Added file of type trait aliases for traits we use

[ci skip]

These are (semi-verbatim) clones of the trait aliases found in C++14 and C++17.

src/utils/type_trait_aliases.hpp

@@ -0,0 +1,91 @@
+/*
+   Copyright (C) 2017 by the Battle for Wesnoth Project http://www.wesnoth.org/
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2 of the License, or
+   (at your option) any later version.
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY.
+
+   See the COPYING file for more details.
+*/
+
+#pragma once
+
+#include "global.hpp"
+
+#include <type_traits>
+
+namespace utils
+{
+//
+// These aliases are available in C++14. Forward to their declarations
+// if building with that.
+//
+#ifdef HAVE_CXX14
+
+using std::add_const_t;
+using std::conditional_t;
+using std::enable_if_t;
+using std::remove_const_t;
+using std::remove_reference_t;
+using std::remove_pointer_t;
+
+#else
+
+// add_const
+template<typename T>
+using add_const_t = typename std::add_const<T>::type;
+
+// conditional
+template<bool B, typename T, typename F>
+using conditional_t = typename std::conditional<B, T, F>::type;
+
+// enable_if
+template<bool B, typename T = void>
+using enable_if_t = typename std::enable_if<B, T>::type;
+
+// remove_const
+template<typename T>
+using remove_const_t = typename std::remove_const<T>::type;
+
+// remove_reference
+template<typename T>
+using remove_reference_t = typename std::remove_reference<T>::type;
+
+// remove_pointer
+template<typename T>
+using remove_pointer_t = typename std::remove_pointer<T>::type;
+
+#endif // HAVE_CXX14
+
+//
+// These aliases are part of the standard starting with C++17.
+// Since we're not even near supporting that, we define them directly.
+// They can be added to a conditional block once we have a define for C++17.
+//
+
+// MSVC 2013 doesn't support constexpr :(
+#if defined(_MSC_VER) && _MSC_VER < 1900
+
+// is_base_of
+template<typename Base, typename Derived>
+static const bool is_base_of_v = std::is_base_of<Base, Derived>::value;
+
+// is_same
+template<typename T, typename U>
+static const bool is_same_v = std::is_same<T, U>::value;
+
+#else
+
+// is_base_of
+template<typename Base, typename Derived>
+inline constexpr bool is_base_of_v = std::is_base_of<Base, Derived>::value;
+
+// is_same
+template<typename T, typename U>
+inline constexpr bool is_same_v = std::is_same<T, U>::value;
+
+#endif
+}