dsl: Patch issue #1578 (Check legality of symbolic derivatives)

devito/finite_differences/derivative.py

@@ -312,6 +312,14 @@ def evaluate(self):
         # Evaluate finite-difference.
         # Note: evaluate and _eval_fd splitted for potential future different
         # types of discretizations.
+        do = self.deriv_order
+        if isinstance(do, int):
+            do = (do,)
+        for i, d in enumerate(self.dims):
+            expr_order = self.expr.time_order if d.is_Time else self.expr.space_order
+            if do[i] > expr_order:
+                raise ValueError("Expression order must be equal or greater than "
+                                 "Derivative order")
         return self._eval_fd(self.expr)
 
     @property

tests/test_derivatives.py

@@ -488,3 +488,16 @@ def test_shifted_grad(self, shift, ndim):
             x0 = (None if shift is None else d + shift[i] * d.spacing if
                   type(shift) is tuple else d + shift * d.spacing)
             assert gi == getattr(f, 'd%s' % d.name)(x0=x0).evaluate
+
+    def test_lower_order_derivatives(self):
+        grid = Grid(tuple([11]*2))
+        u = TimeFunction(name="u", grid=grid, space_order=2, time_order=2)
+        m = TimeFunction(name="m", grid=grid, space_order=1, time_order=1)
+        try:
+            Operator(Eq(u.forward, u.biharmonic(1/m)))
+            Operator(Eq(u.forward, (u * m).dx2))
+            Operator(Eq(u.forward, (u * m).dt2))
+        except ValueError:
+            assert True
+        else:
+            assert False