[Bug libstdc++/107092] New: std::for_each_n and its friends incorrectly accept size parameters that are not convertible to an integer type

["de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107092

            Bug ID: 107092
           Summary: std::for_each_n and its friends incorrectly accept
                    size parameters that are not convertible to an integer
                    type
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

This program shouldn't compile due to [alg.foreach]/16:

> Mandates: The type Size is convertible to an integral type ([conv.integral], [class.conv]).

It's obvious that BadFrom can't be converted to any integral type.

#include <algorithm>
#include <concepts>

struct BadFrom {
    template<std::same_as<double> D>
    operator D() const
    {
        return 42.0;
    }
};


int main()
{
    int arr[42]{};
    std::for_each_n(arr, BadFrom{}, [](int){});
}

However, this is accepted by libstdc++, because the conversion is done by
__size_to_integer overloads in stl_algobase.h (added via PR 87982), and
overloads for floating-point types are (probably) incorrectly provided.

Godbolt link: https://godbolt.org/z/EKvx7dY4e

While I believe the current "is convertible to an integral type" requirement is
defective (as ambiguity is not considered at all), I think libstdc++ should
reject using BadFrom as Size.