Proposed resolution:

This wording is relative to N4727.

1. Change [fs.rec.dir.itr.members] as indicated:

   void pop();
   void pop(error_code& ec);
   

   -26- Effects: If depth() == 0, set *this to recursive_directory_iterator(). Otherwise, cease iteration of the directory currently being iterated over, and continue iteration over the parent directory.

   -?- Postconditions: Any copies of the previous value of *this are no longer required either to be dereferenceable or to be in the domain of ==.

   -27- Throws: As specified in [fs.err.report].

[ 2018-06 Rapperswil: Adopted ]

[ 2018-03-06 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

recursive_directory_iterator::pop is effectively a "supercharged" operator++: it advances the iterator forward as many steps as are necessary to reach the next entry in the parent directory. Just as is the case for operator++, pop must be allowed to invalidate iterator copies to allow efficient implementation. The most efficient fix seems to be borrowing the invalidation wording from [input.iterators] Table 87's specification for the required ++r expression for input iterators.