Refs #14121. Forgot to make method const.

Framework/Geometry/inc/MantidGeometry/Crystal/PointGroup.h

@@ -105,7 +105,7 @@ getLatticeSystemFromString(const std::string &latticeSystem);
 /// when GCC 4.4 is not used anymore.
 struct MANTID_GEOMETRY_DLL CrystalSystemComparator {
   bool operator()(const PointGroup::CrystalSystem &lhs,
-                  const PointGroup::CrystalSystem &rhs);
+                  const PointGroup::CrystalSystem &rhs) const;
 };
 
 typedef std::multimap<PointGroup::CrystalSystem, PointGroup_sptr,

Framework/Geometry/src/Crystal/PointGroup.cpp

@@ -327,8 +327,9 @@ getLatticeSystemFromString(const std::string &latticeSystem) {
   }
 }
 
-bool CrystalSystemComparator::operator()(const PointGroup::CrystalSystem &lhs,
-                                         const PointGroup::CrystalSystem &rhs) {
+bool CrystalSystemComparator::
+operator()(const PointGroup::CrystalSystem &lhs,
+           const PointGroup::CrystalSystem &rhs) const {
   return static_cast<int>(lhs) < static_cast<int>(rhs);
 }