api: fix backward compatibility of Substitution, missing function check

devito/finite_differences/derivative.py

@@ -307,7 +307,8 @@ def _xreplace(self, subs):
         if self in subs:
             new = subs.pop(self)
             try:
-                return new._xreplace(subs)
+                new, flag = new._xreplace(subs)
+                return new, True
             except AttributeError:
                 return new, True
 

devito/finite_differences/tools.py

@@ -265,7 +265,7 @@ def generate_indices(expr, dim, order, side=None, matvec=None, x0=None, nweights
     if nweights > 0:
         do, dw = order + 1 + order % 2, nweights
         if do < dw:
-            raise ValueError(f"More weights ({nweights}) provided than the maximum"
+            raise ValueError(f"More weights ({nweights}) provided than the maximum "
                              f"stencil size ({order + 1}) for order {order} scheme")
         elif do > dw:
             warning(f"Less weights ({nweights}) provided than the stencil size"

devito/types/equation.py

@@ -90,6 +90,7 @@ def _apply_coeffs(cls, expr, coefficients):
         for coeff in coefficients.coefficients:
             derivs = [d for d in retrieve_derivatives(expr)
                       if coeff.dimension in d.dims and
+                      coeff.function in d.expr._functions and
                       coeff.deriv_order == d.deriv_order.get(coeff.dimension, None)]
             if not derivs:
                 continue

tests/test_symbolic_coefficients.py

@@ -344,3 +344,31 @@ def test_spacing(self):
         df_s = f.dx(weights=coeffs1)
 
         assert sp.simplify(df_s.evaluate - df.evaluate) == 0
+
+    def test_backward_compat_mixed(self):
+
+        grid = Grid(shape=(11,))
+        x, = grid.dimensions
+
+        f = Function(name='f', grid=grid, space_order=8)
+        g = Function(name='g', grid=grid, space_order=2)
+
+        coeffs0 = np.arange(0, 9)
+
+        coeffs = Coefficient(1, f, x, coeffs0)
+
+        eq = Eq(f, f.dx * g.dxc, coefficients=Substitutions(coeffs))
+
+        derivs = retrieve_derivatives(eq.rhs)
+
+        assert len(derivs) == 2
+        df = [d for d in derivs if d.expr == f][0]
+        dg = [d for d in derivs if d.expr == g][0]
+
+        assert np.all(df.weights == coeffs0)
+        assert dg.weights is None
+
+        eqe = eq.evaluate
+        assert '7.0*f(x + 3*h_x)' in str(eqe.rhs)
+        assert '0.5*g(x + h_x)' in str(eqe.rhs)
+        assert 'g(x + 2*h_x)' not in str(eqe.rhs)