>Of course it gets more complicated if you have nested loops [...]

I'm not sure I follow why adding an implicit variable which does something that for loops have already been able to do since time immemorial is a good idea. Nor do I see why that's preferable to either restructuring your loop, or (in the very rare situation you want both a range-based for loop with an index) rolling your own solution manually. 

I think the solution here is to write your own code which semantically does what you want it to do, rather than ask for this very specific use-case be added to the standard.