[Bug libstdc++/102064] New: Wrong assignable check in uninitialized_fill and uninitialized_fill_n

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102064

            Bug ID: 102064
           Summary: Wrong assignable check in uninitialized_fill and
                    uninitialized_fill_n
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

In r204615 I added assignable checks to various algos to fix PR
libstdc++/58982.

The checks in uninitialized_fill and uninitialized_fill_n use
is_copy_assignable, which is wrong because the value being filled doesn't have
to be the value type of the output range.

This valid code fails to compile with libstdc++:

#include <memory>
#include <algorithm>

struct X
{
  X() = default;
  X(int) { }
  X(const X&) = default;
  X& operator=(const X&) = default;
  X& operator=(int) = delete;
};

static_assert( std::is_trivial<X>::value, "" );

int main()
{
  unsigned char buf[sizeof(X)];
  std::uninitialized_fill_n((X*)buf, 1, 99);
}


/usr/include/c++/11/bits/stl_algobase.h:924:18: error: use of deleted function
‘X& X::operator=(int)’
  924 |         *__first = __tmp;
      |         ~~~~~~~~~^~~~~~~

The problem is that we decide it's OK to use std::fill_n because X is trivial
and is_copy_assignable<X> is true, but then std::fill_n assigns int to X.

We need to check is_assignable<X&, const int&> instead.