ir: make lower_expr apply the mapper to indices in case function only…

… appear as index

devito/ir/equations/algorithms.py

@@ -160,6 +160,9 @@ def lower_exprs(expressions, **kwargs):
         mapper.update(dimension_map)
         # Add the user-supplied substitutions
         mapper.update(subs)
+        # Apply mapper to itself for nested indexing
+        mapper = {k: uxreplace(v, mapper) for k, v in mapper.items()}
+        # Apply mapper to expression
         processed.append(uxreplace(expr, mapper))
 
     if isinstance(expressions, Iterable):

tests/test_operator.py

@@ -317,6 +317,26 @@ def test_nested_lowering(self):
         assert np.all(u0.data[:2, :2] == 1) and np.all(u0.data[1:3, 1:3] == 1)
         assert np.all(u0.data[2:3, 3] == 2) and np.all(u0.data[3, 2:3] == 2)
 
+    def test_nested_lowering_indexify(self):
+        """
+        Tests that nested function are lowered if only used as index.
+        """
+        grid = Grid(shape=(4, 4), dtype=np.int32)
+        x, y = grid.dimensions
+
+        u0 = Function(name="u0", grid=grid)
+        u1 = Function(name="u1", grid=grid, dtype=np.int32)
+
+        u0.data[:, :] = 2
+        u1.data[:, :] = 1
+
+        eq0 = Eq(u0._subs(x, u1), 2*u0)
+
+        op0 = Operator(eq0)
+        op0.apply()
+        assert np.all(np.all(u0.data[i, :] == 2) for i in [0,2,3])
+        assert np.all(u0.data[1, :] == 4)
+
 
 class TestArithmetic(object):