[Bug libstdc++/109976] New: error: is not a constant expression in std::equal() with _GLIBCXX_DEBUG

["ed at catmur dot uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109976

            Bug ID: 109976
           Summary: error: is not a constant expression in std::equal()
                    with _GLIBCXX_DEBUG
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

#define _GLIBCXX_DEBUG
#include <algorithm>
#include <string>
constexpr bool f(std::string const& l, std::string const& r) {
    return std::equal(l.data(), l.data() + l.size(), r.data(), r.data() +
r.size());
}
constexpr bool b = f("aa", "aa");

In file included from c++/14.0.0/debug/stl_iterator.h:32,
                 from c++/14.0.0/bits/stl_iterator.h:3004,
                 from c++/14.0.0/bits/stl_algobase.h:67,
                 from c++/14.0.0/algorithm:60,
                 from <source>:2:
<source>:7:21:   in 'constexpr' expansion of
'f(std::__cxx11::basic_string<char>(((const char*)"aa"),
std::allocator<char>()), std::__cxx11::basic_string<char>(((const char*)"aa"),
std::allocator<char>()))'
<source>:5:22:   in 'constexpr' expansion of 'std::equal<const char*, const
char*>((& l)->std::__cxx11::basic_string<char>::data(), ((&
l)->std::__cxx11::basic_string<char>::data() + ((sizetype)(&
l)->std::__cxx11::basic_string<char>::size())), (&
r)->std::__cxx11::basic_string<char>::data(), ((&
r)->std::__cxx11::basic_string<char>::data() + ((sizetype)(&
r)->std::__cxx11::basic_string<char>::size())))'
c++/14.0.0/bits/stl_algobase.h:1679:7:   in 'constexpr' expansion of
'__gnu_debug::__valid_range<const char*>(__first1, __last1)'
c++/14.0.0/debug/helper_functions.h:257:31:   in 'constexpr' expansion of
'__gnu_debug::__valid_range_aux<const char*>(__first, __last, (_Integral(),
_Integral()))'
c++/14.0.0/debug/helper_functions.h:189:31:   in 'constexpr' expansion of
'__gnu_debug::__valid_range_aux<const char*>(__first, __last,
(std::__iterator_category<const char*>(__first), std::iterator_traits<const
char*>::iterator_category()))'
c++/14.0.0/debug/helper_functions.h:176:19:   in 'constexpr' expansion of
'__gnu_debug::__valid_range_aux<const char*>(__first, __last,
(std::input_iterator_tag(), std::input_iterator_tag()))'
c++/14.0.0/debug/helper_functions.h:136:20: error: '(((const
char*)(&<anonymous>.std::__cxx11::basic_string<char>::<anonymous>.std::__cxx11::basic_string<char>::<unnamed
union>::_M_local_buf)) == 0)' is not a constant expression
  136 |     { return __ptr == 0; }
      |              ~~~~~~^~~~

This appears to be caused by bug 109975, but I'm filing separately since it 
may be possible to fix in library.

The code is accepted if _GLIBCXX_DEBUG is not defined, since
__gnu_debug::__valid_range is not checked.