From 96a26b44a9bd6512dd916a388c367201bd1c9984 Mon Sep 17 00:00:00 2001
From: John Preston <johnprestonmail@gmail.com>
Date: Fri, 5 Jul 2019 17:08:33 +0200
Subject: [PATCH] Fix build for Xcode.

---
 Telegram/SourceFiles/mtproto/core_types.h | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/Telegram/SourceFiles/mtproto/core_types.h b/Telegram/SourceFiles/mtproto/core_types.h
index 937dd38ced..ffedf9cd7a 100644
--- a/Telegram/SourceFiles/mtproto/core_types.h
+++ b/Telegram/SourceFiles/mtproto/core_types.h
@@ -376,6 +376,15 @@ struct RepeatHelper {
 template <typename Type>
 using Repeat = typename RepeatHelper<Type>::type;
 
+struct InnerHelper {
+	static void Check(...);
+	template <typename Type, typename Result = decltype(std::declval<Type>().v)>
+	static Result Check(const Type&);
+
+	template <typename Type>
+	using type = std::decay_t<decltype(Check(std::declval<Type>()))>;
+};
+
 template <typename Type>
 class conditional {
 public:
@@ -397,12 +406,16 @@ public:
 		return *_value;
 	}
 
-	template <typename Inner = std::decay_t<decltype(std::declval<Type>().v)>>
+	template <
+		typename Inner = InnerHelper::type<Type>,
+		typename = std::enable_if_t<!std::is_same_v<Inner, void>>>
 	Inner value_or(Repeat<Inner> fallback) const {
 		return _value ? _value->v : fallback;
 	}
 
-	template <typename Inner = std::decay_t<decltype(std::declval<Type>().v)>>
+	template <
+		typename Inner = InnerHelper::type<Type>,
+		typename = std::enable_if_t<!std::is_same_v<Inner, void>>>
 	Inner value_or_empty() const {
 		return _value ? _value->v : Inner();
 	}