Change to west-const, since I think that's the Ports-of-Call standard.

ports-of-call/span.hpp

@@ -73,7 +73,7 @@ class span {
 
   // Index operator to obtain mutable access to an element of the range.
   template <typename Index>
-  PORTABLE_FUNCTION constexpr T &operator[](Index const &index) {
+  PORTABLE_FUNCTION constexpr T &operator[](const Index &index) {
     assert(index >= 0);
     assert(static_cast<std::size_t>(index) < size_);
     return *(ptr_ + index);