LWG3749 common_iterator should handle integer-class difference types

Author: burblebee

source/iterators.tex

@@ -5477,7 +5477,7 @@
   template<@\libconcept{input_iterator}@ I, class S>
   struct iterator_traits<common_iterator<I, S>> {
     using iterator_concept = @\seebelow@;
-    using iterator_category = @\seebelow@;
+    using iterator_category = @\seebelow@;  // not always present
     using value_type = iter_value_t<I>;
     using difference_type = iter_difference_t<I>;
     using pointer = @\seebelow@;
@@ -5489,22 +5489,25 @@
 \rSec3[common.iter.types]{Associated types}
 
 \pnum
-The nested \grammarterm{typedef-name}s of the specialization of
-\tcode{iterator_traits} for \tcode{common_iterator<I, S>} are defined as follows.
+The nested \grammarterm{typedef-name} \tcode{iterator_category} of
+the specialization of \tcode{iterator_traits} for \tcode{common_iterator<I, S>}
+is defined if and only if \tcode{iter_difference_t<I>} is an integral type.
+In that case,
+\tcode{iterator_category} denotes \tcode{forward_iterator_tag} if
+the \grammarterm{qualified-id} \tcode{iterator_traits<I>::iterator_category}
+is valid and denotes a type that
+models \tcode{\libconcept{derived_from}<forward_iterator_tag>};
+otherwise it denotes \tcode{input_iterator_tag}.
+
+\pnum
+The remaining nested \grammarterm{typedef-name}s of the specialization of
+\tcode{iterator_traits} for \tcode{common_iterator<I, S>} are defined as follows:
 \begin{itemize}
 \item
 \tcode{iterator_concept} denotes \tcode{forward_iterator_tag}
 if \tcode{I} models \libconcept{forward_iterator};
 otherwise it denotes \tcode{input_iterator_tag}.
 
-\item
-\tcode{iterator_category} denotes
-\tcode{forward_iterator_tag}
-if the \grammarterm{qualified-id} \tcode{iterator_traits<I>::iter\-ator_category}
-is valid and denotes a type that
-models \tcode{\libconcept{derived_from}<forward_iterator_tag>};
-otherwise it denotes \tcode{input_iterator_tag}.
-
 \item
 Let \tcode{a} denote an lvalue of type \tcode{const common_iterator<I, S>}.
 If the expression \tcode{a.operator->()} is well-formed,