Proposed resolution (approved by CWG 2023-02-10):

1. Change and add a new paragraph after 4.1.1 [intro.compliance.general] paragraph 2 as follows:

   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 Clause Clause 5 [lex] through Clause Clause 33 [thread] and Annex D, a conforming implementation shall, within its resource limits as described in Annex B, accept and correctly execute [ Footnote: ... ] 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 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: 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 (15.8 [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 (9.1 [dcl.pre]).

   For classes and class templates, ...

2. Change in 5.1 [lex.separate] paragraph 1 as follows:

   The text of the program is kept in units called source files in this document. A source file together with all the headers (16.4.2.3 [headers]) and source files included (15.3 [cpp.include]) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (15.2 [cpp.cond]) preprocessing directives, is called a preprocessing translation unit.

3. Change in 5.2 [lex.phases] paragraph 7 as follows:

   ... The resulting tokens constitute a translation unit and are syntactically and semantically analyzed and translated as a translation unit.

4. Change in 9.1 [dcl.pre] paragraph 10 as follows:

   In a static_assert-declaration, the constant-expression is contextually converted to bool and the converted expression shall be a constant expression (7.7 [expr.const]). If the value of the expression when so converted is true or the expression is evaluated in the context of a template definition, the declaration has no effect. Otherwise, the static_assert-declaration fails, the program is ill-formed, and the resulting diagnostic message (4.1 [intro.compliance]) should include the text of the string-literal, if one is supplied. [ Example:

     static_assert(sizeof(int) == sizeof(void*), "wrong pointer size");
     static_assert(sizeof(int[2]));   // OK, narrowing allowed
   
     template <class T>
     void f(T t) {
       if constexpr (sizeof(T) == sizeof(int)) {
         use(t);
       } else {
         static_assert(false, "must be int-sized");
       }
     }
   
     void g(char c) {
       f(0); // OK
       f(c); // error: must be int-sized
     }
     
   

   -- end example ]

5. Change in 13.8 [temp.res] paragraph 6 as follows:

   ... The program is ill-formed, no diagnostic required, if:

   • no valid specialization, ignoring static_assert-declarations that fail, can be generated for a template or a substatement of a constexpr if statement (8.5.2 [stmt.if]) within a template and the template is not instantiated, or
   • ...

CWG 2023-02-10

CWG decided to fold P2593R1 (static_assert(false)) into the proposed resolution.

Proposed resolution (February, 2023) [SUPERSEDED]:

1. Change and add a new paragraph after 4.1.1 [intro.compliance.general] paragraph 2 as follows:

   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 Clause Clause 5 [lex] through Clause Clause 33 [thread] and Annex D, a conforming implementation shall, within its resource limits as described in Annex B, accept and correctly execute [ Footnote: ... ] 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 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: 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 (15.8 [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 failed static_assert-declaration (9.1 [dcl.pre]) outside an uninstantiated template.

   For classes and class templates, ...

2. Change in 5.1 [lex.separate] paragraph 1 as follows:

   The text of the program is kept in units called source files in this document. A source file together with all the headers (16.4.2.3 [headers]) and source files included (15.3 [cpp.include]) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (15.2 [cpp.cond]) preprocessing directives, is called a preprocessing translation unit.

3. Change in 5.2 [lex.phases] paragraph 7 as follows:

   ... The resulting tokens constitute a translation unit and are syntactically and semantically analyzed and translated as a translation unit.

4. Change in 9.1 [dcl.pre] paragraph 10 as follows:

   In a static_assert-declaration, the constant-expression is contextually converted to bool and the converted expression shall be a constant expression (7.7 [expr.const]). If the value of the expression when so converted is true, the declaration has no effect. Otherwise, the static_assert-declaration has failed, the program is ill-formed, and the resulting diagnostic message (4.1 [intro.compliance]) should include the text of the string-literal, if one is supplied.

Notes from the November, 2022 meeting

EWG review solicited via cplusplus/papers#1366.

EWG 2023-02-06

EWG agreed with the suggested resolution, but resolved to amend it to treat static_assert similarly.

Suggested resolution [SUPERSEDED]:

Add a new paragraph after 4.1.1 [intro.compliance.general] paragraph 2 as follows:

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 Clause Clause 5 [lex] through Clause Clause 33 [thread] and Annex D, a conforming implementation shall, within its resource limits as described in Annex B, accept and correctly execute [ Footnote: ... ] 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: ... — end note]

Furthermore, a conforming implementation

• shall not accept a translation unit containing a #error preprocessing directive (15.8 [cpp.error]) unless it is part of a group skipped by conditional inclusion (15.2 [cpp.cond]) and
• shall issue at least one diagnostic message for each #warning or #error preprocessing directive not following a #error preprocessing directive, ignoring #warning and #error preprocessing directives appearing in groups skipped by conditional inclusion.

For classes and class templates, ...

[Accepted as a DR at the February, 2023 meeting.]

According to 4.1.1 [intro.compliance.general] bullet 2.2,

a conforming implementation shall issue at least one diagnostic message

for an ill-formed program that is not “no diagnostic required.” The relationship between this diagnostic message and the ones required by the #error and #warning preprocessor directives is not clear. For example, is an implementation required to emit multiple diagnostics if multiple #error directives are encountered, even though conformance requires only one? Could an implementation count the diagnostic emitted by #warning as satisfying the requirement for an ill-formed program?

See also issue 745.