Raises an exception if Downcast fails

Author: tpaviot

src/SWIG_files/common/OccHandle.i

@@ -177,7 +177,12 @@ WRAP_OCC_TRANSIENT(const, TYPE)
     }
 
     opencascade::handle<TYPE>  Handle_ ## TYPE ## _DownCast(const opencascade::handle<Standard_Transient>& t) {
-        return opencascade::handle<TYPE>::DownCast(t);
+        opencascade::handle<TYPE> downcasted_hanle = opencascade::handle<TYPE>::DownCast(t);
+        if (downcasted_hanle.IsNull()) {
+            PyErr_SetString(PyExc_RuntimeError, "Failed to downcast to TYPE.");
+            return NULL;
+        }
+        return downcasted_hanle;
     }
 
     bool Handle_ ## TYPE ## _IsNull(const opencascade::handle<TYPE> & t) {

test/core_wrapper_features_unittest.py

@@ -606,8 +606,8 @@ def test_downcast_curve(self) -> None:
         line = Geom_Line.DownCast(curve)
         self.assertTrue(isinstance(line, Geom_Curve))
         # Hence, it should not be possible to downcast it as a B-Spline curve
-        bspline = Geom_BSplineCurve.DownCast(curve)
-        self.assertTrue(bspline is None)
+        with self.assertRaises(SystemError):
+            Geom_BSplineCurve.DownCast(curve)
 
     def test_return_enum(self) -> None:
         """Check that returned enums are properly handled, whether they're returned