From 8a317e06de88779c60f54da424f482f40199b113 Mon Sep 17 00:00:00 2001
From: Stephen Nicholas Swatman <stephen.nicholas.swatman@cern.ch>
Date: Fri, 12 Jan 2024 22:47:22 +0100
Subject: [PATCH] Fix SFINAE bug in gcc 11

Versions of gcc were incorrectly reporting errors in the `nth_backend`
helper function, even though it is (as far as I can tell) correctly
defined. This commit adds some failsafes.
---
 lib/core/covfie/core/utility/backend_traits.hpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/core/covfie/core/utility/backend_traits.hpp b/lib/core/covfie/core/utility/backend_traits.hpp
index d121182..7b3acf1 100644
--- a/lib/core/covfie/core/utility/backend_traits.hpp
+++ b/lib/core/covfie/core/utility/backend_traits.hpp
@@ -10,6 +10,7 @@
 
 #pragma once
 
+#include <variant>
 #include <cstddef>
 
 namespace covfie::utility {
@@ -30,6 +31,7 @@ struct nth_backend<B, 0, true> {
 
 template <typename B, std::size_t N>
 struct nth_backend<B, N, true> {
+    using type = std::monostate;
 };
 
 template <typename B, bool = B::is_initial>