https://github.com/coin-or/Couenne/pull/57
https://github.com/coin-or/Couenne/commit/fdfe17562c59518b4db478ca51c18ab5e01a59a5 (later found post-patch)

https://bugs.gentoo.org/792798

From debc5de2d0ac9654c01db080448df064b808c56e Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Tue, 15 Jun 2021 04:24:22 +0000
Subject: [PATCH] Fix build with GCC 11
diff --git a/src/problem/CouenneProblem.hpp b/src/problem/CouenneProblem.hpp
index e5b54a5..db28cfd 100644
--- a/src/problem/CouenneProblem.hpp
+++ b/src/problem/CouenneProblem.hpp
@@ -74,7 +74,7 @@ class Nauty;
 #define COUENNE_EPS_SYMM 1e-8
 
   struct myclass0 {
-    inline bool operator() (register const Node &a, register const Node &b) {
+    inline bool operator() (register const Node &a, register const Node &b) const {
 
       return ((               a.get_code  () <  b.get_code  ())                     ||
 	      ((              a.get_code  () == b.get_code  ()                      &&
@@ -120,7 +120,7 @@ class Nauty;
     
       
   struct myclass {
-    inline bool operator() (register const  Node &a, register const Node &b) {
+    inline bool operator() (register const  Node &a, register const Node &b) const {
       return (a.get_index() < b.get_index() );
     }
   };

--- a/src/cut/sdpcuts/CouenneMatrix.hpp
+++ b/src/cut/sdpcuts/CouenneMatrix.hpp
@@ -69,7 +69,7 @@ namespace Couenne {
 
     struct compare_scalars {
       inline bool operator() (register CouenneScalar * const &a, 
-			      register CouenneScalar * const &b)
+			      register CouenneScalar * const &b) const
       {return a -> getIndex () < b -> getIndex ();}
     };
 
-- 
2.32.0