[Bug libstdc++/94164] New: [Regression 10] std::unintialized_fill_n fails to compile

["laurent.stacul at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94164

            Bug ID: 94164
           Summary: [Regression 10] std::unintialized_fill_n fails to
                    compile
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: laurent.stacul at gmail dot com
  Target Milestone: ---

Hello,

I have this piece of code:

#include <memory>

template<typename T>
struct ref {
    typedef T value_type;
    typedef T* pointer;
    typedef T& reference;
    typedef ptrdiff_t difference_type;
    typedef std::random_access_iterator_tag iterator_category;

    int d;

    explicit ref(int =0): d(-1) {}
    operator pointer() {
        if (d == -1) {
            return 0;
        }
        else
        {
            return reinterpret_cast<pointer>(this + d);
        }
    }
    ref& operator++() {
        d += sizeof(value_type);
        return *this;
    }
};

int main(int, const char *[])
{
    ref<char> r;
    std::uninitialized_fill_n(r, 10, 'a');
    return 0;
}

I compile it with the following command (the -std option is not important):

$ g++ -Wall -Wextra -std=gnu++2a -c -o main.o main.cpp

When I compile it with gcc (GCC) 10.0.1 20200305, if fails with the following
error:

/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h: In instantiation of
'constexpr _OutputIterator std::__fill_n_a(_OutputIterator, _Size, const _Tp&,
std::random_access_iterator_tag) [with _OutputIterator = ref<char>; _Size =
int; _Tp = char]':
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:1085:29:   required
from 'constexpr _OI std::fill_n(_OI, _Size, const _Tp&) [with _OI = ref<char>;
_Size = int; _Tp = char]'
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_uninitialized.h:254:29:   required
from 'static _ForwardIterator
std::__uninitialized_fill_n<true>::__uninit_fill_n(_ForwardIterator, _Size,
const _Tp&) [with _ForwardIterator = ref<char>; _Size = int; _Tp = c
har]'
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_uninitialized.h:289:17:   required
from '_ForwardIterator std::uninitialized_fill_n(_ForwardIterator, _Size, const
_Tp&) [with _ForwardIterator = ref<char>; _Size = int; _Tp = char]'
main.cpp:32:41:   required from here
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:1056:20: error: no
matching function for call to '__fill_a(ref<char>&, ref<char>::pointer, const
char&)'
 1056 |       std::__fill_a(__first, __first + __n, __value);
      |       ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:904:5: note: candidate:
'template<class _FIte, class _Tp> constexpr void std::__fill_a(_FIte, _FIte,
const _Tp&)'
  904 |     __fill_a(_FIte __first, _FIte __last, const _Tp& __value)
      |     ^~~~~~~~
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:904:5: note:   template
argument deduction/substitution failed:
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:1056:20: note:  
deduced conflicting types for parameter '_FIte' ('ref<char>' and 'char*')
 1056 |       std::__fill_a(__first, __first + __n, __value);
      |       ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:909:5: note: candidate:
'template<class _Ite, class _Seq, class _Cat, class _Tp> void
std::__fill_a(const __gnu_debug::_Safe_iterator<_Ite, _Seq, _Cat>&, const
__gnu_debug::_Safe_iterator<_Ite, _Seq
, _Cat>&, const _Tp&)'
  909 |     __fill_a(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq, _Cat>&,
      |     ^~~~~~~~
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:909:5: note:   template
argument deduction/substitution failed:
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:1056:20: note:  
'ref<char>' is not derived from 'const __gnu_debug::_Safe_iterator<_Ite, _Seq,
_Cat>'
 1056 |       std::__fill_a(__first, __first + __n, __value);
      |       ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/x86_64-v20.0.13/include/c++/10.0.1/bits/stl_algobase.h:1057:22: error: could
not convert '(__first.ref<char>::operator ref<char>::pointer() +
((sizetype)__n))' from 'ref<char>::pointer' {aka 'char*'} to 'ref<char>'
 1057 |       return __first + __n;
      |              ~~~~~~~~^~~~~
      |                      |
      |                      ref<char>::pointer {aka char*}
make: *** [Makefile:6: main.o] Error 1

Conversely, this code compiles with gcc (GCC) 9.2.1. 

Maybe is it related to
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=6004c17b4d1a2dd1e1936b2e705a5ccfb6e48ab2
but I don' t know if the current bahviour is expected.

Stac