[Bug libstdc++/101965] New: check in charconv is vacuously true

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101965

            Bug ID: 101965
           Summary: check in charconv is vacuously true
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

When investigating PVS-Studio in our codebase, it flagged this check in
charconv:

   322    template<typename _Tp>
   323      __detail::__integer_to_chars_result_type<_Tp>
   324      __to_chars_i(char* __first, char* __last, _Tp __value, int __base =
10)
   325      {
   326        __glibcxx_assert(2 <= __base && __base <= 36);
   327
   328        using _Up = __detail::__unsigned_least_t<_Tp>;
   329        _Up __unsigned_val = __value;
   330
   331        if (__first == __last) [[__unlikely__]]
   332          return { __last, errc::value_too_large };
   333
   334        if (__value == 0)
   335          {
   336            *__first = '0';
   337            return { __first + 1, errc{} };
   338          }
   339
   340        if _GLIBCXX17_CONSTEXPR (std::is_signed<_Tp>::value)
   341          if (__value < 0)
   342            {
   343              if (__builtin_expect(__first != __last, 1))
   344                *__first++ = '-';
   345              __unsigned_val = _Up(~__value) + _Up(1);
   346            }
   347
   348        switch (__base)
   349        {
   350        case 16:

On line 343, __first is never equal to __last. If it were, we would've returned
on line 332.