Error: Unable to evaluate expression (SkewedFlow)

[benzwick]

The SkewedFlow example fails with:

Traceback (most recent call last):
  File "NSfracStep.py", line 178, in <module>
    assemble_first_inner_iter(**vars())
  File "/home/ben/projects/oasis/Oasis-BZ/oasis/solvers/NSfracStep/IPCS_ABCN.py", line 204, in assemble_first_inner_iter
    [bc.apply(A) for bc in bcs['u0']]
  File "/home/ben/projects/oasis/Oasis-BZ/oasis/solvers/NSfracStep/IPCS_ABCN.py", line 204, in <listcomp>
    [bc.apply(A) for bc in bcs['u0']]
RuntimeError:

*** -------------------------------------------------------------------------
*** DOLFIN encountered an error. If you are not able to resolve this issue
*** using the information listed below, you can ask for help at
***
***     fenics-support@googlegroups.com
***
*** Remember to include the error message listed below and, if possible,
*** include a *minimal* running example to reproduce the error.
***
*** -------------------------------------------------------------------------
*** Error:   Unable to evaluate expression.
*** Reason:  Missing eval() function (must be overloaded).
*** Where:   This error was encountered inside Expression.cpp.
*** Process: 0
***
*** DOLFIN version: 2018.1.0
*** Git changeset:  7afe9e1a24edb0e8d5ae51c3419fbb49f44d3d79
*** -------------------------------------------------------------------------

but eval is defined here:

Oasis/oasis/problems/NSfracStep/SkewedFlow.py

Lines 51 to 57 in 3de552c

	# Wrap the boundary function in an Expression to avoid the need to interpolate it back to V
	class MyExp(UserExpression):
	def eval(self, values, x):
	try:
	values[0] = su(x)
	except:
	values[0] = 0

This could be a DOLFIN bug.

[mikaem]

Yes, probably a DOLFIN bug in DirichletBC. It seems to work with

m = project(MyExp(element=V.ufl_element()), V)
bc0 = DirichletBC(V, 0, walls)
bc1 = DirichletBC(V, m, inlet)