Fix a bug when deleting constraint in HiGHS

Author: metab0t

lib/highs_model.cpp

@@ -302,6 +302,8 @@ void POIHighsModel::delete_constraint(const ConstraintIndex &constraint)
 
 	m_linear_constraint_index.delete_index(constraint.index);
 	m_con_names.erase(constraint.index);
+
+	m_n_constraints--;
 }
 
 bool POIHighsModel::is_constraint_active(const ConstraintIndex &constraint)

tests/test_simple_opt.py

@@ -19,6 +19,9 @@ def test_simple_opt(model_interface):
         conexpr, poi.ConstraintSense.GreaterEqual, 10.0, name="con1"
     )
 
+    assert model.number_of_variables() == 2
+    assert model.number_of_constraints(poi.ConstraintType.Linear) == 1
+
     model.optimize()
     status = model.get_model_attribute(poi.ModelAttribute.TerminationStatus)
     assert status == poi.TerminationStatusCode.OPTIMAL
@@ -39,7 +42,9 @@ def test_simple_opt(model_interface):
     assert model.pprint(conexpr) == "1*x+1*y"
 
     model.delete_constraint(con1)
+    assert model.number_of_constraints(poi.ConstraintType.Linear) == 0
     con2 = model.add_linear_constraint(conexpr, poi.ConstraintSense.GreaterEqual, 20.0)
+    assert model.number_of_constraints(poi.ConstraintType.Linear) == 1
     model.optimize()
     status = model.get_model_attribute(poi.ModelAttribute.TerminationStatus)
     assert status == poi.TerminationStatusCode.OPTIMAL