From 0c5a66b6661b35040ee9dc0ea443e059dbfcc13a Mon Sep 17 00:00:00 2001
From: Herb Sutter <herb.sutter@gmail.com>
Date: Fri, 18 Dec 2020 12:02:42 -0800
Subject: [PATCH] Added workaround for non-constexpr `std::exchange`

---
 include/gsl/assert | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/gsl/assert b/include/gsl/assert
index a57d55d..8f71490 100644
--- a/include/gsl/assert
+++ b/include/gsl/assert
@@ -114,7 +114,15 @@ public:
 #else
     constexpr contract_group  (handler h)            : chandler(h ? h : []()noexcept{}) { }
 #endif
+
+#if __cplusplus >= 202002L
     constexpr auto set_handler(handler h) -> handler { return std::exchange(chandler, h ? h : []()noexcept{}); }
+#else
+    constexpr auto set_handler(handler h) -> handler { auto old = chandler;
+                                                       chandler = h ? h : []()noexcept{};
+                                                       return old; 
+                                                     }
+#endif
     constexpr auto get_handler()          -> handler { return chandler; }
 
     constexpr void expects  (bool b) { assertion(b); }