[Bug libstdc++/110149] std::format for pointer arguments allows a '0' option

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110149

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It looks like I forgot to actually format using 'P' though, I only parsed it
from the format string.

This would fix that, and disable the P2519 changes for -std=c++20 and
-std=c++23:

--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2081,19 +2081,29 @@ namespace __format
        if (__finished())
          return __first;

+#if __cplusplus > 202302L || ! defined __STRICT_ANSI__
        // _GLIBCXX_RESOLVE_LIB_DEFECTS
        // P2519R3 Formatting pointers
        __first = __spec._M_parse_zero_fill(__first, __last);
        if (__finished())
          return __first;
+#endif

        __first = __spec._M_parse_width(__first, __last, __pc);

-       if (__first != __last && (*__first == 'p' || *__first == 'P'))
+       if (__first != __last)
          {
-           if (*__first == 'P')
+           if (*__first == 'p')
+             ++__first;
+#if __cplusplus > 202302L || ! defined __STRICT_ANSI__
+           else if (*__first == 'P'))
+           {
+             // _GLIBCXX_RESOLVE_LIB_DEFECTS
+             // P2519R3 Formatting pointers
              __spec._M_type = __format::_Pres_P;
-           ++__first;
+             ++__first;
+           }
+#endif
          }

        if (__finished())
@@ -2113,6 +2123,18 @@ namespace __format
          const int __n = __ptr - __buf;
          __buf[0] = '0';
          __buf[1] = 'x';
+#if __cplusplus > 202302L || ! defined __STRICT_ANSI__
+         if (_M_spec._M_type == _Pres_)
+           {
+             __buf[1] = 'X';
+             for (auto __p = __buf + 2; __p != __res.ptr; ++__p)
+#if __has_builtin(__builtin_toupper)
+               *__p = __builtin_toupper(*__p);
+#else
+               *__p = std::toupper(*__p);
+#endif
+           }
+#endif

          basic_string_view<_CharT> __str;
          if constexpr (is_same_v<_CharT, char>)