Merge 2019-11 LWG Motion 16

P1870R1 forwarding-range<T> is too subtle

Author: zygoloid

papers/nxxxx.md

@@ -63,6 +63,20 @@ and after consulting the paper authors and the LWG chair,
 the corresponding changes were also applied to
 the additional range adaptors listed above.
 
+### LWG motion 16
+
+This paper removed the exposition-only concept *`range-impl`*,
+inlining it into its only remaining user, the `range` concept.
+However, two uses of *`range-impl`* were left behind.
+These have been updated and suitably adjusted
+to refer to `range` instead.
+
+LWG motion 13 ([P1394R4](http://wg21.link/p1394r4))
+added a couple of new uses of
+the exposition-only concept *`forwarding-range`*,
+which was removed by this paper.
+These uses have been replaced with `safe_range`.
+
 ## Feature test macros
 
 The feature test macro `__cpp_nontype_template_parameter_class` has been removed

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
@@ -10786,7 +10786,7 @@
 \item \tcode{extent == dynamic_extent} is \tcode{true}.
 \item \tcode{R} satisfies \tcode{ranges::\libconcept{contiguous_range}} and
   \tcode{ranges::\libconcept{sized_range}}.
-\item Either \tcode{R} satisfies \exposconcept{forwarding-range} or
+\item Either \tcode{R} satisfies \libconcept{safe_range} or
 \tcode{is_const_v<element_type>} is \tcode{true}.
 \item \tcode{remove_cvref_t<R>} is not a specialization of \tcode{span}.
 \item \tcode{remove_cvref_t<R>} is not a specialization of \tcode{array}.
@@ -10805,7 +10805,7 @@
 \item \tcode{R} models \tcode{ranges::\libconcept{contiguous_range}} and
 \tcode{ranges::\libconcept{sized_range}}.
 \item If \tcode{is_const_v<element_type>} is \tcode{false},
-\tcode{R} models \exposconcept{forwarding-range}.
+\tcode{R} models \libconcept{safe_range}.
 \end{itemize}
 
 \pnum