[Bug libstdc++/100479] New: range adaptors handle cached iterators incorrectly

["rs2740 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100479

            Bug ID: 100479
           Summary: range adaptors handle cached iterators incorrectly
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

template<forward_range _Range>
      struct _CachedPosition<_Range>
      {
      private:
        iterator_t<_Range> _M_iter{};

      public:
        constexpr bool
        _M_has_value() const
        { return _M_iter != iterator_t<_Range>{}; }

        constexpr iterator_t<_Range>
        _M_get(const _Range&) const
        {
          __glibcxx_assert(_M_has_value());
          return _M_iter;
        }

        constexpr void
        _M_set(const _Range&, const iterator_t<_Range>& __it)
        {
          __glibcxx_assert(!_M_has_value());
          _M_iter = __it;
        }
      };

- The domain of == for forward iterators is limited to iterators over the same
underlying sequence. While value-initialized forward iterators of the same type
may be compared against each other, comparing them against iterators into other
ranges is not required to be well-defined, so it cannot be used as a sentinel
value.
- The cache cannot be allowed to propagate when the view containing it is
copied/moved, and has to be invalidated when the view containing it is moved.
Any cached iterator points to the original underlying view, and not the new
one; also, moving from a view can change its value (if it is well-defined to
use the moved-from view), so the cache is no longer valid. (Handling this case
correctly was actually the original use case for non-propagating-cache in
range-v3.)