From c23f6a92218bd6191a48dec3b427dd0c869befa7 Mon Sep 17 00:00:00 2001
From: Florian Foinant-Willig <flachyjoe@gmail.com>
Date: Mon, 20 Mar 2023 08:30:12 +0100
Subject: [PATCH] Fix circles distance constraint for concentrics

---
 src/Mod/Sketcher/App/planegcs/Constraints.cpp | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/Mod/Sketcher/App/planegcs/Constraints.cpp b/src/Mod/Sketcher/App/planegcs/Constraints.cpp
index c04e54ef01..3a628de3a6 100644
--- a/src/Mod/Sketcher/App/planegcs/Constraints.cpp
+++ b/src/Mod/Sketcher/App/planegcs/Constraints.cpp
@@ -2666,8 +2666,11 @@ void ConstraintC2CDistance::errorgrad(double *err, double *grad, double *param)
             else if(param == distance()) {
                 drad = (*distance()<0.)?1.0:-1.0;
             }
-
-            *grad = -dlength_ct12 + drad;
+            if (length_ct12>1e-13) {
+                *grad = -dlength_ct12 + drad;
+            } else { // concentric case
+                *grad = drad;
+            }
         }
     }
 }