meta: when_all added and minor changes to selection.h

include/utl/meta/selection.h

@@ -67,6 +67,24 @@ namespace meta{
    using if_ = if_c<If::type::value, Args...>;
 
    //! @}
+
+   /*!
+    * Named type selectors
+    */
+   //! @{
+
+   //! Select the first type of a type sequence
+   template <typename T1, typename ...>
+   struct first_of {
+      using type = T1;
+   };
+   //! Select the second type of a type sequence
+   template <typename T1, typename T2, typename ...>
+   struct second_of {
+      using type = T2;
+   };
+
+   //! @}
 }}
 
 //! @}

include/utl/meta/sfinae.h

@@ -34,13 +34,21 @@ namespace meta {
    //! Tool to enable a partial specialization only if a boolean condition is true.
    //! @{
    namespace detail {
-      template <bool If>
-      struct when_ { };
-      template <> struct when_<true> { using type = void; };
+      template <typename... T>
+      struct dev_null { using type = dev_null; };  //< Same as typelist
+
+      template <bool If>  struct when_ { };
+      template <>         struct when_<true> { using type = void; };
    }
    //! Well formed only if \p If is true
    template <bool If>
    using when = type_<detail::when_<If>>;
+
+   //! Well formed only if all of \p Ifs are \c true
+   template <bool ...Ifs>
+   using when_all = detail::dev_null<
+         when<Ifs>...
+   >;
    //! @}
 
    //! select _Tp if \p If is true, else SFINAE
@@ -59,11 +67,15 @@ namespace meta {
 
    //! Publicly recognized alias template for enable_if
    template<bool If, typename _Tp = void>
-   using enable_if_t = type_<enable_if<If, _Tp>>;
+   using enable_if_t = type_<
+         enable_if<If, _Tp>
+   >;
 
    //! Uniform alias template for use_if
    template<bool If, typename _Tp = void>
-   using use_if_t = type_<enable_if<If, _Tp>>;
+   using use_if_t = type_<
+         enable_if<If, _Tp>
+   >;
    //! @}
 
 }}