Grouping on JSONField subkey throws tuple index out of range error

[Marc DEBUREAUX]

Have this simple model:

class Link(models.Model):
  item = models.ForeignKey("Item", on_delete=models.CASCADE)
  extra = models.JSONField(blank=True, null=True)

And have this simple query:

Link.objects.all().values("extra__key", "item_id").annotate(count=Count("id"))

This throws the following stacktrace error since 5.2 release:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
File ~/python3.12/site-packages/IPython/core/formatters.py:770, in PlainTextFormatter.__call__(self, obj)
    763 stream = StringIO()
    764 printer = pretty.RepresentationPrinter(stream, self.verbose,
    765     self.max_width, self.newline,
    766     max_seq_length=self.max_seq_length,
    767     singleton_pprinters=self.singleton_printers,
    768     type_pprinters=self.type_printers,
    769     deferred_pprinters=self.deferred_printers)
--> 770 printer.pretty(obj)
    771 printer.flush()
    772 return stream.getvalue()

File ~/python3.12/site-packages/IPython/lib/pretty.py:411, in RepresentationPrinter.pretty(self, obj)
    400                         return meth(obj, self, cycle)
    401                 if (
    402                     cls is not object
    403                     # check if cls defines __repr__
   (...)    409                     and callable(_safe_getattr(cls, "__repr__", None))
    410                 ):
--> 411                     return _repr_pprint(obj, self, cycle)
    413     return _default_pprint(obj, self, cycle)
    414 finally:

File ~/python3.12/site-packages/IPython/lib/pretty.py:786, in _repr_pprint(obj, p, cycle)
    784 """A pprint that just redirects to the normal repr function."""
    785 # Find newlines and replace them with p.break_()
--> 786 output = repr(obj)
    787 lines = output.splitlines()
    788 with p.group():

File ~/python3.12/site-packages/django/db/models/query.py:360, in QuerySet.__repr__(self)
    359 def __repr__(self):
--> 360     data = list(self[: REPR_OUTPUT_SIZE + 1])
    361     if len(data) > REPR_OUTPUT_SIZE:
    362         data[-1] = "...(remaining elements truncated)..."

File ~/python3.12/site-packages/django/db/models/query.py:384, in QuerySet.__iter__(self)
    369 def __iter__(self):
    370     """
    371     The queryset iterator protocol uses three nested iterators in the
    372     default case:
   (...)    382            - Responsible for turning the rows into model objects.
    383     """
--> 384     self._fetch_all()
    385     return iter(self._result_cache)

File ~/python3.12/site-packages/django/db/models/query.py:1935, in QuerySet._fetch_all(self)
   1933 def _fetch_all(self):
   1934     if self._result_cache is None:
-> 1935         self._result_cache = list(self._iterable_class(self))
   1936     if self._prefetch_related_lookups and not self._prefetch_done:
   1937         self._prefetch_related_objects()

File ~/python3.12/site-packages/django/db/models/query.py:216, in ValuesIterable.__iter__(self)
    210     names = [
    211         *query.extra_select,
    212         *query.values_select,
    213         *query.annotation_select,
    214     ]
    215 indexes = range(len(names))
--> 216 for row in compiler.results_iter(
    217     chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size
    218 ):
    219     yield {names[i]: row[i] for i in indexes}

File ~/python3.12/site-packages/django/db/models/sql/compiler.py:1571, in SQLCompiler.results_iter(self, results, tuple_expected, chunked_fetch, chunk_size)
   1569 """Return an iterator over the results from executing this query."""
   1570 if results is None:
-> 1571     results = self.execute_sql(
   1572         MULTI, chunked_fetch=chunked_fetch, chunk_size=chunk_size
   1573     )
   1574 fields = [s[0] for s in self.select[0 : self.col_count]]
   1575 converters = self.get_converters(fields)

File ~/python3.12/site-packages/django/db/models/sql/compiler.py:1609, in SQLCompiler.execute_sql(self, result_type, chunked_fetch, chunk_size)
   1607 result_type = result_type or NO_RESULTS
   1608 try:
-> 1609     sql, params = self.as_sql()
   1610     if not sql:
   1611         raise EmptyResultSet

File ~/python3.12/site-packages/django/db/models/sql/compiler.py:765, in SQLCompiler.as_sql(self, with_limits, with_col_aliases)
    763 try:
    764     combinator = self.query.combinator
--> 765     extra_select, order_by, group_by = self.pre_sql_setup(
    766         with_col_aliases=with_col_aliases or bool(combinator),
    767     )
    768     for_update_part = None
    769     # Is a LIMIT/OFFSET clause needed?

File ~/python3.12/site-packages/django/db/models/sql/compiler.py:85, in SQLCompiler.pre_sql_setup(self, with_col_aliases)
     79 def pre_sql_setup(self, with_col_aliases=False):
     80     """
     81     Do any necessary class setup immediately prior to producing SQL. This
     82     is for things that can't necessarily be done in __init__ because we
     83     might not have all the pieces in place at that time.
     84     """
---> 85     self.setup_query(with_col_aliases=with_col_aliases)
     86     order_by = self.get_order_by()
     87     self.where, self.having, self.qualify = self.query.where.split_having_qualify(
     88         must_group_by=self.query.group_by is not None
     89     )

File ~/python3.12/site-packages/django/db/models/sql/compiler.py:74, in SQLCompiler.setup_query(self, with_col_aliases)
     72 if all(self.query.alias_refcount[a] == 0 for a in self.query.alias_map):
     73     self.query.get_initial_alias()
---> 74 self.select, self.klass_info, self.annotation_col_map = self.get_select(
     75     with_col_aliases=with_col_aliases,
     76 )
     77 self.col_count = len(self.select)

File ~/python3.12/site-packages/django/db/models/sql/compiler.py:286, in SQLCompiler.get_select(self, with_col_aliases)
    284 # Reference to a column.
    285 elif isinstance(expression, int):
--> 286     expression = cols[expression]
    287 # ColPairs cannot be aliased.
    288 if isinstance(expression, ColPairs):