compiler: Fix unexpansion w custom coeffs

devito/finite_differences/coefficients.py

@@ -1,7 +1,7 @@
 import numpy as np
 from cached_property import cached_property
 
-from devito.finite_differences import generate_indices
+from devito.finite_differences import Weights, generate_indices
 from devito.finite_differences.tools import numeric_weights, symbolic_weights
 from devito.tools import filter_ordered, as_tuple
 
@@ -268,8 +268,13 @@ def generate_subs(deriv_order, function, index):
         return subs
 
     # Determine which 'rules' are missing
+
     sym = get_sym(functions)
     terms = obj.find(sym)
+    for i in obj.find(Weights):
+        for w in i.weights:
+            terms.update(w.find(sym))
+
     args_present = filter_ordered(term.args[1:] for term in terms)
 
     subs = obj.substitutions

devito/finite_differences/differentiable.py

@@ -637,6 +637,15 @@ def spacings(self):
 
     weights = Array.initvalue
 
+    def _xreplace(self, rule):
+        if self in rule:
+            return rule[self]
+        elif not rule:
+            return self
+        else:
+            weights, changed = zip(*[i._xreplace(rule) for i in self.weights])
+            return self.func(initvalue=weights, function=None), any(changed)
+
 
 class IndexDerivative(IndexSum):
 

tests/test_unexpansion.py

@@ -21,6 +21,23 @@ def test_backward_dt2(self):
         assert_structure(op, ['t,x,y'], 't,x,y')
 
 
+class TestSymbolicCoefficients(object):
+
+    def test_fallback_to_default(self):
+        grid = Grid(shape=(8, 8, 8))
+
+        u = TimeFunction(name='u', grid=grid, coefficients='symbolic',
+                         space_order=4, time_order=2)
+
+        eq = Eq(u.forward, u.dx2 + 1)
+
+        op = Operator(eq, opt=('advanced', {'expand': False}))
+
+        # Ensure all symbols have been resolved
+        op.arguments(dt=1, time_M=10)
+        op.cfunction
+
+
 class Test1Pass(object):
 
     def test_v0(self):