Proposed resolution (approved by CWG 2024-01-19):

1. Change in 4.1.1 [intro.compliance.general] bullet 2.3 as follows:

   • ...
   • Otherwise, if a program contains
     • a violation of any diagnosable rule or,
     • a preprocessing translation unit with a #warning preprocessing directive (15.8 [cpp.error]), 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.

2. Change in 4.1.1 [intro.compliance.general] paragraph 2 as follows:

   Furthermore, a conforming implementation shall not accept

   • a preprocessing translation unit containing a #error preprocessing directive (15.8 [cpp.error]), or
   • 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]).

3. 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, as modified by the implementation-defined behavior of any conditionally-supported-directives (15.1 [cpp.pre]) and pragmas (15.9 [cpp.pragma]), if any, is called a preprocessing translation unit.

4. Change in 15.8 [cpp.error] as follows:

   A preprocessing directive of either of the following forms the form

   # error pp-tokensopt new-line
   

   renders the program ill-formed. A preprocessing directive of the form

   # warning pp-tokensopt new-line
   

   causes requires the implementation to produce a at least one diagnostic message for the preprocessing translation unit (4.1.1 [intro.compliance.general]) that.

   Recommended practice: Any diagnostic message caused by either of these directives should include the specified sequence of preprocessing tokens; the #error directive renders the program ill-formed.

CWG 2023-03-03

Permit that #warning can be ignored if another diagnostic is produced.

Suggested resolution [SUPERSEDED]:

1. Change in 4.1.1 [intro.compliance.general] paragraph 2 as follows:

   Furthermore, a conforming implementation

   • shall not accept a preprocessing translation unit containing a #error preprocessing directive (15.8 [cpp.error]), and
   • 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]).

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 or by the implementation-defined behavior of any conditionally-supported-directives (15.1 [cpp.pre]), is called a preprocessing translation unit.

[Accepted as a DR at the March, 2024 meeting.]

The resolution for issue 2518 disallows existing implementation practice, as detailed below:

• The requirement to issue a diagnostic message for each appearance of #warning conflicts with existing implementations having a diagnostic limit. Furthermore, an implementation with an on-the-fly incremental preprocessor is no longer allowed to abort the compilation when an error deemed fatal happens (e.g. a #include file that does not exist).
• The wording does not allow for implementation-specific mechanisms for creating a preprocessing translation unit, such as vendor-specific forms of #if-like directives and #pragma once.