5 Lexical conventions [lex]

5.13 Literals [lex.literal]

5.13.8 User-defined literals [lex.ext]

1
#
If a token matches both user-defined-literal and another literal kind, it is treated as the latter.
[Example 1: 
123_km is a user-defined-literal, but 12LL is an integer-literal.
— end example]
The syntactic non-terminal preceding the ud-suffix in a user-defined-literal is taken to be the longest sequence of characters that could match that non-terminal.
2
#
A user-defined-literal is treated as a call to a literal operator or literal operator template ([over.literal]).
To determine the form of this call for a given user-defined-literal L with ud-suffix X, first let S be the set of declarations found by unqualified lookup for the literal-operator-id whose literal suffix identifier is X ([basic.lookup.unqual]).
S shall not be empty.
3
#
If L is a user-defined-integer-literal, let n be the literal without its ud-suffix.
If S contains a literal operator with parameter type unsigned long long, the literal L is treated as a call of the form operator ""X(nULL)
Otherwise, S shall contain a raw literal operator or a numeric literal operator template ([over.literal]) but not both.
If S contains a raw literal operator, the literal L is treated as a call of the form operator ""X("n")
Otherwise (S contains a numeric literal operator template), L is treated as a call of the form operator ""X<'', '', ... ''>() where n is the source character sequence .
[Note 1: 
The sequence can only contain characters from the basic character set.
— end note]
4
#
If L is a user-defined-floating-point-literal, let f be the literal without its ud-suffix.
If S contains a literal operator with parameter type long double, the literal L is treated as a call of the form operator ""X(fL)
Otherwise, S shall contain a raw literal operator or a numeric literal operator template ([over.literal]) but not both.
If S contains a raw literal operator, the literal L is treated as a call of the form operator ""X("f")
Otherwise (S contains a numeric literal operator template), L is treated as a call of the form operator ""X<'', '', ... ''>() where f is the source character sequence .
[Note 2: 
The sequence can only contain characters from the basic character set.
— end note]
5
#
If L is a user-defined-string-literal, let str be the literal without its ud-suffix and let len be the number of code units in str (i.e., its length excluding the terminating null character).
If S contains a literal operator template with a non-type template parameter for which str is a well-formed template-argument, the literal L is treated as a call of the form operator ""X<str>()
Otherwise, the literal L is treated as a call of the form operator ""X(str, len)
6
#
If L is a user-defined-character-literal, let ch be the literal without its ud-suffix.
S shall contain a literal operator whose only parameter has the type of ch and the literal L is treated as a call of the form operator ""X(ch)
7
#
[Example 2: long double operator ""_w(long double); std::string operator ""_w(const char16_t*, std::size_t); unsigned operator ""_w(const char*); int main() { 1.2_w; // calls operator ""_w(1.2L) u"one"_w; // calls operator ""_w(u"one", 3) 12_w; // calls operator ""_w("12") "two"_w; // error: no applicable literal operator } — end example]
8
#
In translation phase 6 ([lex.phases]), adjacent string-literals are concatenated and user-defined-string-literals are considered string-literals for that purpose.
During concatenation, ud-suffixes are removed and ignored and the concatenation process occurs as described in [lex.string].
At the end of phase 6, if a string-literal is the result of a concatenation involving at least one user-defined-string-literal, all the participating user-defined-string-literals shall have the same ud-suffix and that suffix is applied to the result of the concatenation.
9
#
[Example 3: int main() { L"A" "B" "C"_x; // OK, same as L"ABC"_x "P"_x "Q" "R"_y; // error: two different ud-suffixes } — end example]