Annex C (informative) Compatibility [diff]

C.1 C++ and ISO C++ 2017 [diff.cpp17]

C.1.2 [lex]: lexical conventions [diff.cpp17.lex]

Affected subclauses: [lex.pptoken], [module.unit], [module.import], [cpp.pre], [cpp.module], and [cpp.import]

Change: New identifiers with special meaning.

Rationale: Required for new features.

Effect on original feature: Logical lines beginning with module or import may be interpreted differently in this revision of C++.

[Example 1: class module {}; module m1; // was variable declaration; now module-declaration module *m2; // variable declaration class import {}; import j1; // was variable declaration; now module-import-declaration ::import j2; // variable declaration — end example]

Affected subclause: [lex.header]

Change: header-name tokens are formed in more contexts.

Rationale: Required for new features.

Effect on original feature: When the identifier import is followed by a < character, a header-name token may be formed.

[Example 2: template<typename> class import {}; import<int> f(); // ill-formed; previously well-formed ::import<int> g(); // OK — end example]

Affected subclause: [lex.key]

Change: New keywords.

Rationale: Required for new features.

(3.1)
The char8_t keyword is added to differentiate the types of ordinary and UTF-8 literals ([lex.string]).
(3.2)
The concept keyword is added to enable the definition of concepts ([temp.concept]).
(3.3)
The consteval keyword is added to declare immediate functions ([dcl.constexpr]).
(3.4)
The constinit keyword is added to prevent unintended dynamic initialization ([dcl.constinit]).
(3.5)
The co_await, co_yield, and co_return keywords are added to enable the definition of coroutines ([dcl.fct.def.coroutine]).
(3.6)
The requires keyword is added to introduce constraints through a requires-clause ([temp.pre]) or a requires-expression ([expr.prim.req]).

Effect on original feature: Valid C++ 2017 code using char8_t, concept, consteval, constinit, co_await, co_yield, co_return, or requires as an identifier is not valid in this revision of C++.

Affected subclause: [lex.operators]

Change: New operator <=>.

Rationale: Necessary for new functionality.

Effect on original feature: Valid C++ 2017 code that contains a <= token immediately followed by a > token may be ill-formed or have different semantics in this revision of C++: namespace N { struct X {}; bool operator<=(X, X); template<bool(X, X)> struct Y {}; Y<operator<=> y; // ill-formed; previously well-formed }

Affected subclause: [lex.literal]

Change: Type of UTF-8 string and character literals.

Rationale: Required for new features.

The changed types enable function overloading, template specialization, and type deduction to distinguish ordinary and UTF-8 string and character literals.

Effect on original feature: Valid C++ 2017 code that depends on UTF-8 string literals having type “array of const char” and UTF-8 character literals having type “char” is not valid in this revision of C++.

const auto *u8s = u8"text"; // u8s previously deduced as const char*; now deduced as const char8_t* const char *ps = u8s; // ill-formed; previously well-formed auto u8c = u8'c'; // u8c previously deduced as char; now deduced as char8_t char *pc = &u8c; // ill-formed; previously well-formed std::string s = u8"text"; // ill-formed; previously well-formed void f(const char *s); f(u8"text"); // ill-formed; previously well-formed template<typename> struct ct; template<> struct ct<char> { using type = char; }; ct<decltype(u8'c')>::type x; // ill-formed; previously well-formed.