tmp/tmpz0x3f5ah/{from.md → to.md}
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| 1 |
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### [[lex]]: lexical conventions <a id="diff.cpp17.lex">[[diff.cpp17.lex]]</a>
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**Change:** New identifiers with special meaning. **Rationale:**
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Required for new features. **Effect on original feature:** Logical lines
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beginning with `module` or `import` may be interpreted differently in
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this International Standard.
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[*Example 1*:
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``` cpp
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class module {};
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module m1; // was variable declaration; now module-declaration
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module *m2; // variable declaration
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class import {};
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import j1; // was variable declaration; now import-declaration
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::import j2; // variable declaration
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```
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— *end example*]
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**Change:** *header-name* tokens are formed in more contexts.
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**Rationale:** Required for new features. **Effect on original
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feature:** When the identifier `import` is followed by a `<` character,
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a *header-name* token may be formed.
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[*Example 2*:
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``` cpp
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template<typename> class import {};
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import<int> f(); // ill-formed; previously well-formed
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::import<int> g(); // OK
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```
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— *end example*]
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**Change:** New keywords. **Rationale:** Required for new features.
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- The `char8_t` keyword is added to differentiate the types of ordinary
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and UTF-8 literals [[lex.string]].
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- The `concept` keyword is added to enable the definition of concepts
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[[temp.concept]].
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- The `consteval` keyword is added to declare immediate functions
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[[dcl.constexpr]].
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- The `constinit` keyword is added to prevent unintended dynamic
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initialization [[dcl.constinit]].
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- The `co_await`, `co_yield`, and `co_return` keywords are added to
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enable the definition of coroutines [[dcl.fct.def.coroutine]].
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- The `requires` keyword is added to introduce constraints through a
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*requires-clause* [[temp.pre]] or a *requires-expression*
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[[expr.prim.req]].
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Valid C++17 code using `char8_t`, `concept`, `consteval`, `constinit`,
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`co_await`, `co_yield`, `co_return`, or `requires` as an identifier is
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not valid in this International Standard.
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**Change:** New operator `<=>`. **Rationale:** Necessary for new
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functionality. **Effect on original feature:** Valid C++17 code that
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contains a `<=` token immediately followed by a `>` token may be
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ill-formed or have different semantics in this International Standard:
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``` cpp
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namespace N {
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struct X {};
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bool operator<=(X, X);
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template<bool(X, X)> struct Y {};
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Y<operator<=> y; // ill-formed; previously well-formed
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}
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```
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**Change:** Type of UTF-8 string and character literals. **Rationale:**
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Required for new features. The changed types enable function
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overloading, template specialization, and type deduction to distinguish
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ordinary and UTF-8 string and character literals. **Effect on original
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feature:** Valid C++17 code that depends on UTF-8 string literals having
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type “array of `const char`” and UTF-8 character literals having type
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“`char`” is not valid in this International Standard.
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``` cpp
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| 80 |
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const auto *u8s = u8"text"; // u8s previously deduced as const char*; now deduced as const char8_t*
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| 81 |
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const char *ps = u8s; // ill-formed; previously well-formed
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auto u8c = u8'c'; // u8c previously deduced as char; now deduced as char8_t
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char *pc = &u8c; // ill-formed; previously well-formed
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| 86 |
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std::string s = u8"text"; // ill-formed; previously well-formed
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void f(const char *s);
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| 89 |
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f(u8"text"); // ill-formed; previously well-formed
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| 90 |
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| 91 |
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template<typename> struct ct;
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| 92 |
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template<> struct ct<char> {
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| 93 |
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using type = char;
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};
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| 95 |
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ct<decltype(u8'c')>::type x; // ill-formed; previously well-formed.
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| 96 |
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```
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