LWG3420 cpp17-iterator should check that the type looks like an iterator first

Author: burblebee

source/iterators.tex

@@ -856,11 +856,11 @@
 \begin{codeblock}
 template<class I>
 concept @\defexposconcept{cpp17-iterator}@ =
-  @\libconcept{copyable}@<I> && requires(I i) {
+  requires(I i) {
     {   *i } -> @\exposconcept{can-reference}@;
     {  ++i } -> @\libconcept{same_as}@<I&>;
     { *i++ } -> @\exposconcept{can-reference}@;
-  };
+  } && @\libconcept{copyable}@<I>;
 
 template<class I>
 concept @\defexposconcept{cpp17-input-iterator}@ =