Using external function in nlpsol codegen

[LinusTxtonomy]

Hi,
I am building an optimization problem that should use external code as part of the objective function. I have built a small example that reproduces the error:

from casadi import *

x = MX.sym('x', 2)
slack = MX.sym('slack')

radius = MX.sym('radius')

J = x[0] + x[1]
obj_function = Function('objective', [x], [J])
obj_function.generate('J.c')
external_obj_function = external('objective', 'J.so', {'enable_fd': True})
J = external_obj_function(x)

g = x[0]**2 + x[1]**2 - radius + slack

prob = {'f': J, 'x': vertcat(x, slack), 'g': g, 'p': radius}

solver = nlpsol('solver', 'ipopt', prob, {'ipopt.print_level': 5})

solver.generate_dependencies('nlp.c')

res = solver(lbg = 0, ubg = 0, lbx = [-inf, -inf, 0], ubx = [inf, inf, inf], p = [4])
print(res)

The objective function code generates and builds without issues but I get compiler errors when trying to compile the generated nlp.c file even though the optimization problem runs fine inside the python example:

nlp.c: In function ‘nlp_smoothing_diff_err’:
nlp.c:211:55: error: ‘static_cast’ undeclared (first use in this function)
  211 |     wk = casadi_smoothing_diff_weights(k, yb, yc, yf, static_cast<casadi_real*>(0));
      |                                                       ^~~~~~~~~~~
nlp.c:211:55: note: each undeclared identifier is reported only once for each function it appears in
nlp.c:24:21: error: expected expression before ‘double’
   24 | #define casadi_real double
      |                     ^~~~~~
nlp.c:211:67: note: in expansion of macro ‘casadi_real’
  211 |     wk = casadi_smoothing_diff_weights(k, yb, yc, yf, static_cast<casadi_real*>(0));
      |                                                                   ^~~~~~~~~~~
nlp.c: In function ‘nlp_f6’:
nlp.c:812:39: error: expected ‘)’ before ‘;’ token
  812 |         if (mid = casadi_f1_checkout();
      |            ~                          ^
      |                                       )

If I manually fix the errors to atleast make the code compile I get this error when loading nlpsol with external code:

symbol lookup error: nlp.so: undefined symbol: objective_incref

How do I link external code to other external code?
I am using the latest casadi version 3.6.5
Regards,
Linus