[intro] - C++20 → C++23

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 # General principles <a id="intro">[[intro]]</a>
 ## Implementation compliance <a id="intro.compliance">[[intro.compliance]]</a>
 The set of *diagnosable rules* consists of all syntactic and semantic
 rules in this document except for those rules containing an explicit
 notation that “no diagnostic is required” or which are described as
 resulting in “undefined behavior”.
 Although this document states only requirements on C++ implementations,
 those requirements are often easier to understand if they are phrased as
 requirements on programs, parts of programs, or execution of programs.
 Such requirements have the following meaning:
-- If a program contains no violations of the rules in this document, a
-  conforming implementation shall, within its resource limits, accept
- and correctly execute[^2] that program.
-- If a program contains a violation of any diagnosable rule or an
-  occurrence of a construct described in this document as
-  “conditionally-supported” when the implementation does not support
-  that construct, a conforming implementation shall issue at least one
-  diagnostic message.
 - If a program contains a violation of a rule for which no diagnostic is
   required, this document places no requirement on implementations with
   respect to that program.
 [*Note 1*: During template argument deduction and substitution, certain
 constructs that in other contexts require a diagnostic are treated
 differently; see  [[temp.deduct]]. — *end note*]
 For classes and class templates, the library Clauses specify partial
 definitions. Private members [[class.access]] are not specified, but
 each implementation shall supply them to complete the definitions
 according to the description in the library Clauses.
 For functions, function templates, objects, and values, the library
 Clauses specify declarations. Implementations shall supply definitions
 consistent with the descriptions in the library Clauses.
-The names defined in the library have namespace scope
-[[basic.namespace]]. A C++ translation unit [[lex.phases]] obtains
-access to these names by including the appropriate standard library
 header or importing the appropriate standard library named header unit
 [[using.headers]].
 The templates, classes, functions, and objects in the library have
 external linkage [[basic.link]]. The implementation provides definitions
 for standard library entities, as necessary, while combining translation
 units to form a complete C++ program [[lex.phases]].
 Two kinds of implementations are defined: a *hosted implementation* and
-a *freestanding implementation*. For a hosted implementation, this
-document defines the set of available libraries. A freestanding
-implementation is one in which execution may take place without the
-benefit of an operating system, and has an *implementation-defined* set
-of libraries that includes certain language-support libraries
-[[compliance]].
 A conforming implementation may have extensions (including additional
 library functions), provided they do not alter the behavior of any
 well-formed program. Implementations are required to diagnose programs
 that use such extensions that are ill-formed according to this document.
 Having done so, however, they can compile and execute such programs.
 Each implementation shall include documentation that identifies all
 conditionally-supported constructs that it does not support and defines
-all locale-specific characteristics.[^3]
 ### Abstract machine <a id="intro.abstract">[[intro.abstract]]</a>
 The semantic descriptions in this document define a parameterized
 nondeterministic abstract machine. This document places no requirement
 on the structure of conforming implementations. In particular, they need
 not copy or emulate the structure of the abstract machine. Rather,
 conforming implementations are required to emulate (only) the observable
-behavior of the abstract machine as explained below.[^4]
 Certain aspects and operations of the abstract machine are described in
 this document as implementation-defined (for example, `sizeof(int)`).
 These constitute the parameters of the abstract machine. Each
 implementation shall include documentation describing its
-characteristics and behavior in these respects.[^5] Such documentation
-shall define the instance of the abstract machine that corresponds to
-that implementation (referred to as the “corresponding instance” below).
 Certain other aspects and operations of the abstract machine are
 described in this document as unspecified (for example, order of
 evaluation of arguments in a function call [[expr.call]]). Where
 possible, this document defines a set of allowable behaviors. These
@@ -119,11 +133,11 @@ The least requirements on a conforming implementation are:
 These collectively are referred to as the *observable behavior* of the
 program.
 [*Note 2*: More stringent correspondences between abstract and actual
-semantics may be defined by each implementation. — *end note*]
 ## Structure of this document <a id="intro.structure">[[intro.structure]]</a>
 [[lex]] through [[cpp]] describe the C++ programming language. That
 description includes detailed syntactic specifications in a form
@@ -141,14 +155,10 @@ implementations.
 [[diff]] summarizes the evolution of C++ since its first published
 description, and explains in detail the differences between C++ and C.
 Certain features of C++ exist solely for compatibility purposes;
 [[depr]] describes those features.
-Throughout this document, each example is introduced by “” and
-terminated by “”. Each note is introduced by “” or “” and terminated by
-“”. Examples and notes may be nested.
 ## Syntax notation <a id="syntax">[[syntax]]</a>
 In the syntax notation used in this document, syntactic categories are
 indicated by *italic* type, and literal words and characters in
 `constant` `width` type. Alternatives are listed on separate lines
@@ -174,103 +184,82 @@ the following rules:
 - *X-seq* is one or more *X*’s without intervening delimiters (e.g.,
   *declaration-seq* is a sequence of declarations).
 - *X-list* is one or more *X*’s separated by intervening commas (e.g.,
   *identifier-list* is a sequence of identifiers separated by commas).
-## Acknowledgments <a id="intro.ack">[[intro.ack]]</a>
-The C++ programming language as described in this document is based on
-the language as described in Chapter R (Reference Manual) of Stroustrup:
-*The C++ Programming Language* (second edition, Addison-Wesley
-Publishing Company, ISBN 0-201-53992-6, copyright ©1991 AT&T). That, in
-turn, is based on the C programming language as described in Appendix A
-of Kernighan and Ritchie: *The C Programming Language* (Prentice-Hall,
-1978, ISBN 0-13-110163-3, copyright ©1978 AT&T).
-Portions of the library Clauses of this document are based on work by
-P.J. Plauger, which was published as *The Draft Standard C++ Library*
-(Prentice-Hall, ISBN 0-13-117003-1, copyright ©1995 P.J. Plauger).
-POSIX® is a registered trademark of the Institute of Electrical and
-Electronic Engineers, Inc.
-ECMAScript® is a registered trademark of Ecma International.
-Unicode® is a registered trademark of Unicode, Inc.
-All rights in these originals are reserved.
 <!-- Link reference definitions -->
-[basic.def.odr]: basic.md#basic.def.odr
 [basic.link]: basic.md#basic.link
-[basic.namespace]: dcl.md#basic.namespace
-[basic.types]: basic.md#basic.types
 [class.access]: class.md#class.access
 [class.copy.assign]: class.md#class.copy.assign
 [class.ctor]: class.md#class.ctor
 [class.derived]: class.md#class.derived
 [compliance]: library.md#compliance
 [conv.lval]: expr.md#conv.lval
 [cpp]: cpp.md#cpp
-[cpp.replace]: cpp.md#cpp.replace
-[dcl.decl]: dcl.md#dcl.decl
-[dcl.fct]: dcl.md#dcl.fct
 [dcl.ptr]: dcl.md#dcl.ptr
 [dcl.ref]: dcl.md#dcl.ref
-[definitions]: library.md#definitions
 [defns.well.formed]: #defns.well.formed
 [depr]: future.md#depr
-[depr.locale.stdcvt]: future.md#depr.locale.stdcvt
 [diff]: compatibility.md#diff
 [diff.library]: compatibility.md#diff.library
 [expr.ass]: expr.md#expr.ass
 [expr.call]: expr.md#expr.call
 [expr.const]: expr.md#expr.const
 [expr.post.incr]: expr.md#expr.post.incr
 [expr.pre.incr]: expr.md#expr.pre.incr
-[expr.throw]: expr.md#expr.throw
 [gram]: grammar.md#gram
 [implimits]: limits.md#implimits
 [intro]: #intro
 [intro.abstract]: #intro.abstract
-[intro.ack]: #intro.ack
 [intro.compliance]: #intro.compliance
 [intro.defs]: #intro.defs
 [intro.execution]: basic.md#intro.execution
-[intro.object]: basic.md#intro.object
 [intro.refs]: #intro.refs
 [intro.scope]: #intro.scope
 [intro.structure]: #intro.structure
 [lex]: lex.md#lex
-[lex.charset]: lex.md#lex.charset
 [lex.phases]: lex.md#lex.phases
 [library]: library.md#library
-[stmt.block]: stmt.md#stmt.block
 [support]: support.md#support
 [syntax]: #syntax
-[temp.arg]: temp.md#temp.arg
 [temp.deduct]: temp.md#temp.deduct
 [thread]: thread.md#thread
 [using.headers]: library.md#using.headers
-[^1]: With the qualifications noted in [[support]] through [[thread]]
     and in [[diff.library]], the C standard library is a subset of the
     C++ standard library.
-[^2]: “Correct execution” can include undefined behavior, depending on
     the data being processed; see [[intro.defs]] and
     [[intro.execution]].
-[^3]: This documentation also defines implementation-defined behavior;
     see  [[intro.abstract]].
-[^4]: This provision is sometimes called the “as-if” rule, because an
     implementation is free to disregard any requirement of this document
     as long as the result is *as if* the requirement had been obeyed, as
     far as can be determined from the observable behavior of the
     program. For instance, an actual implementation need not evaluate
     part of an expression if it can deduce that its value is not used
     and that no side effects affecting the observable behavior of the
     program are produced.
-[^5]: This documentation also includes conditionally-supported
-    constructs and locale-specific behavior. See  [[intro.compliance]].

 # General principles <a id="intro">[[intro]]</a>
 ## Implementation compliance <a id="intro.compliance">[[intro.compliance]]</a>
+### General <a id="intro.compliance.general">[[intro.compliance.general]]</a>
 The set of *diagnosable rules* consists of all syntactic and semantic
 rules in this document except for those rules containing an explicit
 notation that “no diagnostic is required” or which are described as
 resulting in “undefined behavior”.
 Although this document states only requirements on C++ implementations,
 those requirements are often easier to understand if they are phrased as
 requirements on programs, parts of programs, or execution of programs.
 Such requirements have the following meaning:
+- If a program contains no violations of the rules in [[lex]] through
+ [[thread]] and [[depr]], a conforming implementation shall, within its
+ resource limits as described in [[implimits]], accept and correctly
+  execute[^3] that program.
 - If a program contains a violation of a rule for which no diagnostic is
   required, this document places no requirement on implementations with
   respect to that program.
+- Otherwise, if a program contains a violation of any diagnosable rule
+  or an occurrence of a construct described in this document as
+  “conditionally-supported” when the implementation does not support
+  that construct, a conforming implementation shall issue at least one
+  diagnostic message.
 [*Note 1*: During template argument deduction and substitution, certain
 constructs that in other contexts require a diagnostic are treated
 differently; see  [[temp.deduct]]. — *end note*]
+Furthermore, a conforming implementation
+- shall not accept a preprocessing translation unit containing a
+  `#error` preprocessing directive [[cpp.error]],
+- shall issue at least one diagnostic message for each `#warning` or
+  `#error` preprocessing directive not following a `#error`
+  preprocessing directive in a preprocessing translation unit, and
+- shall not accept a translation unit with a *static_assert-declaration*
+  that fails [[dcl.pre]].
 For classes and class templates, the library Clauses specify partial
 definitions. Private members [[class.access]] are not specified, but
 each implementation shall supply them to complete the definitions
 according to the description in the library Clauses.
 For functions, function templates, objects, and values, the library
 Clauses specify declarations. Implementations shall supply definitions
 consistent with the descriptions in the library Clauses.
+A C++ translation unit [[lex.phases]] obtains access to the names
+defined in the library by including the appropriate standard library
 header or importing the appropriate standard library named header unit
 [[using.headers]].
 The templates, classes, functions, and objects in the library have
 external linkage [[basic.link]]. The implementation provides definitions
 for standard library entities, as necessary, while combining translation
 units to form a complete C++ program [[lex.phases]].
 Two kinds of implementations are defined: a *hosted implementation* and
+a *freestanding implementation*. A freestanding implementation is one in
+which execution may take place without the benefit of an operating
+system. A hosted implementation supports all the facilities described in
+this document, while a freestanding implementation supports the entire
+C++ language described in [[lex]] through [[cpp]] and the subset of the
+library facilities described in [[compliance]].
 A conforming implementation may have extensions (including additional
 library functions), provided they do not alter the behavior of any
 well-formed program. Implementations are required to diagnose programs
 that use such extensions that are ill-formed according to this document.
 Having done so, however, they can compile and execute such programs.
 Each implementation shall include documentation that identifies all
 conditionally-supported constructs that it does not support and defines
+all locale-specific characteristics.[^4]
 ### Abstract machine <a id="intro.abstract">[[intro.abstract]]</a>
 The semantic descriptions in this document define a parameterized
 nondeterministic abstract machine. This document places no requirement
 on the structure of conforming implementations. In particular, they need
 not copy or emulate the structure of the abstract machine. Rather,
 conforming implementations are required to emulate (only) the observable
+behavior of the abstract machine as explained below.[^5]
 Certain aspects and operations of the abstract machine are described in
 this document as implementation-defined (for example, `sizeof(int)`).
 These constitute the parameters of the abstract machine. Each
 implementation shall include documentation describing its
+characteristics and behavior in these respects.[^6]
+Such documentation shall define the instance of the abstract machine
+that corresponds to that implementation (referred to as the
+“corresponding instance” below).
 Certain other aspects and operations of the abstract machine are
 described in this document as unspecified (for example, order of
 evaluation of arguments in a function call [[expr.call]]). Where
 possible, this document defines a set of allowable behaviors. These
 These collectively are referred to as the *observable behavior* of the
 program.
 [*Note 2*: More stringent correspondences between abstract and actual
+semantics can be defined by each implementation. — *end note*]
 ## Structure of this document <a id="intro.structure">[[intro.structure]]</a>
 [[lex]] through [[cpp]] describe the C++ programming language. That
 description includes detailed syntactic specifications in a form
 [[diff]] summarizes the evolution of C++ since its first published
 description, and explains in detail the differences between C++ and C.
 Certain features of C++ exist solely for compatibility purposes;
 [[depr]] describes those features.
 ## Syntax notation <a id="syntax">[[syntax]]</a>
 In the syntax notation used in this document, syntactic categories are
 indicated by *italic* type, and literal words and characters in
 `constant` `width` type. Alternatives are listed on separate lines
 - *X-seq* is one or more *X*’s without intervening delimiters (e.g.,
   *declaration-seq* is a sequence of declarations).
 - *X-list* is one or more *X*’s separated by intervening commas (e.g.,
   *identifier-list* is a sequence of identifiers separated by commas).
 <!-- Link reference definitions -->
+[algorithm.stable]: library.md#algorithm.stable
+[basic.fundamental]: basic.md#basic.fundamental
 [basic.link]: basic.md#basic.link
 [class.access]: class.md#class.access
 [class.copy.assign]: class.md#class.copy.assign
 [class.ctor]: class.md#class.ctor
 [class.derived]: class.md#class.derived
 [compliance]: library.md#compliance
 [conv.lval]: expr.md#conv.lval
 [cpp]: cpp.md#cpp
+[cpp.error]: cpp.md#cpp.error
+[dcl.pre]: dcl.md#dcl.pre
 [dcl.ptr]: dcl.md#dcl.ptr
 [dcl.ref]: dcl.md#dcl.ref
+[defns.repositional.stream]: #defns.repositional.stream
 [defns.well.formed]: #defns.well.formed
 [depr]: future.md#depr
 [diff]: compatibility.md#diff
 [diff.library]: compatibility.md#diff.library
 [expr.ass]: expr.md#expr.ass
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 [expr.post.incr]: expr.md#expr.post.incr
 [expr.pre.incr]: expr.md#expr.pre.incr
 [gram]: grammar.md#gram
 [implimits]: limits.md#implimits
+[input.output]: input.md#input.output
 [intro]: #intro
 [intro.abstract]: #intro.abstract
 [intro.compliance]: #intro.compliance
+[intro.compliance.general]: #intro.compliance.general
 [intro.defs]: #intro.defs
 [intro.execution]: basic.md#intro.execution
 [intro.refs]: #intro.refs
 [intro.scope]: #intro.scope
 [intro.structure]: #intro.structure
 [lex]: lex.md#lex
 [lex.phases]: lex.md#lex.phases
 [library]: library.md#library
+[localization]: localization.md#localization
+[re]: re.md#re
+[strings]: strings.md#strings
 [support]: support.md#support
 [syntax]: #syntax
 [temp.deduct]: temp.md#temp.deduct
 [thread]: thread.md#thread
 [using.headers]: library.md#using.headers
+[^1]: POSIX is a registered trademark of the Institute of Electrical and
+    Electronic Engineers, Inc. This information is given for the
+    convenience of users of this document and does not constitute an
+    endorsement by ISO or IEC of this product.
+[^2]: With the qualifications noted in [[support]] through [[thread]]
     and in [[diff.library]], the C standard library is a subset of the
     C++ standard library.
+[^3]: “Correct execution” can include undefined behavior, depending on
     the data being processed; see [[intro.defs]] and
     [[intro.execution]].
+[^4]: This documentation also defines implementation-defined behavior;
     see  [[intro.abstract]].
+[^5]: This provision is sometimes called the “as-if” rule, because an
     implementation is free to disregard any requirement of this document
     as long as the result is *as if* the requirement had been obeyed, as
     far as can be determined from the observable behavior of the
     program. For instance, an actual implementation need not evaluate
     part of an expression if it can deduce that its value is not used
     and that no side effects affecting the observable behavior of the
     program are produced.
+[^6]: This documentation also includes conditionally-supported
+    constructs and locale-specific behavior. See
+    [[intro.compliance.general]].

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