string:
" d-char-sequenceopt ( r-char-sequenceopt ) d-char-sequenceopt "

r-char-sequence:
r-char
r-char-sequence r-char

r-char:
any member of the source character set, except a right parenthesis ) followed by
the initial d-char-sequence (which may be empty) followed by a double quote ".

d-char-sequence:
d-char
d-char-sequence d-char

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) before 5.13.5 [lex.string] paragraph 1:

The kind of a string-literal, its type, and its associated character encoding are determined by its encoding prefix and sequence of s-chars or r-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]

Encoding
prefix
Kind Type Associated
character 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 to 5.13.5 [lex.string] paragraph 1:

A string-literal that has an R in the prefix is a raw string literal. The d-char-sequence serves as a delimiter. The terminating d-char-sequence of a raw-string is the same sequence of characters as the initial d-char-sequence. A d-char-sequence shall consist of at most 16 characters.

No changes to 5.13.5 [lex.string] paragraph 2:

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

No changes to 5.13.5 [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 to 5.13.5 [lex.string] paragraph 4:

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

Delete 5.13.5 [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, a string-literal that does not begin with an encoding-prefix is 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.

Delete 5.13.5 [lex.string] paragraph 6:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

A string-literal that 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 to 5.13.5 [lex.string] paragraph 7:

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

Delete 5.13.5 [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.

A string-literal that 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 single c-char may 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 ]

Delete 5.13.5 [lex.string] paragraph 9:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

A string-literal that 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.

Delete 5.13.5 [lex.string] paragraph 10:
Drafting Note: The contents of paragraphs 5, 6, 8, 9, and 10 were incorporated into new paragraphs.

A string-literal that 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 in 5.13.5 [lex.string] paragraph 11:

In translation phase 6 ([lex.phases]), adjacent string-literals are concatenated. If both string-literals have the same encoding-prefix, the resulting concatenated string-literal has that encoding-prefix. If one string-literal has no encoding-prefix, it is treated as a string-literal of the same encoding-prefix as 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 a string-literal has been translated into a value from the appropriate character setafter the string literal contents have been encoded in the string-literal's associated character encoding), a string-literal's initial rawness has no effect on the interpretation or well-formedness of the concatenation. — end note ] Table 11 has 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:
"\xA" "B"
contains the two characters '\xA' and 'B' after concatenation (and not the single hexadecimal character '\xAB'). — end example ]

Change in 5.13.5 [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, iIn translation phase 76 ([lex.phases]), after adjacent string-literals are concatenated, '\0'a null character is appended to the resultevery string-literal so that programs that scan a string can find its end.

Delete 5.13.5 [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 and universal-character-names in non-raw string literals have the same meaning as in character 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 a universal-character-name in a UTF-16 string literal may yield a surrogate pair. In a narrow string literal, a universal-character-name may map to more than one char or char8_t element due to multibyte 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' or L'\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 a char16_t string literal is the number of code units, not the number of characters. — end note ] [ Note: Any universal-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 each universal-character-name, plus one for the terminating '\0'.

Change in 5.13.5 [lex.string] paragraph 14:
Drafting note: Wording for string literal object initialization has been moved to a new paragraph.

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

Add a new paragraph (Z) after 5.13.5 [lex.string] paragraph 14:

String literal objects are initialized with the sequence of code unit values corresponding to the string-literal's sequence of s-chars (for a non-raw string literal) and r-chars (for a raw string literal) in order as follows:
(Z.1) — The sequence of characters denoted by each contiguous sequence of basic-s-chars, r-chars, simple-escape-sequences ([lex.ccon]), and universal-character-names ([lex.charset]) is encoded to a code unit sequence using the string-literal's associated character encoding. If a character lacks representation in the associated character encoding, then:
— If the string-literal's encoding-prefix is absent or L, then the string-literal is conditionally-supported and an implementation defined code unit sequence is encoded.
— Otherwise, the string-literal is ill-formed.
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) — Each numeric-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 the string-literal's array element type, then the value is V.
— Otherwise, if the string-literal's encoding-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 the string-literal's array element type, then the value is the unique value of the string-literal's array element type T that is congruent to V modulo 2N, where N is the width of T.
— Otherwise, the string-literal is ill-formed.
When encoding a stateful character encoding, these sequences should have no effect on encoding state.
(Z.3) — Each conditional-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.