LWG2974 Diagnose out of bounds tuple_element/variant_alternative

Author: Dawn Perchik

source/utilities.tex

@@ -133,8 +133,7 @@
   template <size_t I, class T> class tuple_element;
 
   template <class T1, class T2> struct tuple_size<pair<T1, T2>>;
-  template <class T1, class T2> struct tuple_element<0, pair<T1, T2>>;
-  template <class T1, class T2> struct tuple_element<1, pair<T1, T2>>;
+  template <size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>>;
 
   template<size_t I, class T1, class T2>
     constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>&) noexcept;
@@ -1287,18 +1286,14 @@
 
 \indexlibrary{\idxcode{tuple_element}}%
 \begin{itemdecl}
-tuple_element<0, pair<T1, T2>>::type
+tuple_element<I, pair<T1, T2>>::type
 \end{itemdecl}
 \begin{itemdescr}
-\pnum\textit{Value:} The type \tcode{T1}.
-\end{itemdescr}
+\pnum
+\requires \tcode{I < 2}. The program is ill-formed if \tcode{I} is out of bounds.
 
-\indexlibrary{\idxcode{tuple_element}}%
-\begin{itemdecl}
-tuple_element<1, pair<T1, T2>>::type
-\end{itemdecl}
-\begin{itemdescr}
-\pnum\textit{Value:} The type T2.
+\pnum
+\textit{Value:} The type \tcode{T1} if \tcode{I == 0}, otherwise the type \tcode{T2}.
 \end{itemdescr}
 
 \indexlibrarymember{get}{pair}%
@@ -5030,6 +5025,7 @@
 \begin{itemdescr}
 \pnum
 \requires \tcode{I < sizeof...(Types)}.
+The program is ill-formed if \tcode{I} is out of bounds.
 
 \pnum
 \textit{Value:} The type $\tcode{T}_I$.