[libsigc++2/variadic_bind] tuple_transform_each(): Avoid use of std::make_tuple().

[Murray Cumming <murrayc src gnome org>]

commit 2e3f2b1f2a8f06baf33a1a9a7d508d0df31d5c88
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Feb 19 12:39:50 2016 +0100

    tuple_transform_each(): Avoid use of std::make_tuple().
    
    To preserve std::ref()ed elements.
    
    test_bind now builds and passes.

 sigc++/tuple_transform_each.h |   20 +++++++++++---------
 1 files changed, 11 insertions(+), 9 deletions(-)
---
diff --git a/sigc++/tuple_transform_each.h b/sigc++/tuple_transform_each.h
index 78da544..9ec9cfb 100644
--- a/sigc++/tuple_transform_each.h
+++ b/sigc++/tuple_transform_each.h
@@ -93,11 +93,11 @@ struct tuple_transform_each_impl {
   template <typename T_current, typename T_original>
   static decltype(auto)
   tuple_transform_each(T_current&& t, T_original& t_original) {
-    using element_type = typename std::tuple_element<index, std::decay_t<T_current>>::type;
-
-    auto& from = std::get<index>(t_original);
-    const auto element = T_transformer<element_type>::transform(from);
-    const auto t_element = std::make_tuple(element);
+    using from_element_type = typename std::tuple_element<index, std::decay_t<T_current>>::type;
+    using to_element_type = typename std::result_of<decltype (
+      &T_transformer<from_element_type>::transform)(from_element_type&)>::type;
+    const auto t_element =
+      std::tuple<to_element_type>(T_transformer<from_element_type>::transform(std::get<index>(t_original)));
 
     const auto t_start = tuple_start<index>(std::forward<T_current>(t));
 
@@ -124,10 +124,12 @@ struct tuple_transform_each_impl<T_transformer, 0> {
   tuple_transform_each(T_current&& t, T_original& t_original) {
     constexpr std::size_t index = 0;
 
-    using element_type = typename std::tuple_element<index, T_original>::type;
-    const auto element =
-      T_transformer<element_type>::transform(std::get<index>(t_original));
-    const auto tuple_element = std::make_tuple(element);
+    using from_element_type = typename std::tuple_element<index, T_original>::type;
+    using to_element_type = typename std::result_of<decltype (
+      &T_transformer<from_element_type>::transform)(from_element_type&)>::type;
+    const auto tuple_element =
+      std::tuple<to_element_type>(T_transformer<from_element_type>::transform(std::get<index>(t_original)));
+
     const auto tuple_rest = tuple_cdr(std::forward<T_current>(t));
     return std::tuple_cat(tuple_element, tuple_rest);
   }