From: tr.17687@z991.linuxsc.com   
      
   Kaz Kylheku <046-301-5902@kylheku.com> writes:   
      
   > On 2025-12-20, Tim Rentsch  wrote:   
   >   
   >> identifiers).  Hence any other predefined macro name may be used,   
   >> definedly, simply by using #undef and then #define for the macro   
   >> name in question (in particular, under C23 rules, but not earlier   
   >> versions of the C standard).   
   >   
   > If that interpretation is accurate, it is a serious problem.   
      
   Yes, very serious.  I suspect the change is simply an oversight   
   on the part of those responsible for the wording in the C23   
   standard.   
      
   > The implementation must be prepared for the situation that any of its   
   > internal macros are undefined, or undefined and redefined.   
   >   
   > That means that other macros cannot rely on those macros.   
   > Some documented macro foo() cannot rely on a __foo_impl()   
   > macro because the program could #undef that.   
      
   Probably there are other ways to work around the problem,   
   but it does seem better just to avoid the problem altogether   
   by forbidding definitions (or #undef) of any reserved   
   identifiers, including macro names.   
      
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