[Bug libstdc++/100903] Bogus "zero as null pointer constant" warning

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100903

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to M Welinder from comment #0)
> The standard is crazy-strict here.  Anything other than a literal 0 is
> undefined behaviour.  Allowed: 0, 0LL, 0'0'0'0, 0x0.

I don't think 0LL is allowed. A literal 0 has type int, and 0LL doesn't. I'm
unsure about 0'0 and 0x0.

>  No good: +0, nullptr,
> (void*)0, 0+0, '\0', (0,nullptr).  gcc incorrectly allows nullptr and
> (0,nullptr), but rejects the others.

gcc is not incorrect, the code that uses nullptr or (0, nullptr) is incorrect.
No diagnostic is required for violations of that rule.

> The library tries to do that with a construct that mainly allows null
> pointer constants.  That leads to the warning when you actually supply the 0
> you are supposed to.
> 
> Suggestion: just use an int argument.  That's wrong in different ways (and
> might cause warnings with 0LL which is allowed), but it matches 0 better.

Using int would allow 1 and 100 though. That seems much worse than allowing
other spellings of zero.

However, since I added the <compare> header gcc gained support for consteval
functions, which would allow us to reject non-zero values.

Something like this should work:

    struct __unspec
    {
      template<same_as<int> _Tp>
        consteval __unspec(_Tp __z) noexcept
        {
          if (__z != 0) throw; // comparison category types only compare to 0
        }
    };

This results in errors like:

cmp.cc:6:13: error: no match for 'operator<' (operand types are
'std::partial_ordering' and 'long int')
    6 |   return po < 0L;
      |          ~~ ^ ~~
      |          |    |
      |          |    long int
      |          std::partial_ordering

and:

/home/jwakely/gcc/14/include/c++/14.0.0/compare:59:25: error: expression
'<throw-expression>' is not a constant expression
   59 |           if (__z != 0) throw; // comparison category types only
compare to 0
      |                         ^~~~~