Merge 2018-11 LWG Motion 8

P0655R1 visit<R>: Explicit Return Type for visit

Fixes #2414

Author: zygoloid

source/utilities.tex

@@ -3775,6 +3775,8 @@
   // \ref{variant.visit}, visitation
   template<class Visitor, class... Variants>
     constexpr @\seebelow@ visit(Visitor&&, Variants&&...);
+  template <class R, class Visitor, class... Variants>
+    constexpr R visit(Visitor&&, Variants&&...);
 
   // \ref{variant.monostate}, class \tcode{monostate}
   struct monostate;
@@ -4862,6 +4864,8 @@
 \begin{itemdecl}
 template<class Visitor, class... Variants>
   constexpr @\seebelow@ visit(Visitor&& vis, Variants&&... vars);
+template <class R, class Visitor, class... Variants>
+  constexpr R visit(Visitor&& vis, Variants&&... vars);
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4874,6 +4878,11 @@
 \begin{codeblock}
 @\placeholder{INVOKE}@(std::forward<Visitor>(vis), get<m>(std::forward<Variants>(vars))...) // see \ref{func.require}
 \end{codeblock}
+for the first form and
+\begin{codeblock}
+@\placeholder{INVOKE}@<R>(std::forward<Visitor>(vis), get<m>(std::forward<Variants>(vars))...) // see \ref{func.require}
+\end{codeblock}
+for the second form.
 
 \pnum
 \requires
@@ -4883,8 +4892,9 @@
 
 \pnum
 \returns $e(\tcode{m})$, where \tcode{m} is the pack for which
-$\tcode{m}_i$ is \tcode{vars$_i$.index()} for
-all $0 \leq i < n$. The return type is $\tcode{decltype(}e(\tcode{m})\tcode{)}$.
+$\tcode{m}_i$ is \tcode{vars$_i$.index()} for all $0 \leq i < n$.
+The return type is $\tcode{decltype(}e(\tcode{m})\tcode{)}$
+for the first form.
 
 \pnum
 \throws