Description: GCC 11 now enforces that comparison objects be invocable as const.
--- a/Rendering/Label/vtkLabelHierarchy.cxx
+++ b/Rendering/Label/vtkLabelHierarchy.cxx
@@ -525,1 +525,1 @@
-                    const vtkHierarchyNode & b)
+                    const vtkHierarchyNode & b) const
--- a/Rendering/Label/vtkLabelHierarchyPrivate.h
+++ b/Rendering/Label/vtkLabelHierarchyPrivate.h
@@ -66,1 +66,1 @@
-    bool operator () ( const vtkIdType& a, const vtkIdType& b )
+    bool operator () ( const vtkIdType& a, const vtkIdType& b ) const
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