[Bug libstdc++/94017] New: std::ranges::fill_n uses memset incorrectly

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94017

            Bug ID: 94017
           Summary: std::ranges::fill_n uses memset incorrectly
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: ppalka at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

#include <algorithm>

int main()
{
  int c[1] = { -1 };
  std::ranges::fill(c, 'a');
  if (*c != 'a')
    __builtin_abort();
}

This fails, because it uses memset to set a single byte of the int.

It also fails to compile with volatile input (related to PR 94013):


#include <algorithm>

int main()
{
  volatile char c[1] = { -1 };
  std::ranges::fill(c, 'a');
}

In file included from
/home/jwakely/gcc/10/include/c++/10.0.1/bits/ranges_algo.h:35,
                 from /home/jwakely/gcc/10/include/c++/10.0.1/algorithm:64,
                 from fill.cc:1:
/home/jwakely/gcc/10/include/c++/10.0.1/bits/ranges_algobase.h: In
instantiation of 'constexpr _Out std::ranges::__fill_n_fn::operator()(_Out,
std::iter_difference_t<_Iter>, const _Tp&) const [with _Tp = char; _Out =
volatile char*; std::iter_difference_t<_Iter> = long int]':
/home/jwakely/gcc/10/include/c++/10.0.1/bits/ranges_algobase.h:555:27:  
required from 'constexpr _Out std::ranges::__fill_fn::operator()(_Out, _Sent,
const _Tp&) const [with _Tp = char; _Out = volatile char*; _Sent = volatile
char*]'
/home/jwakely/gcc/10/include/c++/10.0.1/bits/ranges_algobase.h:576:16:  
required from 'constexpr std::ranges::borrowed_iterator_t<_Range>
std::ranges::__fill_fn::operator()(_Range&&, const _Tp&) const [with _Tp =
char; _Range = volatile char (&)[1]; std::ranges::borrowed_iterator_t<_Range> =
volatile char*]'
fill.cc:6:28:   required from here
/home/jwakely/gcc/10/include/c++/10.0.1/bits/ranges_algobase.h:522:23: error:
invalid conversion from 'volatile void*' to 'void*' [-fpermissive]
  522 |      __builtin_memset(__first, static_cast<unsigned char>(__value),
__n);
      |                       ^~~~~~~
      |                       |
      |                       volatile void*
<built-in>: note:   initializing argument 1 of 'void* __builtin_memset(void*,
int, long unsigned int)'