Merge branch 'feature/void_t' into 'master'

void_t, real_t, field_t in dune/common/typetraits.hh

Contains:
 - void_t (+voider)

 - template <class T>
   using field_t = typename FieldTraits<T>::field_type;

 - template &lt;class T&gt;
   using real_t = typename FieldTraits<T>::real_type;

Requires C++11.

See merge request !20

README.md

@@ -28,7 +28,7 @@ dune-common depends on the following software packages
   icc (C/C++) >= 13.0
   Clang >= 3.2
 
-The following software is recommend but optional:
+The following software is recommended but optional:
 
 - MPI (either OpenMPI, lam, or mpich suffice)
 

dune/common/typetraits.hh

@@ -531,6 +531,38 @@ namespace Dune
 
 #endif // defined(DOXYGEN) or HAVE_IS_INDEXABLE_SUPPORT
 
+  namespace detail
+  {
+    ///
+    /**
+     * @internal
+     * @brief Helper to make void_t work with gcc versions prior to gcc 5.0.
+     *
+     * This was not a compiler bug, but an accidental omission in the C++11 standard (see N3911, CWG issue 1558).
+     * It is not clearly specified what happens
+     * with unused template arguments in template aliases. The developers of GCC decided to ignore them, thus making void_t equivalent to void.
+     * With gcc 5.0 this was changed and the voider-hack is no longer needed.
+     */
+    template <class...>
+    struct voider
+    {
+      using type = void;
+    };
+  }
+
+  template <class> struct FieldTraits;
+
+  //! Is void for all valid input types (see N3911). The workhorse for C++11 SFINAE-techniques.
+  template <class... Types>
+  using void_t = typename detail::voider<Types...>::type;
+
+  //! Convenient access to FieldTraits<Type>::field_type.
+  template <class Type>
+  using field_t = typename FieldTraits<Type>::field_type;
+
+  //! Convenient access to FieldTraits<Type>::real_type.
+  template <class Type>
+  using real_t = typename FieldTraits<Type>::real_type;
 
   /** @} */
 }