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

Change in 5.11 [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 (15.12 [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.