[Bug libstdc++/70303] New: Value-initialized debug iterators

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70303

            Bug ID: 70303
           Summary: Value-initialized debug iterators
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

Since C++14 requires value-initialized Forward iterators to compare equal, and
subtraction/ordering of RandomAccess iterators is based on equality, this
program should run to completion:

#define _GLIBCXX_DEBUG

#include <cassert>
#include <vector>

int main() {
  using I = std::vector<int>::iterator;
  assert(I{} == I{});
  assert(!(I{} != I{}));
  assert(I{} - I{} == 0);
  assert(!(I{} < I{}));
  assert(!(I{} > I{}));
  assert(I{} <= I{});
  assert(I{} >= I{});
}

instead of reporting "Error: attempt to compare a singular iterator to a
singular iterator." for any of the listed iterator operations.