> C23 furthermore gives the compiler license to use an unreachable annotation on one code path to justify removing, without notice or warning, an entirely different code path that is not marked unreachable: see the discussion of puts() in Example 1 on page 316 of N3054.9

I don't agree with that description at all. Here's the code:

  1 if (argc <= 2)
  2   unreachable();
  3 else
  4   return printf("%s: we see %s", argv[0], argv[1]);
  5 return puts("this should never be reached");

The only code path that's "entirely different" is lines 1,4,5 and in that case of course you remove a return that's after a return.

And the other valid code path is 1,2,5, which has `puts` after `unreachable`.

To need `puts` you have to imagine a code path that gets past the "if" without taking either branch?

Maybe the author means something by "code path" that's very different from how I interpret it?

I would be pretty surprised if the above code means something different from:

  if (argc <= 2) {
    unreachable();
    return puts("this should never be reached");
  } else {
    return printf("%s: we see %s", argv[0], argv[1]);
    return puts("this should never be reached");
  }