P1870R1 forwarding-range<T> is too subtle

Also fixes NB US 279, US 279, and GB 280 (C++20 CD).

Author: Dawn Perchik

source/containers.tex

@@ -10513,6 +10513,9 @@
   template<class ElementType, size_t Extent = dynamic_extent>
     class span;
 
+  template<class ElementType, size_t Extent>
+    inline constexpr bool enable_safe_range<span<ElementType, Extent>> = true;
+
   // \ref{span.objectrep}, views of object representation
   template<class ElementType, size_t Extent>
     span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
@@ -10625,9 +10628,6 @@
     constexpr const_reverse_iterator crbegin() const noexcept;
     constexpr const_reverse_iterator crend() const noexcept;
 
-    friend constexpr iterator begin(span s) noexcept { return s.begin(); }
-    friend constexpr iterator end(span s) noexcept { return s.end(); }
-
   private:
     pointer data_;              // \expos
     size_type size_;            // \expos