ircd::util: Add pre-c++2a identity functor template.

include/ircd/util/identity.h

@@ -0,0 +1,34 @@
+// The Construct
+//
+// Copyright (C) The Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2020 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+#pragma once
+#define HAVE_IRCD_UTIL_IDENTITY_H
+
+namespace ircd
+{
+	inline namespace util
+	{
+		struct identity;
+	}
+}
+
+struct ircd::util::identity
+{
+	template<class T>
+	constexpr T&& operator()(T&& t) const noexcept;
+};
+
+template<class T>
+constexpr inline T&&
+ircd::util::identity::operator()(T&& t)
+const noexcept
+{
+	return std::forward<T>(t);
+}

include/ircd/util/util.h

@@ -31,6 +31,7 @@ namespace ircd
 #define IRCD_UNIQUE(a)      IRCD_CONCAT(a, __COUNTER__)
 
 #include "typography.h"
+#include "identity.h"
 #include "unit_literal.h"
 #include "construction.h"
 #include "unwind.h"