Throws error when calling add_linear_constraint for a quadratic expression

Author: metab0t

include/pyoptinterface/solver_common.hpp

@@ -38,6 +38,10 @@ class OnesideLinearConstraintMixin
 	                                                ConstraintSense sense, CoeffT rhs,
 	                                                const char *name = nullptr)
 	{
+		if (function.degree() >= 2)
+		{
+			throw std::runtime_error("add_linear_constraint expects linear expression but receives a quadratic expression.");
+		}
 		ScalarAffineFunction f(function);
 		return get_base()->add_linear_constraint(f, sense, rhs, name);
 	}

tests/test_simple_opt.py

@@ -1,5 +1,6 @@
 import pyoptinterface as poi
 from pytest import approx
+import pytest
 
 
 def test_simple_opt(model_interface):
@@ -134,3 +135,13 @@ def test_constraint_primal_dual(model_interface):
 
     dual_val = model.get_constraint_attribute(con1, poi.ConstraintAttribute.Dual)
     assert dual_val == approx(0.5)
+
+
+def test_add_quadratic_expr_as_linear_throws_error(model_interface):
+    model = model_interface
+
+    xs = model.add_m_variables(10)
+    x2_sum = poi.quicksum(x * x for x in xs.flat)
+
+    with pytest.raises(RuntimeError, match="add_linear_constraint"):
+        model.add_linear_constraint(x2_sum <= 1.0)