Bug report

Mypy infers object for a bounded type variable when combining covariant generic arguments, although their common upper bound is valid. This reproduces without Django or plugins.

Reproducer

from typing import Generic, TypeVar
class Model: pass
class Person(Model): pass
class Audit(Model): pass
M = TypeVar("M", bound=Model, covariant=True)
class QuerySet(Generic[M]): pass
Q = TypeVar("Q", bound=QuerySet[Model], covariant=True)
L = TypeVar("L", bound=str, covariant=True)
P = TypeVar("P", bound=Person)
class Prefetch(Generic[L, Q]):
    def __init__(self, lookup: L, queryset: Q) -> None: ...
def accept(*items: Prefetch[L, Q]) -> None: ...
def example(people: QuerySet[P], audits: QuerySet[Audit]) -> None:
    accept(Prefetch("left", queryset=people), Prefetch("audit", queryset=audits))

Expected: the call is accepted; Q = QuerySet[Model] satisfies both arguments and its bound.

Actual:

error: Value of type variable "Q" of "accept" cannot be "object"  [type-var]

Annotating each argument as Prefetch[str, QuerySet[Model]] before the call succeeds. Using concrete QuerySet[Person] instead of QuerySet[P], or removing the L parameter from Prefetch, also succeeds. Merely assigning the arguments to unannotated intermediate variables does not help.

Environment: Python 3.14.6; reproduced on mypy 2.4.1 and unmodified master d8a43fbcf21e30bd7855b5635bdd2f07ffbb5d95, without plugins or project configuration (--no-incremental --cache-dir=/dev/null --config-file=/dev/null).

Originally reported as typeddjango/django-stubs#3770. The draft stubs PR bypasses this inference and is being withdrawn in favor of reporting the checker issue.

Investigation and reproducer prepared with AI assistance.