`Iterable[Any]` incorrectly matches `Iterable[Never]` overload, inferring `Never` instead of the element type
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Description
Bug Report
When a class (e.g., Box in our example below) has two __init__ overloads — one taking Iterable[Never] (binding self to Box[Never]) and one taking Iterable[_T] (binding self to Box[_T]) — passing a value whose iteration type is Any incorrectly resolves to the Iterable[Never] overload, yielding Box[Never] instead of Box[_T].
To Reproduce
Gist URL: https://gist.github.com/mypy-play/d326ca961e53d4c9d085cfda05f3e455
Playground URL: https://mypy-play.net/?gist=d326ca961e53d4c9d085cfda05f3e455
# any_never_bug.py
from enum import IntEnum
from typing import Generic, Iterable, Never, TypeVar, overload, reveal_type
_T = TypeVar("_T")
class Box(Generic[_T]):
@overload
def __init__(self: "Box[Never]", val: Iterable[Never]) -> None: ...
@overload
def __init__(self: "Box[_T]", val: Iterable[_T]) -> None: ...
def __init__(self, val: object) -> None: ...
class Color(IntEnum):
RED = 1
GREEN = 2
reveal_type(Box(Color)) # Expected: Box[Color], mypy gives: Box[Never]
% mypy --config-file /dev/null any_never_bug.py
/dev/null: No [mypy] section in config file
any_never_bug.py:21: note: Revealed type is "any_never_bug.Box[Never]"
Success: no issues found in 1 source file
Expected Behavior
Expected: Box[Color]
Actual: Box[Never]
Notes:
My best guess on root cause is that EnumMeta.__iter__ is typed in typeshed as returning Iterator[Any], so Color (the class) is seen as Iterable[Any]. I'm guessing that since Any is consistent with Never, mypy selects the Iterable[Never] overload rather than the more general Iterable[_T] one. However, Iterable[Any] should probably not be considered assignable to Iterable[Never]. I'm pretty sure Never is the bottom type and nothing except Never itself should be a subtype of it. Pyright and Pyrefly both correctly infer Box[Color] for this case.
Your Environment
- Mypy version used: mypy 1.20.2 (compiled: yes)
- Mypy command-line flags: None
- Mypy configuration options from
mypy.ini(and other config files): None - Python version used: Python 3.11.15
Ref: astral-sh/ty#3330
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by running the any_never_bug.py reproducer with mypy --config-file /dev/null and confirm the revealed type. Trace overload resolution for Iterable[Any] against the Iterable[Never] and Iterable[_T] signatures, then verify that the result is Box[Color] without regressing the shown overload behavior.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers, devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100