Ensure compile_ufl output has correct free indices

When an expression given to compile_ufl does not have points in the
input PointSet in its expression tree, the resulting gem expression has
missing free indices. This attempts to fix that.

tsfc/driver.py

@@ -377,7 +377,11 @@ def compile_expression_dual_evaluation(expression, to_element, *,
             quad_rule = QuadratureRule(point_set, to_element._weights)
             config["quadrature_rule"] = quad_rule
 
+        # evaluate expression at the points specified by point_set
         expr, = fem.compile_ufl(expression, **config, point_sum=False)
+        # the point set free indices should now be free indices of the complied
+        # expression
+        assert all(i in expr.free_indices for i in point_set.indices)
         shape_indices = tuple(gem.Index() for _ in expr.shape)
         basis_indices = point_set.indices
         ir = gem.Indexed(expr, shape_indices)
@@ -395,7 +399,11 @@ def compile_expression_dual_evaluation(expression, to_element, *,
                 point_set = PointSet(pts)
                 config = kernel_cfg.copy()
                 config.update(point_set=point_set)
+                # evaluate expression at the points specified by point_set
                 expr, = fem.compile_ufl(expression, **config, point_sum=False)
+                # the point set free indices should now be free indices of the
+                # complied expression
+                assert all(i in expr.free_indices for i in point_set.indices)
                 expr = gem.partial_indexed(expr, shape_indices)
                 expr_cache[pts] = expr, point_set
             weights = collections.defaultdict(list)

tsfc/fem.py

@@ -697,6 +697,15 @@ def compile_ufl(expression, interior_facet=False, point_sum=False, **kwargs):
 
     # Translate UFL to GEM, lowering finite element specific nodes
     result = map_expr_dags(context.translator, expressions)
+    # Check that any indices from the PointSet have made it into the resulting expression
+    import functools
+    import operator
+    for i in range(len(result)):
+        expansion_indices = tuple(j for j in context.point_indices if j not in result[i].free_indices)
+        # blow up shape to include expansion_indices using deltas - these will
+        # eventually be cancelled out but ensure the our result has the expected
+        # free_indices and shape.
+        result[i] = functools.reduce(operator.mul, (gem.Delta(j, j) for j in expansion_indices), result[i])
     if point_sum:
         result = [gem.index_sum(expr, context.point_indices) for expr in result]
     return result