From e653eb3e8b00785eb49cc3c600e3b0867e4ed0a2 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sun, 24 May 2020 06:14:18 +0300
Subject: [PATCH] Use an internal forward_from_tuple as the std:: one isn't
 constexpr in C++11 or libstdc++ 5

---
 include/boost/mp11/tuple.hpp | 32 +++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/include/boost/mp11/tuple.hpp b/include/boost/mp11/tuple.hpp
index 5475211..ae74303 100644
--- a/include/boost/mp11/tuple.hpp
+++ b/include/boost/mp11/tuple.hpp
@@ -93,48 +93,54 @@ template<class Tp, class F> BOOST_MP11_CONSTEXPR F tuple_for_each( Tp && tp, F &
 namespace detail
 {
 
-template<std::size_t J, class... Tp>
-BOOST_MP11_CONSTEXPR auto tp_extract( Tp&&... tp )
-    -> decltype( std::forward_as_tuple( std::get<J>( std::forward<Tp>( tp ) )... ) )
+// std::forward_as_tuple is not constexpr in C++11 or libstdc++ 5.x
+template<class... T> BOOST_MP11_CONSTEXPR auto tp_forward_r( T&&... t ) -> std::tuple<T&&...>
 {
-    return std::forward_as_tuple( std::get<J>( std::forward<Tp>( tp ) )... );
+    return std::tuple<T&&...>( std::forward<T>( t )... );
 }
 
-template<class... T> BOOST_MP11_CONSTEXPR auto tp_forward( T&&... t ) -> std::tuple<T...>
+template<class... T> BOOST_MP11_CONSTEXPR auto tp_forward_v( T&&... t ) -> std::tuple<T...>
 {
     return std::tuple<T...>( std::forward<T>( t )... );
 }
 
+template<std::size_t J, class... Tp>
+BOOST_MP11_CONSTEXPR auto tp_extract( Tp&&... tp )
+    -> decltype( tp_forward_r( std::get<J>( std::forward<Tp>( tp ) )... ) )
+{
+    return tp_forward_r( std::get<J>( std::forward<Tp>( tp ) )... );
+}
+
 #if !BOOST_MP11_WORKAROUND( BOOST_MP11_MSVC, < 1900 )
 
 template<class F, class... Tp, std::size_t... J>
 BOOST_MP11_CONSTEXPR auto tuple_transform_impl( integer_sequence<std::size_t, J...>, F const& f, Tp&&... tp )
-    -> decltype( tp_forward( tuple_apply( f, tp_extract<J>( std::forward<Tp>(tp)... ) )... ) )
+    -> decltype( tp_forward_v( tuple_apply( f, tp_extract<J>( std::forward<Tp>(tp)... ) )... ) )
 {
-    return tp_forward( tuple_apply( f, tp_extract<J>( std::forward<Tp>(tp)... ) )... );
+    return tp_forward_v( tuple_apply( f, tp_extract<J>( std::forward<Tp>(tp)... ) )... );
 }
 
 #else
 
 template<class F, class Tp1, std::size_t... J>
 BOOST_MP11_CONSTEXPR auto tuple_transform_impl( integer_sequence<std::size_t, J...>, F const& f, Tp1&& tp1 )
-    -> decltype( tp_forward( f( std::get<J>( std::forward<Tp1>(tp1) ) )... ) )
+    -> decltype( tp_forward_v( f( std::get<J>( std::forward<Tp1>(tp1) ) )... ) )
 {
-    return tp_forward( f( std::get<J>( std::forward<Tp1>(tp1) ) )... );
+    return tp_forward_v( f( std::get<J>( std::forward<Tp1>(tp1) ) )... );
 }
 
 template<class F, class Tp1, class Tp2, std::size_t... J>
 BOOST_MP11_CONSTEXPR auto tuple_transform_impl( integer_sequence<std::size_t, J...>, F const& f, Tp1&& tp1, Tp2&& tp2 )
-    -> decltype( tp_forward( f( std::get<J>( std::forward<Tp1>(tp1) ), std::get<J>( std::forward<Tp2>(tp2) ) )... ) )
+    -> decltype( tp_forward_v( f( std::get<J>( std::forward<Tp1>(tp1) ), std::get<J>( std::forward<Tp2>(tp2) ) )... ) )
 {
-    return tp_forward( f( std::get<J>( std::forward<Tp1>(tp1) ), std::get<J>( std::forward<Tp2>(tp2) ) )... );
+    return tp_forward_v( f( std::get<J>( std::forward<Tp1>(tp1) ), std::get<J>( std::forward<Tp2>(tp2) ) )... );
 }
 
 template<class F, class Tp1, class Tp2, class Tp3, std::size_t... J>
 BOOST_MP11_CONSTEXPR auto tuple_transform_impl( integer_sequence<std::size_t, J...>, F const& f, Tp1&& tp1, Tp2&& tp2, Tp3&& tp3 )
-    -> decltype( tp_forward( f( std::get<J>( std::forward<Tp1>(tp1) ), std::get<J>( std::forward<Tp2>(tp2) ), std::get<J>( std::forward<Tp3>(tp3) ) )... ) )
+    -> decltype( tp_forward_v( f( std::get<J>( std::forward<Tp1>(tp1) ), std::get<J>( std::forward<Tp2>(tp2) ), std::get<J>( std::forward<Tp3>(tp3) ) )... ) )
 {
-    return tp_forward( f( std::get<J>( std::forward<Tp1>(tp1) ), std::get<J>( std::forward<Tp2>(tp2) ), std::get<J>( std::forward<Tp3>(tp3) ) )... );
+    return tp_forward_v( f( std::get<J>( std::forward<Tp1>(tp1) ), std::get<J>( std::forward<Tp2>(tp2) ), std::get<J>( std::forward<Tp3>(tp3) ) )... );
 }
 
 #endif // !BOOST_MP11_WORKAROUND( BOOST_MP11_MSVC, < 1900 )