Rope iterators (both const and mutable) do not meet the requirements of an Input Iterator, because their dereference operator only operates on non-const objects. Table 72 of the C++98 Standard requires the valid expression *a, where a is a const iterator, but this valid expression does not work with a non-const operator*.

To fix, make operator* and everything it relies on "const". This might have an unfortunate ripple effect; an alternative (but ugly) approach would be to add a new operator* to _Rope_iterator and _Rope_const_iterator that const_cast's and forwards:

  reference
  operator*() const
  {
    return *const_cast<_Rope_iterator&>(*this);
  }

  reference
  operator*() const
  {
    return *const_cast<_Rope_const_iterator&>(*this);
  }

This problem is very unlikely to cause problems in user code, but it makes ConceptGCC sad.