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Telegram/lib_base/base/overload.h

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+// This file is part of Desktop App Toolkit,
+// a set of libraries for developing nice desktop applications.
+//
+// For license and copyright information please follow this link:
+// https://github.com/desktop-app/legal/blob/master/LEGAL
+//
+#pragma once
+
+namespace base {
+namespace details {
+
+// This implementation was taken from range-v3 library.
+// It was modified so that more than one of passed function objects
+// could be called with an argument list and the first one that
+// matches gets used instead of a compile-time ambiguity error.
+//
+// This allows to write "default" visitor handlers with [](auto&&) syntax.
+
+template <typename ...Args>
+constexpr bool is_callable_v = rpl::details::is_callable_plain_v<Args...>;
+
+template <typename ...Args>
+struct overloaded;
+
+template <>
+struct overloaded<> {
+};
+
+template <typename First, typename ...Rest>
+struct overloaded<First, Rest...>
+	: private First
+	, private overloaded<Rest...> {
+
+private:
+	using Others = overloaded<Rest...>;
+
+public:
+	overloaded() = default;
+
+	constexpr overloaded(First first, Rest... rest)
+	: First(std::move(first))
+	, Others(std::move(rest)...) {
+	}
+
+	template <typename... Args>
+	auto operator()(Args&&... args)
+	-> decltype(std::declval<First&>()(std::forward<Args>(args)...)) {
+		return static_cast<First&>(*this)(std::forward<Args>(args)...);
+	}
+
+	template <typename... Args>
+	auto operator()(Args&&... args) const
+	-> decltype(std::declval<const First&>()(std::forward<Args>(args)...)) {
+		return static_cast<const First&>(*this)(std::forward<Args>(args)...);
+	}
+
+	template <
+		typename... Args,
+		typename = std::enable_if_t<!is_callable_v<First&, Args&&...>>>
+	auto operator()(Args&&... args)
+	-> decltype(std::declval<Others&>()(std::forward<Args>(args)...)) {
+		return static_cast<Others&>(*this)(std::forward<Args>(args)...);
+	}
+
+	template <
+		typename... Args,
+		typename = std::enable_if_t<!is_callable_v<const First&, Args&&...>>>
+	auto operator()(Args&&... args) const
+	-> decltype(std::declval<const Others&>()(std::forward<Args>(args)...)) {
+		return static_cast<const Others&>(*this)(std::forward<Args>(args)...);
+	}
+
+};
+
+} // namespace details
+
+template <typename Function>
+Function overload(Function &&function) {
+	return std::forward<Function>(function);
+}
+
+template <typename ...Functions, typename = std::enable_if_t<(sizeof...(Functions) > 1)>>
+auto overload(Functions ...functions) {
+	return details::overloaded<Functions...>(std::move(functions)...);
+}
+
+} // namespace base
+