Ticket #12578: Make directory iterators able to detect when a copy has advanced to the end

[mlimber]

Motivating example is using an iterator (e.g., testing to see if it is at the end) after a copy of it has been advanced to the end:

auto di = fs::directory_iterator( "." );
const auto end = fs::directory_iterator();
const auto dist = std::distance( di, end ); // implicitly iterate a copy of di to the end
                                            // which makes di undetectably invalid.
std::cout << "File count: " << dist << std::endl;
while( di != end ) // crash! Also with: for( const auto& e : di ), etc.
{
  std::cout << *di++ << std::endl;
}

For details, see Ticket #12578.

[Beman]

I've applied essentially the same fix as your pull request, but with some added comments and a different set of test cases. By the way, the motivating example you give does a post-increment on an iterator than will be the end iterator the last time through the loop.

Thanks for the bug report!

--Beman

[mlimber]

Fixed the increment. Thanks for processing it so quickly!