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Also rename .visit() with .match() in MTP types.
Also add base::match_method() and base::match() for base::variant.
Also add base::match() and .match() for base::optional_variant.

Telegram/SourceFiles/base/match_method.h

@@ -0,0 +1,28 @@
+/*
+This file is part of Telegram Desktop,
+the official desktop application for the Telegram messaging service.
+
+For license and copyright information please follow this link:
+https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
+*/
+#pragma once
+
+#include <rpl/details/callable.h>
+
+namespace base {
+
+template <typename Data, typename Method, typename ...Methods>
+inline decltype(auto) match_method(
+		Data &&data,
+		Method &&method,
+		Methods &&...methods) {
+	if constexpr (rpl::details::is_callable_plain_v<Method, Data&&>) {
+		return std::forward<Method>(method)(std::forward<Data>(data));
+	} else {
+		return match_method(
+			std::forward<Data>(data),
+			std::forward<Methods>(methods)...);
+	}
+}
+
+} // namespace base

Telegram/SourceFiles/base/optional.h

@@ -109,6 +109,15 @@ class optional_variant {
 		return _impl.template get_unchecked<T>();
 	}
 
+	template <typename ...Methods>
+	decltype(auto) match(Methods &&...methods) {
+		return base::match(_impl, std::forward<Methods>(methods)...);
+	}
+	template <typename ...Methods>
+	decltype(auto) match(Methods &&...methods) const {
+		return base::match(_impl, std::forward<Methods>(methods)...);
+	}
+
 private:
 	variant<none_type, Types...> _impl;
 
@@ -124,6 +133,20 @@ inline const T *get_if(const optional_variant<Types...> *v) {
 	return (v && v->template is<T>()) ? &v->template get_unchecked<T>() : nullptr;
 }
 
+template <typename ...Types, typename ...Methods>
+inline decltype(auto) match(
+		optional_variant<Types...> &value,
+		Methods &&...methods) {
+	return value.match(std::forward<Methods>(methods)...);
+}
+
+template <typename ...Types, typename ...Methods>
+inline decltype(auto) match(
+		const optional_variant<Types...> &value,
+		Methods &&...methods) {
+	return value.match(std::forward<Methods>(methods)...);
+}
+
 template <typename Type>
 class optional;
 

Telegram/SourceFiles/base/variant.h

@@ -8,6 +8,9 @@ For license and copyright information please follow this link:
 #pragma once
 
 #include <mapbox/variant.hpp>
+#include <rpl/details/type_list.h>
+#include "base/match_method.h"
+#include "base/assertion.h"
 
 // We use base::variant<> alias and base::get_if() helper while we don't have std::variant<>.
 namespace base {
@@ -25,20 +28,79 @@ inline const T *get_if(const variant<Types...> *v) {
 	return (v && v->template is<T>()) ? &v->template get_unchecked<T>() : nullptr;
 }
 
-// Simplified visit
-template <typename Method, typename... Types>
-inline auto visit(Method &&method, const variant<Types...> &value) {
-	return value.match(std::forward<Method>(method));
-}
+namespace type_list = rpl::details::type_list;
+
+template <typename ...Types>
+struct normalized_variant {
+	using list = type_list::list<Types...>;
+	using distinct = type_list::distinct_t<list>;
+	using type = std::conditional_t<
+		type_list::size_v<distinct> == 1,
+		type_list::get_t<0, distinct>,
+		type_list::extract_to_t<distinct, base::variant>>;
+};
+
+template <typename ...Types>
+using normalized_variant_t
+	= typename normalized_variant<Types...>::type;
+
+template <typename TypeList, typename Variant, typename ...Methods>
+struct match_helper;
+
+template <
+	typename Type,
+	typename ...Types,
+	typename Variant,
+	typename ...Methods>
+struct match_helper<type_list::list<Type, Types...>, Variant, Methods...> {
+	static decltype(auto) call(Variant &value, Methods &&...methods) {
+		if (const auto v = get_if<Type>(&value)) {
+			return match_method(
+				*v,
+				std::forward<Methods>(methods)...);
+		}
+		return match_helper<
+			type_list::list<Types...>,
+			Variant,
+			Methods...>::call(
+				value,
+				std::forward<Methods>(methods)...);
+	}
+};
+
+template <
+	typename Type,
+	typename Variant,
+	typename ...Methods>
+struct match_helper<type_list::list<Type>, Variant, Methods...> {
+	static decltype(auto) call(Variant &value, Methods &&...methods) {
+		if (const auto v = get_if<Type>(&value)) {
+			return match_method(
+				*v,
+				std::forward<Methods>(methods)...);
+		}
+		Unexpected("Valueless variant in base::match().");
+	}
+};
 
-template <typename Method, typename... Types>
-inline auto visit(Method &&method, variant<Types...> &value) {
-	return value.match(std::forward<Method>(method));
+template <typename ...Types, typename ...Methods>
+inline decltype(auto) match(
+		variant<Types...> &value,
+		Methods &&...methods) {
+	return match_helper<
+		type_list::list<Types...>,
+		variant<Types...>,
+		Methods...>::call(value, std::forward<Methods>(methods)...);
 }
 
-template <typename Method, typename... Types>
-inline auto visit(Method &&method, variant<Types...> &&value) {
-	return value.match(std::forward<Method>(method));
+template <typename ...Types, typename ...Methods>
+inline decltype(auto) match(
+		const variant<Types...> &value,
+		Methods &&...methods) {
+	return match_helper<
+		type_list::list<Types...>,
+		const variant<Types...>,
+		Methods...>::call(value, std::forward<Methods>(methods)...);
 }
 
 } // namespace base

Telegram/SourceFiles/codegen/scheme/codegen_scheme.py

@@ -635,7 +635,7 @@ def addTextSerializeInit(lst, dct):
 
     switchLines += '\tcase mtpc_' + name + ': '; # for by-type-id type constructor
     getters += '\tconst MTPD' + name + ' &c_' + name + '() const;\n'; # const getter
-    visitor += '\tcase mtpc_' + name + ': return VisitData(c_' + name + '(), std::forward<Callback>(callback), std::forward<Callbacks>(callbacks)...);\n';
+    visitor += '\tcase mtpc_' + name + ': return base::match_method(c_' + name + '(), std::forward<Method>(method), std::forward<Methods>(methods)...);\n';
 
     forwards += 'class MTPD' + name + ';\n'; # data class forward declaration
     if (len(prms) > len(trivialConditions)):
@@ -771,14 +771,14 @@ def addTextSerializeInit(lst, dct):
     typesText += getters;
     if (withType):
       typesText += '\n';
-      typesText += '\ttemplate <typename Callback, typename ...Callbacks>\n';
-      typesText += '\tdecltype(auto) visit(Callback &&callback, Callbacks &&...callbacks) const;\n';
-      visitorMethods += 'template <typename Callback, typename ...Callbacks>\n';
-      visitorMethods += 'decltype(auto) MTP' + restype + '::visit(Callback &&callback, Callbacks &&...callbacks) const {\n';
+      typesText += '\ttemplate <typename Method, typename ...Methods>\n';
+      typesText += '\tdecltype(auto) match(Method &&method, Methods &&...methods) const;\n';
+      visitorMethods += 'template <typename Method, typename ...Methods>\n';
+      visitorMethods += 'decltype(auto) MTP' + restype + '::match(Method &&method, Methods &&...methods) const {\n';
       visitorMethods += '\tswitch (_type) {\n';
       visitorMethods += visitor;
       visitorMethods += '\t}\n';
-      visitorMethods += '\tUnexpected("Type in MTP' + restype + '::visit.");\n';
+      visitorMethods += '\tUnexpected("Type in MTP' + restype + '::match.");\n';
       visitorMethods += '}\n\n';
 
   typesText += '\n\tuint32 innerLength() const;\n'; # size method