From ea0f6b9a12f4b62d7cde9af27c26ea2b2992d26c Mon Sep 17 00:00:00 2001
From: John Preston <johnprestonmail@gmail.com>
Date: Fri, 29 Sep 2017 13:55:25 +0300
Subject: [PATCH] Use 'if constexpr ()' instead of tag dispatch.

---
 Telegram/SourceFiles/base/optional.h        | 25 ++----
 Telegram/SourceFiles/core/utils.h           | 19 ++---
 Telegram/SourceFiles/rpl/combine.h          | 37 +++-----
 Telegram/SourceFiles/rpl/consumer.h         | 79 ++++++++---------
 Telegram/SourceFiles/rpl/details/callable.h | 94 +++++----------------
 5 files changed, 78 insertions(+), 176 deletions(-)

diff --git a/Telegram/SourceFiles/base/optional.h b/Telegram/SourceFiles/base/optional.h
index c0d45c8d0..ff623eb7c 100644
--- a/Telegram/SourceFiles/base/optional.h
+++ b/Telegram/SourceFiles/base/optional.h
@@ -187,29 +187,14 @@ optional_wrap_once_t<Type> make_optional(Type &&value) {
 	return optional_wrap_once_t<Type> { std::forward<Type>(value) };
 }
 
-template <typename Type, typename Method>
-inline auto optional_chain(
-	const optional<Type> &value,
-	Method &method,
-	std::false_type)
--> optional_chain_result_t<decltype(method(*value))> {
-	return value ? make_optional(method(*value)) : none;
-}
-
-template <typename Type, typename Method>
-inline auto optional_chain(
-	const optional<Type> &value,
-	Method &method,
-	std::true_type)
--> optional_chain_result_t<decltype(method(*value))> {
-	return value ? (method(*value), true) : false;
-}
-
 template <typename Type, typename Method>
 inline auto operator|(const optional<Type> &value, Method method)
 -> optional_chain_result_t<decltype(method(*value))> {
-	using is_void_return = std::is_same<decltype(method(*value)), void>;
-	return optional_chain(value, method, is_void_return {});
+	if constexpr (std::is_same_v<decltype(method(*value)), void>) {
+		return value ? (method(*value), true) : false;
+	} else {
+		return value ? make_optional(method(*value)) : none;
+	}
 }
 
 } // namespace base
diff --git a/Telegram/SourceFiles/core/utils.h b/Telegram/SourceFiles/core/utils.h
index f6bfd88ed..030453f16 100644
--- a/Telegram/SourceFiles/core/utils.h
+++ b/Telegram/SourceFiles/core/utils.h
@@ -43,25 +43,16 @@ inline span<const char> make_span(const QByteArray &cont) {
 #endif // OS_MAC_OLD
 
 namespace base {
-namespace internal {
-
-template <typename D, typename T>
-inline constexpr D up_cast_helper(std::true_type, T object) {
-	return object;
-}
-
-template <typename D, typename T>
-inline constexpr D up_cast_helper(std::false_type, T object) {
-	return nullptr;
-}
-
-} // namespace internal
 
 template <typename D, typename T>
 inline constexpr D up_cast(T object) {
 	using DV = std::decay_t<decltype(*D())>;
 	using TV = std::decay_t<decltype(*T())>;
-	return internal::up_cast_helper<D>(std::integral_constant<bool, std::is_base_of<DV, TV>::value || std::is_same<DV, TV>::value>(), object);
+	if constexpr (std::is_base_of_v<DV, TV>) {
+		return object;
+	} else {
+		return nullptr;
+	}
 }
 
 template <typename Container, typename T>
diff --git a/Telegram/SourceFiles/rpl/combine.h b/Telegram/SourceFiles/rpl/combine.h
index 2d16bf943..410e0f7d8 100644
--- a/Telegram/SourceFiles/rpl/combine.h
+++ b/Telegram/SourceFiles/rpl/combine.h
@@ -257,16 +257,6 @@ template <typename ...Args>
 constexpr bool combine_producers_with_mapper_v
 	 = combine_producers_with_mapper<Args...>::value;
 
-template <
-	typename ...Values,
-	typename ...Errors,
-	typename ...Generators>
-inline decltype(auto) combine_helper(
-		std::true_type,
-		producer<Values, Errors, Generators> &&...producers) {
-	return combine_implementation(std::move(producers)...);
-}
-
 template <typename ...Producers, std::size_t ...I>
 inline decltype(auto) combine_call(
 		std::index_sequence<I...>,
@@ -275,18 +265,6 @@ inline decltype(auto) combine_call(
 		argument_mapper<I>::call(std::move(producers)...)...);
 }
 
-template <typename ...Args>
-inline decltype(auto) combine_helper(
-		std::false_type,
-		Args &&...args) {
-	constexpr auto kProducersCount = sizeof...(Args) - 1;
-	return combine_call(
-		std::make_index_sequence<kProducersCount>(),
-		std::forward<Args>(args)...)
-		| map(argument_mapper<kProducersCount>::call(
-			std::forward<Args>(args)...));
-}
-
 } // namespace details
 
 template <
@@ -295,9 +273,18 @@ template <
 		details::combine_just_producers_v<Args...>
 	|| details::combine_producers_with_mapper_v<Args...>>>
 inline decltype(auto) combine(Args &&...args) {
-	return details::combine_helper(
-		details::combine_just_producers<Args...>(),
-		std::forward<Args>(args)...);
+	if constexpr (details::combine_just_producers_v<Args...>) {
+		return details::combine_implementation(std::move(args)...);
+	} else if constexpr (details::combine_producers_with_mapper_v<Args...>) {
+		constexpr auto kProducersCount = sizeof...(Args) - 1;
+		return details::combine_call(
+			std::make_index_sequence<kProducersCount>(),
+			std::forward<Args>(args)...)
+				| map(details::argument_mapper<kProducersCount>::call(
+					std::forward<Args>(args)...));
+	} else {
+		static_assert(false_(args...), "Bad combine() call.");
+	}
 }
 
 namespace details {
diff --git a/Telegram/SourceFiles/rpl/consumer.h b/Telegram/SourceFiles/rpl/consumer.h
index b4791280e..3766c9cd3 100644
--- a/Telegram/SourceFiles/rpl/consumer.h
+++ b/Telegram/SourceFiles/rpl/consumer.h
@@ -86,6 +86,10 @@ struct is_type_erased_handlers<type_erased_handlers<Value, Error>>
 	: std::true_type {
 };
 
+template <typename Handlers>
+constexpr bool is_type_erased_handlers_v
+	= is_type_erased_handlers<Handlers>::value;
+
 template <typename Value, typename Error, typename OnNext, typename OnError, typename OnDone>
 class consumer_handlers final
 	: public type_erased_handlers<Value, Error> {
@@ -285,7 +289,8 @@ namespace details {
 
 template <typename Value, typename Error, typename Handlers>
 class consumer_base {
-	using is_type_erased = is_type_erased_handlers<Handlers>;
+	static constexpr bool is_type_erased
+		= is_type_erased_handlers_v<Handlers>;
 
 public:
 	template <
@@ -345,44 +350,22 @@ private:
 
 	mutable std::shared_ptr<Handlers> _handlers;
 
-	bool handlers_put_next(Value &&value, std::true_type) const {
-		return _handlers->put_next(std::move(value));
+	bool handlers_put_next(Value &&value) const {
+		if constexpr (is_type_erased) {
+			return _handlers->put_next(std::move(value));
+		} else {
+			return _handlers->Handlers::put_next(std::move(value));
+		}
 	}
-	bool handlers_put_next_copy(
-			const Value &value,
-			std::true_type) const {
-		return _handlers->put_next_copy(value);
+	bool handlers_put_next_copy(const Value &value) const {
+		if constexpr (is_type_erased) {
+			return _handlers->put_next_copy(value);
+		} else {
+			return _handlers->Handlers::put_next_copy(value);
+		}
 	}
-	void handlers_put_error(Error &&error, std::true_type) const {
-		return std::exchange(_handlers, nullptr)->put_error(std::move(error));
-	}
-	void handlers_put_error_copy(
-			const Error &error,
-			std::true_type) const {
-		return std::exchange(_handlers, nullptr)->put_error_copy(error);
-	}
-	void handlers_put_done(std::true_type) const {
-		return std::exchange(_handlers, nullptr)->put_done();
-	}
-
-	bool handlers_put_next(Value &&value, std::false_type) const {
-		return _handlers->Handlers::put_next(std::move(value));
-	}
-	bool handlers_put_next_copy(
-			const Value &value,
-			std::false_type) const {
-		return _handlers->Handlers::put_next_copy(value);
-	}
-	void handlers_put_error(Error &&error, std::false_type) const {
-		return std::exchange(_handlers, nullptr)->Handlers::put_error(std::move(error));
-	}
-	void handlers_put_error_copy(
-			const Error &error,
-			std::false_type) const {
-		return std::exchange(_handlers, nullptr)->Handlers::put_error_copy(error);
-	}
-	void handlers_put_done(std::false_type) const {
-		return std::exchange(_handlers, nullptr)->Handlers::put_done();
+	std::shared_ptr<Handlers> take_handlers() const {
+		return std::exchange(_handlers, nullptr);
 	}
 
 	template <
@@ -408,7 +391,7 @@ template <typename Value, typename Error, typename Handlers>
 inline bool consumer_base<Value, Error, Handlers>::put_next(
 		Value &&value) const {
 	if (_handlers) {
-		if (handlers_put_next(std::move(value), is_type_erased())) {
+		if (handlers_put_next(std::move(value))) {
 			return true;
 		}
 		_handlers = nullptr;
@@ -420,7 +403,7 @@ template <typename Value, typename Error, typename Handlers>
 inline bool consumer_base<Value, Error, Handlers>::put_next_copy(
 		const Value &value) const {
 	if (_handlers) {
-		if (handlers_put_next_copy(value, is_type_erased())) {
+		if (handlers_put_next_copy(value)) {
 			return true;
 		}
 		_handlers = nullptr;
@@ -432,7 +415,11 @@ template <typename Value, typename Error, typename Handlers>
 inline void consumer_base<Value, Error, Handlers>::put_error(
 		Error &&error) const {
 	if (_handlers) {
-		handlers_put_error(std::move(error), is_type_erased());
+		if constexpr (is_type_erased) {
+			take_handlers()->put_error(std::move(error));
+		} else {
+			take_handlers()->Handlers::put_error(std::move(error));
+		}
 	}
 }
 
@@ -440,14 +427,22 @@ template <typename Value, typename Error, typename Handlers>
 inline void consumer_base<Value, Error, Handlers>::put_error_copy(
 		const Error &error) const {
 	if (_handlers) {
-		handlers_put_error_copy(error, is_type_erased());
+		if constexpr (is_type_erased) {
+			take_handlers()->put_error_copy(error);
+		} else {
+			take_handlers()->Handlers::put_error_copy(error);
+		}
 	}
 }
 
 template <typename Value, typename Error, typename Handlers>
 inline void consumer_base<Value, Error, Handlers>::put_done() const {
 	if (_handlers) {
-		handlers_put_done(is_type_erased());
+		if constexpr (is_type_erased) {
+			take_handlers()->put_done();
+		} else {
+			take_handlers()->Handlers::put_done();
+		}
 	}
 }
 
diff --git a/Telegram/SourceFiles/rpl/details/callable.h b/Telegram/SourceFiles/rpl/details/callable.h
index 40807eaa2..5932d464c 100644
--- a/Telegram/SourceFiles/rpl/details/callable.h
+++ b/Telegram/SourceFiles/rpl/details/callable.h
@@ -99,54 +99,19 @@ struct is_callable<Method, Arg>
 template <typename Method, typename ...Args>
 constexpr bool is_callable_v = is_callable<Method, Args...>::value;
 
-enum class CallableArgTag {
-	Plain,
-	Tuple,
-	Empty,
-};
-template <CallableArgTag Arg>
-using callable_arg_tag = std::integral_constant<CallableArgTag, Arg>;
-
-template <typename Method, typename Arg>
-inline decltype(auto) callable_helper(
-		Method &&method,
-		Arg &&arg,
-		callable_arg_tag<CallableArgTag::Plain>) {
-	return std::forward<Method>(method)(std::forward<Arg>(arg));
-}
-
-template <typename Method, typename Arg>
-inline decltype(auto) callable_helper(
-		Method &&method,
-		Arg &&arg,
-		callable_arg_tag<CallableArgTag::Tuple>) {
-	return std::apply(
-		std::forward<Method>(method),
-		std::forward<Arg>(arg));
-}
-
-template <typename Method, typename Arg>
-inline decltype(auto) callable_helper(
-		Method &&method,
-		Arg &&,
-		callable_arg_tag<CallableArgTag::Empty>) {
-	return std::forward<Method>(method)();
-}
-
 template <typename Method, typename Arg>
 inline decltype(auto) callable_invoke(Method &&method, Arg &&arg) {
-	// #TODO if constexpr
-	constexpr auto kTag = is_callable_plain_v<Method, Arg>
-		? CallableArgTag::Plain
-		: is_callable_tuple_v<Method, Arg>
-		? CallableArgTag::Tuple
-		: is_callable_v<Method>
-		? CallableArgTag::Empty
-		: throw "Bad callable_invoke instance.";
-	return callable_helper(
-		std::forward<Method>(method),
-		std::forward<Arg>(arg),
-		callable_arg_tag<kTag>());
+	if constexpr (is_callable_plain_v<Method, Arg>) {
+		return std::forward<Method>(method)(std::forward<Arg>(arg));
+	} else if constexpr (is_callable_tuple_v<Method, Arg>) {
+		return std::apply(
+			std::forward<Method>(method),
+			std::forward<Arg>(arg));
+	} else if constexpr (is_callable_v<Method>) {
+		return std::forward<Method>(method)();
+	} else {
+		static_assert(false_(method, arg), "Bad callable_invoke() call.");
+	}
 }
 
 template <typename Method, typename Arg>
@@ -167,39 +132,18 @@ constexpr bool allows_const_ref_v = (sizeof(test_allows_const_ref(
 	std::declval<Method>(),
 	std::declval<Arg>())) == sizeof(true_t));
 
-template <typename Method, typename Arg>
-struct allows_const_ref
-	: std::bool_constant<
-		allows_const_ref_v<Method, Arg>> {
-};
-
-template <typename Method, typename Arg>
-inline decltype(auto) const_ref_call_helper(
-		Method &&method,
-		const Arg &arg,
-		std::true_type) {
-	return callable_invoke(std::forward<Method>(method), arg);
-}
-
-template <typename Method, typename Arg>
-inline decltype(auto) const_ref_call_helper(
-		Method &&method,
-		const Arg &arg,
-		std::false_type) {
-	auto copy = arg;
-	return callable_invoke(
-		std::forward<Method>(method),
-		std::move(copy));
-}
-
 template <typename Method, typename Arg>
 inline decltype(auto) const_ref_call_invoke(
 		Method &&method,
 		const Arg &arg) {
-	return const_ref_call_helper(
-		std::forward<Method>(method),
-		arg,
-		allows_const_ref<Method, Arg>());
+	if constexpr (allows_const_ref_v<Method, Arg>) {
+		return callable_invoke(std::forward<Method>(method), arg);
+	} else {
+		auto copy = arg;
+		return callable_invoke(
+			std::forward<Method>(method),
+			std::move(copy));
+	}
 }
 
 } // namespace details