Util: Add a y-combinator template class

src/core/CMakeLists.txt

@@ -31,4 +31,5 @@ add_files(
     smallvec_type.hpp
     string_compare_type.hpp
     tinystring_type.hpp
+    y_combinator.hpp
 )

src/core/y_combinator.hpp

@@ -0,0 +1,36 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD 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, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file y_combinator.hpp Y-combinator template implementation to support recursive lambdas and similar. */
+
+#ifndef Y_COMBINATOR_HPP
+#define Y_COMBINATOR_HPP
+
+#include <functional>
+#include <utility>
+
+/* Based on C++ std library proposal: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0200r0.html */
+
+template<class F>
+class y_combinator_result {
+	F func_;
+public:
+	template<class T>
+	explicit y_combinator_result(T &&func): func_(std::forward<T>(func)) {}
+
+	template<class ...Args>
+	decltype(auto) operator()(Args &&...args) {
+		return func_(std::ref(*this), std::forward<Args>(args)...);
+	}
+};
+
+template<class F>
+decltype(auto) y_combinator(F &&func) {
+	return y_combinator_result<std::decay_t<F>>(std::forward<F>(func));
+}
+
+#endif /* Y_COMBINATOR_HPP */