#define _GLIBCXX_DEBUG 1
#include <unordered_set>

int main()
{
  std::unordered_set<int>::iterator it{};
  it == it;
}

This aborts at runtime:

/home/jwakely/gcc/11/include/c++/11.0.0/debug/safe_iterator.h:461:
In function:
    bool __gnu_debug::operator==(const _Self&, const _Self&)

Error: attempt to compare a singular iterator to a singular iterator.

Objects involved in the operation:
    iterator "__lhs" @ 0x0x7ffc0e6d5940 {
      type = std::__detail::_Node_iterator<int, true, false> (mutable iterator);
      state = singular;
    }
    iterator "__rhs" @ 0x0x7ffc0e6d5940 {
      type = std::__detail::_Node_iterator<int, true, false> (mutable iterator);
      state = singular;
    }
Aborted (core dumped)


It seems that https://wg21.link/n3644 was not implemented for the debug mode iterators.