PyDolfin VariationalProblem does not provide a common_cell for forms with invalid cells

[Martin Sandve Alnæs]

Dolfin does not always pass a common_cell to jit if there are missing cells in the form.

Using this code:

from dolfin import *
parameters["form_compiler"]["name"] = "sfc"
mesh = UnitInterval(10)
V = FunctionSpace(mesh, "CG", 1)
v = TestFunction(V)
u = TrialFunction(V)
f = Expression("x[0]")
a = v*u*dx
L = v*f*dx
problem = VariationalProblem(a, L)

I printed arguments from sfc.jit:

common cell = None

str(form) =
{ v_{-2} * <Expression ()> } * dx0

repr(form) =
Form([Integral(Product(Argument(FiniteElement('Lagrange', Cell('interval', Space(1)), 1, None), -2), Coefficient(FiniteElement('Lagrange', Cell(None, None), None, None), 0)), Measure('cell', 0, None))])

Thus the form compiler must analyse the input form to detect whether PyDOLFIN has given an ill defined form and find the common cell itself before preprocess.

The above code works with ffc, from which I infer that ffc must detect a common cell.

Is this intended behaviour?

If it was up to me I would rip out common_cell once and for all, it leads to a lot of clutter...