api: prevent factorization for symbolic coefficients

devito/passes/equations/linearity.py

@@ -187,6 +187,11 @@ def _(expr):
     if not derivs:
         return reuse_if_untouched(expr, args)
 
+    # Cannot factorize derivatives with symbolic coefficients since
+    # they may have different coefficient values at evaluation
+    if any(d._uses_symbolic_coefficients for d in derivs):
+        return reuse_if_untouched(expr, args)
+
     # Map by type of derivative
     # Note: `D0(a) + D1(b) == D(a + b)` <=> `D0` and `D1`'s metadata match,
     # i.e. they are the same type of derivative

tests/test_symbolic_coefficients.py

@@ -6,6 +6,7 @@
                     Dimension, solve, Operator, NODE)
 from devito.finite_differences import Differentiable
 from devito.tools import as_tuple
+from devito.passes.equations.linearity import factorize_derivatives
 
 _PRECISION = 9
 
@@ -344,3 +345,17 @@ def test_with_timefunction(self, stagger):
         Operator([eq_f, eq_g])(t_m=0, t_M=1)
 
         assert np.allclose(f.data[-1], -g.data[-1], atol=1e-7)
+
+    def test_collect_w_custom_coeffs(self):
+        grid = Grid(shape=(11, 11, 11))
+        p = TimeFunction(name='p', grid=grid, space_order=8, time_order=2,
+                         coefficients='symbolic')
+
+        q = TimeFunction(name='q', grid=grid, space_order=8, time_order=2,
+                         coefficients='symbolic')
+
+        expr = p.dx2 + q.dx2
+        collected = factorize_derivatives(expr)
+        assert collected == expr
+        assert collected.is_Add
+        Operator([Eq(p.forward, expr)])(time_M=2)  # noqa