I was wondering if pointer size is still a problem. Hopefully this explanation is clearer:

  int compare_char (const char *a, const char *b) {
          return strcmp(a, b);
  }
  
  int compare_void (const void *a1, const void *b1) {
          const char *a = a1;
          const char *b = b1;
          return strcmp (a, b);
  }
  
  int (*call_void)(const void *a1, const void *b1) = &compare_char; //not valid in current C standard

Why can't the standard be changed so that it is valid to set call_void to &compare_char, so we don't need to write the longer compare_void? Are there architectures are still in active use where not all pointers are the same size in plain C (not worrying about C++) that would disallow this?

AFAIK, x86, ARM, MIPS, SPARC, Alpha, and SuperH would all work fine calling compare_char though call_void. There could be some other issues, like near/far on 16-bit x86, but that would be orthogonal to implicit function pointer void* casting, and could still occur if call_void was set to compare_void. Do one of the more obscure embedded CPUs still in use have a varying pointer size?