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.

gem/gem.py

@@ -792,10 +792,6 @@ def __new__(cls, i, j):
         assert isinstance(i, IndexBase)
         assert isinstance(j, IndexBase)
 
-        # \delta_{i,i} = 1
-        if i == j:
-            return one
-
         # Fixed indices
         if isinstance(i, int) and isinstance(j, int):
             return Literal(int(i == j))

tsfc/fem.py

@@ -699,4 +699,17 @@ def compile_ufl(expression, interior_facet=False, point_sum=False, **kwargs):
     result = map_expr_dags(context.translator, expressions)
     if point_sum:
         result = [gem.index_sum(expr, context.point_indices) for expr in result]
+    # Check that any indices from the PointSet have made it into the resulting expression
+    for i in range(len(result)):
+        expansion_indices = ()
+        for index in context.point_indices:
+            if index not in result[i].free_indices:
+                expansion_indices = expansion_indices + (index,)
+        if expansion_indices != ():
+            # blow up shape to include expansion_indices
+            deltas = gem.Delta(expansion_indices[0], expansion_indices[0])
+            # Use deltas to increase the possible rank of our expression
+            for index in expansion_indices[1:]:
+                deltas *= gem.Delta(index, index)
+            result[i] = gem.Indexed(gem.ComponentTensor(result[i] * deltas, expansion_indices), expansion_indices)
     return result