ufl >= 2024 and Constant having a domain.

[dr-robertk]

In dune-fem we have not been able to upgrade to ufl beyond 22.2.0 because of the Constant (now having a domain) and the domain check being enforced in domain.py:221: (join_domains).

In our case this is, for example, the case when a Constant representing the time step dt is used in two different forms.

I think this is also related to the discussion in #267.

The following code triggers the problem:

import basix.ufl
import ufl

# Bulk problem: u - tau laplace u = u_n + tau f.
# but only setting up right hand side forcing 'tau f'
#
bc_el = basix.ufl.element("Lagrange", "triangle", 1, shape=(2, ))
bdomain = ufl.Mesh(bc_el)

# issue here: constant defined on bulk domain
tau = ufl.Constant(bdomain)

bel = basix.ufl.element("Lagrange", "triangle", 1)
bV = ufl.FunctionSpace(bdomain, bel)
bL = tau*ufl.inner(ufl.Coefficient(bV), ufl.TestFunction(bV)) * ufl.dx

#############################################################

# Surface problem: w - tau laplace w = w_n + tau g
# but only setting up right hand side forcing 'tau g'
sc_el = basix.ufl.element("Lagrange", "triangle", 1, shape=(3, ))
sdomain = ufl.Mesh(sc_el)

# issue here: can't use constant from bulk problem - using 'sdomain' works
# tau = ufl.Constant(sdomain)

sel = basix.ufl.element("Lagrange", "triangle", 1)
sV = ufl.FunctionSpace(sdomain, sel)

# This line results in
# ValueError: Found domains with different geometric dimensions.
sL = tau*ufl.inner(ufl.Coefficient(sV), ufl.TestFunction(sV)) * ufl.dx