TypeIs and TypeGuard fail to narrow type[C]
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Description
Bug Report
A TypeIs/TypeGuard function fails to narrow a type[A] expression. A plain if guard block treats the branch as unreachable instead. A comprehension filter keeps the unnarrowed type instead.
To Reproduce
from typing_extensions import TypeIs
class A: ...
class M: ...
class C(M, A): ...
def is_m(t: type) -> TypeIs[type[M]]:
return issubclass(t, M)
cls: type[A] = C
if is_m(cls):
reveal_type(cls)
Expected Behavior
reveal_type(cls) reports type[<subclass of "A" and "M">].
Actual Behavior
Success: no issues found in 1 source file
No reveal_type note at all -- the branch is silently treated as unreachable.
Second repro (comprehension)
from typing import Type
from typing_extensions import TypeIs
class A: ...
class M: ...
class B(A): ...
class C(M, A): ...
def is_m(t: type) -> TypeIs[Type[M]]:
return issubclass(t, M)
alist: list[Type[A]] = [B, C]
mlist: list[Type[M]] = [cls for cls in alist if is_m(cls)]
reveal_type(mlist)
Expected Behavior
No error, mlist is list[type[M]].
Actual Behavior
error: List comprehension has incompatible type List[type[A]]; expected List[type[M]] [misc]
Environment
- Mypy version used: mypy 2.2.0+dev (main branch)
- Mypy command-line flags: none
- Mypy configuration options from
mypy.ini(and other config files): nothing special - Python version used: 3.12
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
No project file or test is named. Start by searching the type-narrowing tests and implementation for TypeIs, TypeGuard, conditional branches, and comprehension filters, then run both reproductions with mypy. Done means the first reveal_type reports the expected intersection type and the second infers list[type[M]] without an error.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers, devtools
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 55/100