Proposed resolution:

This wording is relative to N4901.

1. Modify [special.mem.concepts] as indicated:

   template<class I>
   concept nothrow-input-iterator = // exposition only
     input_iterator<I> &&
     is_lvalue_reference_v<iter_reference_t<I>> &&
     same_as<remove_cvref_t<iter_reference_t<I>>, iter_value_t<I>>;
   

   -2- A type I models nothrow-input-iterator only if no exceptions are thrown from increment, copy construction, move construction, copy assignment, move assignment, or indirection through valid iterators.