[Bug libstdc++/96042] New: Reference type of std::ranges::iota is __int128 with -std=c++2a?!

[gcc-bugs at marehr dot dialup.fu-berlin.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96042

            Bug ID: 96042
           Summary: Reference type of std::ranges::iota is __int128 with
                    -std=c++2a?!
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc-bugs at marehr dot dialup.fu-berlin.de
  Target Milestone: ---

The following code

```c++
#include <ranges>

using iota_view = std::ranges::iota_view<size_t>;
using reference_t = std::ranges::range_difference_t<iota_view>;

static_assert(std::same_as<reference_t, __int128>); // why?

// but std::numeric_limits is not specialised which indicates that we use
__STRICT_ANSI__

using t = std::numeric_limits<__int128>;
static_assert(t::is_specialized);
```

https://godbolt.org/z/-XFyYN

I don't fully understand what `__int128` and `__STRICT_ANSI__` means, but I
think it is unexpected that I get a type that is not described in the standard.

I know that `std::indirectly_readable` allows (compiler) implementation defined
signed integer types, but from what I have seen, `__int128` is normally only
supported / exposed in the gnu standard library if `__STRICT_ANSI__` is not
defined.

Since `-std=c++2a` does not provide a specialisation for `std::numeric_limits`,
I assume that this means, that `__STRICT_ANSI__` is not defined in that mode,
and I think this is inconsistent behaviour.

Related issues in the range-v3 library:
* https://github.com/ericniebler/range-v3/issues/1514
* https://github.com/ericniebler/range-v3/issues/1516