Element mapping is not provided to ufl.Form.compute_form_data()

[Martin Sandve Alnæs]

FFC does not provide an element mapping to ufl.Form.compute_form_data(). This is needed when used from PyDOLFIN, to make forms well defined when coefficients are missing cells.

I have some failing code, will need to simplify it before I post it here.

[Martin Sandve Alnæs (martinal)]

The last line can be replaced by the slightly simpler
norm = assemble(grad(f)[0]*dx)
to get the same bug.

[Martin Sandve Alnæs (martinal)]

Actually, I would like to see PyDOLFIN construct the element mapping. It is dolfin that constructs the undefined cells, and it's just not right that the form compilers should have to work around that.

[Martin Sandve Alnæs (martinal)]

In an upcoming commit, the element mapping is now constructed automatically by ufl in preprocess in the case of invalid cells. Thus ffc (and sfc) do not need to pass a mapping unless you want to override things like degree and family.

Note that there may still be issues with the above code snippet, but I'll close this bug and report eventual other bugs separately if necessary.