A somewhat complicated solution that necessitated adding a new custom
function to `tsg-python`. See the comments in `python.tsg` for why this
was necessary.
Surprisingly, the new parser did not support these constructs (and the
relevant test was missing this case), so on files that required the new
parser we were unable to parse this construct.
To fix it, we add `list_pattern` (not to be confused with
`pattern_list`) as a `tree-sitter-python` node that results in a `List`
node in the AST.
That is, the `*T` in `def foo(*args : *T): ...`.
This is apparently a piece of syntax we did not support correctly until
now.
In terms of the grammar, we simply add `list_splat` as a possible
alternative for `type` (which could previously only be an `expression`).
We also update `python.tsg` to not specify `expression` those places (as
the relevant stanzas will then not work for `list_splat`s).
This syntax is not supported by the old parser, hence we only add a new
parser test for it.
This caused a dataset check error on the `python/cpython` database, as
we had a `DictUnpacking` node whose parent was not a `dict_item_list`,
but rather an `expr_list`.
Investigating a bit further revealed that this was because in a
construction like
```python
class C[T](base, foo=bar, **kwargs): ...
```
we were mistakenly adding `**kwargs` to the same list as `base` (which
is just a list of expressions), rather than the same list as `foo=bar`
(which is a list of dictionary items)
The ultimate cause of this was the use of `! name` in `python.tsg` to
distinguish between bases and keyword arguments (only the latter of
which have the `name` field). Because `dictionary_splat` doesn't have a
`name` field either, these were mistakenly put in the wrong list,
leading to the error.
Also, because our previous test of `class` statements did not include a
`**kwargs` construction, we were not checking that the new parser
behaved correctly in this case. For the most part this was not a
problem, but on files that use syntax not supported by the old parser
(like type parameters on classes), this became an issue. This is also
why we did not see this error previously.
To fix this, we added `! value` (which is a field present on
`dictionary_splat` nodes) as a secondary filter, and added a third
stanza to handle `dictionary_splat` nodes.
