Add default scalar type cast

[schnellerhase]

Fixes FEniCS/ufl#399.

Caused by default scalar type being complex in the ufl dolfinx integration test. In this case form, function and coefficients are constructed from default type (complex), but the boundary condition from a float value. This caused a mismatch.

Tested at https://github.com/FEniCS/ufl/actions/runs/16607427384/job/46982707793

[garth-wells]

Maybe fem.dirichletbc should be handling the cast. It's pretty natural to cast a float to complex.

[schnellerhase]

So, changing from extracting the dtype from the value passed to dirichletbc, to instead extracting it from the function space (on which we want to apply it) and casting the value (array) to its dtype?

[jorgensd]

So, changing from extracting the dtype from the value passed to dirichletbc, to instead extracting it from the function space (on which we want to apply it) and casting the value (array) to its dtype?

The function space doesn’t know the dtype? It is only the function we are applying it to that knows the dtype.

[schnellerhase]

Ah right, function space only knows the geometry dtype. Then I don't know what to change it to.

[jorgensd]

I guess this goes back to Constant not having a default dtype. I raised this a long time ago, and here is the discussion: #1997

[garth-wells]

Ah right, function space only knows the geometry dtype. Then I don't know what to change it to.

I'd suggest (i) moving bc closer to where it's used, and (ii) getting the dtype from the Form it acts on. default_scalar_type is something a bit ugly we need to live with but should use as little as possible.

[schnellerhase]

ufl CI rerun https://github.com/FEniCS/ufl/actions/runs/16607427384/job/47052249727.

[jorgensd]

Ah right, function space only knows the geometry dtype. Then I don't know what to change it to.

I'd suggest (i) moving bc closer to where it's used, and (ii) getting the dtype from the Form it acts on. default_scalar_type is something a bit ugly we need to live with but should use as little as possible.

That has to be slightly adapted to work with bc.set:

dolfinx/python/dolfinx/fem/bcs.py

Line 134 in d1caaf2

def set(