Proposed resolution (approved by CWG 2024-03-20):

Change in 5.10 [lex.name] paragraph 3 and add bullets as follows:

In addition, some identifiers appearing as a token or preprocessing-token are reserved for use by C++ implementations and shall not be used otherwise; no diagnostic is required.

• Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter, other than those specified in this document (for example, __cplusplus (15.11 [cpp.predefined])), is reserved to the implementation for any use.
• Each identifier that begins with an underscore is reserved to the implementation for use as a name in the global namespace.

CWG 2024-03-20

The exceptions should not be specified using a specific list, which might become stale in the future.

Proposed resolution (approved by CWG 2024-01-19) [SUPERSEDED]:

Change in 5.10 [lex.name] paragraph 3 and add bullets as follows:

In addition, some identifiers appearing as a token or preprocessing-token are reserved for use by C++ implementations and shall not be used otherwise; no diagnostic is required.

• Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use, except for
  • the macros specified in 15.11 [cpp.predefined],
  • the function-local predefined variable (9.5.1 [dcl.fct.def.general]), and
  • the macros and identifiers declared in the standard library as specified in Clause 17 [support] through Clause 33 [thread], and Clause Annex D [depr]. [ Note: This affects macros in 17.3.2 [version.syn], 31.13.1 [cstdio.syn], and D.11 [depr.c.macros] as well as identifiers in 17.2.2 [cstdlib.syn]. -- end note ]
• Each identifier that begins with an underscore is reserved to the implementation for use as a name in the global namespace.

Subclause 5.10 [lex.name] paragraph 3 specifies:

In addition, some identifiers appearing as a token or preprocessing-token are reserved for use by C++ implementations and shall not be used otherwise; no diagnostic is required.

• Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use.
• Each identifier that begins with an underscore is reserved to the implementation for use as a name in the global namespace.

That implies that uses of standard-specified predefined macros (15.11 [cpp.predefined]) or feature-test macros (17.3.2 [version.syn]) make the program ill-formed. This does not appear to be the intent.