LWG3502 elements_view should not be allowed to return dangling references

Author: burblebee

source/ranges.tex

@@ -6310,10 +6310,13 @@
       { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
     };
 
+  template<class T, size_t N>
+  concept @\defexposconcept{returnable-element}@ = is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;
 
   template<@\libconcept{input_range}@ V, size_t N>
     requires @\libconcept{view}@<V> && @\exposconcept{has-tuple-element}@<range_value_t<V>, N> &&
-             @\exposconcept{has-tuple-element}@<remove_reference_t<range_reference_t<V>>, N>
+             @\exposconcept{has-tuple-element}@<remove_reference_t<range_reference_t<V>>, N> &&
+             @\exposconcept{returnable-element}@<range_reference_t<V>, N>
   class elements_view : public view_interface<elements_view<V, N>> {
   public:
     elements_view() = default;
@@ -6374,12 +6377,16 @@
 namespace std::ranges {
   template<@\libconcept{input_range}@ V, size_t N>
     requires @\libconcept{view}@<V> && @\exposconcept{has-tuple-element}@<range_value_t<V>, N> &&
-             @\exposconcept{has-tuple-element}@<remove_reference_t<range_reference_t<V>>, N>
+             @\exposconcept{has-tuple-element}@<remove_reference_t<range_reference_t<V>>, N> &&
+             @\exposconcept{returnable-element}@<range_reference_t<V>, N>
   template<bool Const>
   class elements_view<V, N>::@\exposid{iterator}@ {                 // \expos
     using @\exposid{Base}@ = @\exposid{maybe-const}@<Const, V>;                 // \expos
 
-    iterator_t<@\exposid{Base}@> @\exposid{current_}@ = iterator_t<@\exposid{Base}@>();
+    iterator_t<@\exposid{Base}@> @\exposid{current_}@ = iterator_t<@\exposid{Base}@>();    // \expos
+
+    static constexpr decltype(auto) @\exposid{get-element}@(const iterator_t<@\exposid{Base}@>& i);     // \expos
+
   public:
     using iterator_category = typename iterator_traits<iterator_t<@\exposid{Base}@>>::iterator_category;
     using value_type = remove_cvref_t<tuple_element_t<N, range_value_t<@\exposid{Base}@>>>;
@@ -6395,7 +6402,7 @@
     constexpr iterator_t<@\exposid{Base}@> base() &&;
 
     constexpr decltype(auto) operator*() const
-    { return get<N>(*@\exposid{current_}@); }
+    { return @\exposid{get-element}@(@\exposid{current_}@); }
 
     constexpr @\exposid{iterator}@& operator++();
     constexpr void operator++(int);
@@ -6411,7 +6418,7 @@
 
     constexpr decltype(auto) operator[](difference_type n) const
       requires @\libconcept{random_access_range}@<@\exposid{Base}@>
-    { return get<N>(*(@\exposid{current_}@ + n)); }
+    { return @\exposid{get-element}@(@\exposid{current_}@ + n); }
 
     friend constexpr bool operator==(const @\exposid{iterator}@& x, const @\exposid{iterator}@& y)
       requires @\libconcept{equality_comparable}@<iterator_t<@\exposid{Base}@>>;
@@ -6439,6 +6446,26 @@
 }
 \end{codeblock}
 
+\indexlibrarymember{\exposid{get-element}}{elements_view::iterator}%
+\begin{itemdecl}
+static constexpr decltype(auto) @\exposid{get-element}@(const iterator_t<@\exposid{Base}@>& i);     // \expos
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to:
+\begin{codeblock}
+if constexpr (is_reference_v<range_reference_t<@\exposid{Base}@>>) {
+  return get<N>(*i);
+}
+else {
+  using E = remove_cv_t<tuple_element_t<N, range_reference_t<@\exposid{Base}@>>>;
+  return static_cast<E>(get<N>(*i));
+}
+\end{codeblock}
+\end{itemdescr}
+
 \indexlibraryctor{elements_view::iterator}%
 \begin{itemdecl}
 constexpr explicit @\exposid{iterator}@(iterator_t<@\exposid{Base}@> current);
@@ -6717,7 +6744,8 @@
 namespace std::ranges {
   template<@\libconcept{input_range}@ V, size_t N>
     requires @\libconcept{view}@<V> && @\placeholder{has-tuple-element}@<range_value_t<V>, N> &&
-             @\placeholder{has-tuple-element}@<remove_reference_t<range_reference_t<V>>, N>
+             @\placeholder{has-tuple-element}@<remove_reference_t<range_reference_t<V>>, N> &&
+             @\exposconcept{returnable-element}@<range_reference_t<V>, N>
   template<bool Const>
   class elements_view<V, N>::@\exposid{sentinel}@ {                 // \expos
   private: