In [38]: a = ak.from_iter([[1, 2, 3], [4], [5, 6]])

In [39]: tt = ak.Array(a.layout.to_typetracer(forget_length=True))

In [40]: ak.num(tt, axis=1)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[40], line 1
----> 1 ak.num(tt, axis=1)

File ~/.pyenv/versions/3.11.1/envs/dev/lib/python3.11/site-packages/awkward/operations/ak_num.py:71, in num(array, axis, highlevel, behavior)
      9 """
     10 Args:
     11     array: Array-like data (anything #ak.to_layout recognizes).
   (...)
     65     <Array [[1.1, 2.2, 3.3], None, [7.7]] type='3 * option[var * float64]'>
     66 """
     67 with ak._errors.OperationErrorContext(
     68     "ak.num",
     69     dict(array=array, axis=axis, highlevel=highlevel, behavior=behavior),
     70 ):
---> 71     return _impl(array, axis, highlevel, behavior)

File ~/.pyenv/versions/3.11.1/envs/dev/lib/python3.11/site-packages/awkward/operations/ak_num.py:100, in _impl(array, axis, highlevel, behavior)
     95     elif layout.is_leaf:
     96         raise ak._errors.wrap_error(
     97             np.AxisError(f"axis={axis} exceeds the depth of this array ({depth})")
     98         )
--> 100 out = ak._do.recursively_apply(layout, action, behavior, numpy_to_regular=True)
    102 return ak._util.wrap(out, behavior, highlevel)

File ~/.pyenv/versions/3.11.1/envs/dev/lib/python3.11/site-packages/awkward/_do.py:33, in recursively_apply(layout, action, behavior, depth_context, lateral_context, allow_records, keep_parameters, numpy_to_regular, return_simplified, return_array, function_name)
     18 def recursively_apply(
     19     layout: Content | Record,
     20     action: ActionType,
   (...)
     29     function_name: str | None = None,
     30 ) -> Content | Record | None:
     32     if isinstance(layout, Content):
---> 33         return layout._recursively_apply(
     34             action,
     35             behavior,
     36             1,
     37             copy.copy(depth_context),
     38             lateral_context,
     39             {
     40                 "allow_records": allow_records,
     41                 "keep_parameters": keep_parameters,
     42                 "numpy_to_regular": numpy_to_regular,
     43                 "return_simplified": return_simplified,
     44                 "return_array": return_array,
     45                 "function_name": function_name,
     46             },
     47         )
     49     elif isinstance(layout, Record):
     50         out = recursively_apply(
     51             layout._array,
     52             action,
   (...)
     61             function_name,
     62         )

File ~/.pyenv/versions/3.11.1/envs/dev/lib/python3.11/site-packages/awkward/contents/listoffsetarray.py:1996, in ListOffsetArray._recursively_apply(self, action, behavior, depth, depth_context, lateral_context, options)
   1986     def continuation():
   1987         content._recursively_apply(
   1988             action,
   1989             behavior,
   (...)
   1993             options,
   1994         )
-> 1996 result = action(
   1997     self,
   1998     depth=depth,
   1999     depth_context=depth_context,
   2000     lateral_context=lateral_context,
   2001     continuation=continuation,
   2002     behavior=behavior,
   2003     backend=self._backend,
   2004     options=options,
   2005 )
   2007 if isinstance(result, Content):
   2008     return result

File ~/.pyenv/versions/3.11.1/envs/dev/lib/python3.11/site-packages/awkward/operations/ak_num.py:93, in _impl.<locals>.action(layout, depth, **kwargs)
     90 posaxis = ak._util.maybe_posaxis(layout, axis, depth)
     92 if posaxis == depth and layout.is_list:
---> 93     return ak.contents.NumpyArray(layout.stops.data - layout.starts.data)
     95 elif layout.is_leaf:
     96     raise ak._errors.wrap_error(
     97         np.AxisError(f"axis={axis} exceeds the depth of this array ({depth})")
     98     )

TypeError: unsupported operand type(s) for -: 'TypeTracerArray' and 'TypeTracerArray'