Apply P0663R0 PR for 318: common_iterator::operator-> does not specif…

…y its return type

Fixes #318.

iterators.tex

@@ -4029,35 +4029,40 @@
 \indexlibrary{\idxcode{operator->}!\idxcode{common_iterator}}%
 \indexlibrary{\idxcode{common_iterator}!\idxcode{operator->}}%
 \begin{itemdecl}
-@\seebelow@ operator->() const requires Readable<I>;
+decltype(auto) operator->() const requires Readable<I>;
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
 \requires \tcode{!is_sentinel}
 
 \pnum
-\effects Given an object \tcode{i} of type \tcode{I}
+\effects Equivalent to:
 \begin{itemize}
-\item if \tcode{I} is a pointer type or if the expression
-      \tcode{i.operator->()} is well-formed, this function returns
-      \tcode{iter}.
-\item Otherwise, if the expression \tcode{*iter} is a glvalue, this function
-      is equivalent to \tcode{return addressof(*iter);}
-\item Otherwise, this function returns a proxy object of an unspecified type
-      equivalent to the following:
-      \begin{codeblock}
-      class proxy {               // \expos
-        value_type_t<I> keep_;
-        proxy(reference_t<I>&& x)
-          : keep_(std::move(x)) {}
-      public:
-        const value_type_t<I>* operator->() const {
-          return addressof(keep_);
-        }
-      };
-      \end{codeblock}
-      that is initialized with \tcode{*iter}.
+\item
+If \tcode{I} is a pointer type or if the expression \tcode{i.operator->()} is
+well-formed, \tcode{return iter;}
+
+\item
+Otherwise, if the expression \tcode{*iter} is a glvalue:
+\begin{codeblock}
+auto&& tmp = *iter;
+return addressof(tmp);
+\end{codeblock}
+
+\item
+Otherwise, \tcode{return proxy(*iter);} where proxy is the exposition-only class:
+\begin{codeblock}
+class proxy {               // \expos
+  value_type_t<I> keep_;
+  proxy(reference_t<I>&& x)
+    : keep_(std::move(x)) {}
+public:
+  const value_type_t<I>* operator->() const {
+    return addressof(keep_);
+  }
+};
+\end{codeblock}
 \end{itemize}
 \end{itemdescr}