Use mapbox::util::variant in a submodule.

While we don't have C++17 std::variant<> use a BSD licensed
alternative from mapbox team in a base:: wrapper.

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "third_party/GSL"]
 	path = third_party/GSL
 	url = https://github.com/Microsoft/GSL.git
+[submodule "third_party/variant"]
+	path = third_party/variant
+	url = https://github.com/mapbox/variant

README.md

@@ -32,6 +32,7 @@ The source code is published under GPLv3 with OpenSSL exception, the license is
 * Opus codec ([BSD license](http://www.opus-codec.org/license/))
 * FFmpeg ([LGPL](https://www.ffmpeg.org/legal.html))
 * Guideline Support Library ([MIT License](https://github.com/Microsoft/GSL/blob/master/LICENSE))
+* Mapbox Variant ([BSD license](https://github.com/mapbox/variant/blob/master/LICENSE))
 * Open Sans font ([Apache License 2.0](http://www.apache.org/licenses/LICENSE-2.0.html))
 
 ## Build instructions

Telegram/SourceFiles/core/variant.h

@@ -0,0 +1,36 @@
+/*
+This file is part of Telegram Desktop,
+the official desktop version of Telegram messaging app, see https://telegram.org
+
+Telegram Desktop 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, either version 3 of the License, or
+(at your option) any later version.
+
+It 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.
+
+In addition, as a special exception, the copyright holders give permission
+to link the code of portions of this program with the OpenSSL library.
+
+Full license: https://github.com/telegramdesktop/tdesktop/blob/master/LICENSE
+Copyright (c) 2014-2017 John Preston, https://desktop.telegram.org
+*/
+#pragma once
+
+#include <mapbox/variant.hpp>
+
+// We use base::variant<> alias and base::get_if() helper while we don't have std::variant<>.
+namespace base {
+
+template <typename... Types>
+using variant = mapbox::util::variant<Types...>;
+
+template <typename T, typename... Types>
+inline T *get_if(variant<Types...> *v) {
+	return (v && v->is<T>()) ? &v->get_unchecked<T>() : nullptr;
+}
+
+} // namespace base

Telegram/gyp/Telegram.gyp

@@ -96,6 +96,7 @@
       '<(minizip_loc)',
       '<(sp_media_key_tap_loc)',
       '<(submodules_loc)/GSL/include',
+      '<(submodules_loc)/variant/include',
     ],
     'sources': [
       '<@(qrc_files)',
@@ -178,6 +179,7 @@
       '<(src_loc)/core/type_traits.h',
       '<(src_loc)/core/utils.cpp',
       '<(src_loc)/core/utils.h',
+      '<(src_loc)/core/variant.h',
       '<(src_loc)/core/version.h',
       '<(src_loc)/core/virtual_method.h',
       '<(src_loc)/core/zlib_help.h',

third_party/variant

@@ -0,0 +1 @@
+Subproject commit 916139a2e51e125816efce6e19d428385601273f