ak.concatenate with axis=1 fails for more than 127 regular-length arrays

[jpivarski]

Version of Awkward Array

HEAD

Description and code to reproduce

>>> ak.concatenate([ak.Array([i])[:, np.newaxis] for i in range(127)], axis=1)
<Array [[0, 1, 2, 3, 4, ..., 122, 123, 124, 125, 126]] type='1 * 127 * int64'>

but

>>> ak.concatenate([ak.Array([i])[:, np.newaxis] for i in range(128)], axis=1)
Traceback (most recent call last):
...
KeyError: ('awkward_UnionArray_regular_index_getsize', <class 'numpy.int64'>, <class 'numpy.int64'>)

This error occurred while calling

    ak.concatenate(
        [<Array [[0]] type='1 * 1 * int64'>, <Array [[1]] type='1 * 1 * int64...
        axis = 1
    )

[jpivarski]

ak.concatenate is going into UnionArray here:

awkward/src/awkward/operations/ak_concatenate.py

Line 278 in 4db7973

index = ak.contents.UnionArray.regular_index(tags, backend=backend)

which is part of this if-condition:

awkward/src/awkward/operations/ak_concatenate.py

Lines 233 to 238 in 4db7973

	if all(
	(isinstance(x, ak.contents.Content) and x.is_regular)
	or (isinstance(x, ak.contents.NumpyArray) and x.data.ndim > 1)
	or not isinstance(x, ak.contents.Content)
	for x in inputs
	):

Note the x.is_regular. If the array were not regular, then we don't hit the bug:

>>> ak.concatenate([ak.Array([[i]]) for i in range(128)], axis=1)
<Array [[0, 1, 2, 3, 4, ..., 123, 124, 125, 126, 127]] type='1 * var * int64'>
>>> ak.concatenate([ak.Array([[i]]) for i in range(129)], axis=1)
<Array [[0, 1, 2, 3, 4, ..., 124, 125, 126, 127, 128]] type='1 * var * int64'>

I think #3207 handled the variable-length list case but not the regular-length list case.