Reorder requirements in concept Iterator

[ericniebler]

When passing a std::vector<MoveOnly> to sort, overload resolution causes the evaluation of Iterator<std::vector<MoveOnly>>, which tests the vector type for assignability. Vectors of move-only types are not assignable, but their assignment operators are not properly constrained. That leads to a hard error rather than a concept check failure.

The problem can be mostly avoided by testing for dereferencability first in concept Iterator, as shows in the Proposed Resolution below.

Proposed Resolution

Change concept Iterator ([iterators.iterator]) as follows:

 template <class I>
 concept bool Iterator() {
-   return WeaklyIncrementable<I>() &&
-     requires(I i) {
-       { *i } -> auto&&; // Requires: i is dereferenceable
-     };
+   return requires(I i) {
+     { *i } -> auto&&; // Requires: i is dereferenceable
+   } && WeaklyIncrementable<I>();
 }