Merge 2022-07 LWG Motion 13

P2278R4 cbegin should always return a constant iterator

Author: tkoeppe

source/containers.tex

@@ -17373,7 +17373,9 @@
     using reference = element_type&;
     using const_reference = const element_type&;
     using iterator = @\impdefx{type of \tcode{span::iterator}}@;        // see \ref{span.iterators}
+    using const_iterator = std::const_iterator<iterator>;
     using reverse_iterator = std::reverse_iterator<iterator>;
+    using const_reverse_iterator = std::const_iterator<reverse_iterator>;
     static constexpr size_type extent = Extent;
 
     // \ref{span.cons}, constructors, copy, and assignment
@@ -17425,8 +17427,12 @@
     // \ref{span.iterators}, iterator support
     constexpr iterator begin() const noexcept;
     constexpr iterator end() const noexcept;
+    constexpr const_iterator cbegin() const noexcept { return begin(); }
+    constexpr const_iterator cend() const noexcept { return end(); }
     constexpr reverse_iterator rbegin() const noexcept;
     constexpr reverse_iterator rend() const noexcept;
+    constexpr const_reverse_iterator crbegin() const noexcept { return rbegin(); }
+    constexpr const_reverse_iterator crend() const noexcept { return rend(); }
 
   private:
     pointer data_;              // \expos