api: prevent derivative shortcut with incompatible fd order

devito/finite_differences/differentiable.py

@@ -105,7 +105,15 @@ def is_TimeDependent(self):
 
     @cached_property
     def _fd(self):
-        return dict(ChainMap(*[getattr(i, '_fd', {}) for i in self._args_diff]))
+        # Filter out all args with fd order too high
+        fd_args = []
+        for f in self._args_diff:
+            try:
+                if f.space_order <= self.space_order and f.time_order <= self.time_order:
+                    fd_args.append(f)
+            except AttributeError:
+                pass
+        return dict(ChainMap(*[getattr(i, '_fd', {}) for i in fd_args]))
 
     @cached_property
     def _symbolic_functions(self):

tests/test_derivatives.py

@@ -457,6 +457,21 @@ def test_all_shortcuts(self, so):
         for fd in g._fd:
             assert getattr(g, fd)
 
+        for d in grid.dimensions:
+            assert 'd%s' % d.name in f._fd
+            assert 'd%s' % d.name in g._fd
+            for o in range(2, min(7, so+1)):
+                assert 'd%s%s' % (d.name, o) in f._fd
+                assert 'd%s%s' % (d.name, o) in g._fd
+
+    def test_shortcuts_mixed(self):
+        grid = Grid(shape=(10,))
+        f = Function(name='f', grid=grid, space_order=2)
+        g = Function(name='g', grid=grid, space_order=4)
+        assert 'dx4' not in (f*g)._fd
+        assert 'dx4' not in (f+g)._fd
+        assert 'dx4' not in (g*f.dx)._fd
+
     def test_transpose_simple(self):
         grid = Grid(shape=(4, 4))