ValueError when using check_bounds=False in a model with DiscreteUniform

[ricardoV94]

I was poking around to see if we should change the docstrings of check_bounds in pm.Model to mention this might not be safe in the presence of discrete variables and I found this strange error:

with pm.Model(check_bounds=False) as m:
    y = pm.DiscreteUniform('y', 0, 5)
    trace = pm.sample()

Traceback (most recent call last):
  File "/home/ricardo/Documents/Projects/pymc3-venv/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3418, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-60-ca8c3478b1dc>", line 3, in <module>
    trace = pm.sample()
  File "/home/ricardo/Documents/Projects/pymc3/pymc3/sampling.py", line 515, in sample
    step = assign_step_methods(model, step, step_kwargs=kwargs)
  File "/home/ricardo/Documents/Projects/pymc3/pymc3/sampling.py", line 217, in assign_step_methods
    return instantiate_steppers(model, steps, selected_steps, step_kwargs)
  File "/home/ricardo/Documents/Projects/pymc3/pymc3/sampling.py", line 138, in instantiate_steppers
    step = step_class(vars=vars, **args)
  File "/home/ricardo/Documents/Projects/pymc3/pymc3/step_methods/metropolis.py", line 187, in __init__
    self.delta_logp = delta_logp(model.logpt, vars, shared)
  File "/home/ricardo/Documents/Projects/pymc3/pymc3/step_methods/metropolis.py", line 888, in delta_logp
    logp1 = pm.CallableTensor(logp0)(inarray1)
  File "/home/ricardo/Documents/Projects/pymc3/pymc3/aesaraf.py", line 308, in __call__
    (oldinput,) = inputvars(self.tensor)
ValueError: not enough values to unpack (expected 1, got 0)

I don't see why changing the check_bounds flag would lead to any issues in sampling. I can call the logp method for any value and I get a finite (and wrong) evaluation as would be expected from removing the bounds:

np.array([m.logp({'y': y}) for y in np.arange(-10, 10)])
array([-1.79175947, -1.79175947, -1.79175947, -1.79175947, -1.79175947,
       -1.79175947, -1.79175947, -1.79175947, -1.79175947, -1.79175947,
       -1.79175947, -1.79175947, -1.79175947, -1.79175947, -1.79175947,
       -1.79175947, -1.79175947, -1.79175947, -1.79175947, -1.79175947])

I tried with a couple of other Discrete Variables and I had no issues either. Obviously it works as expected with the flag set to True

with pm.Model(check_bounds=True) as m:
    y = pm.DiscreteUniform('y', 0, 5)
    trace = pm.sample()