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Mypy does not check bounds of variable type aliases #13086
Description
Bug Report
When having defined a TypeAlias dependent on a TypeVar, mypy does not allays that instances of these are actually allowed. That is, that they respect the bounds or restrictions of the used TypeVar. This is in contrast to the behavior for generic types, which are checked.
To Reproduce
from typing import Generic, TypeVar, TypeAlias
T = TypeVar("T", bound=bytes | str)
U = TypeVar("U", bytes, str)
BoundedTuple: TypeAlias = tuple[T]
RestrictedTuple: TypeAlias = tuple[U]
i_am_illegal1: BoundedTuple[int] # Passes, even though this type shouldn't be possible!
i_am_illegal2: RestrictedTuple[int] # Passes, even though this type shouldn't be possible!
class Bounded(Generic[T]):
pass
class Restricted(Generic[U]):
pass
i_am_illegal3: Bounded[int] # Behaves as expected, resulting in `error: Type argument "int" of "Bounded" must be a subtype of "Union[bytes, str]"`
i_am_illegal4: Restricted[int] # Behaves as expected, resulting in `error: Value of type variable "U" of "Restricted" cannot be "int"`https://mypy-play.net/?mypy=latest&python=3.10&flags=strict&gist=f5763650eba54f25b51553dbdd58179f
Expected Behavior
I expect four errors instead of two from the example code above.
Actual Behavior
As can be seen, I was allowed to define types that shouldn't be allowed, given the restrictions of the type variables in use.
Your Environment
- Mypy version used: 0.961
- Mypy command-line flags: --strict
- Python version used: 3.10
Activity
AlexWaygood commentedon Jul 7, 2022
According to PEP 484, if
T = TypeVar("T", bound=complex | str),Tcan be solved as beingintwhere necessary, since PEP 484 states that type checkers should considerintandfloatto be implicit subtypes ofcomplex. In "The Numeric Tower", the PEP states:This explains why mypy does not emit an error for your first and third example.
Your second example does seem to be buggy to me, however. Constrained
TypeVars should only be solveable with one of the exact constraints, i.e., subtypes should not be permitted. So I believe you're correct when you say that mypy should be emitting an error for the following:[-]Mypy does not check bounds of all generic types[/-][+]Mypy does not check bounds of variable type aliases[/+]harahu commentedon Jul 7, 2022
Good catch! Appreciate it. Changed the example, but the first example also remains faulty, so it seems this is an issue for variable type aliases, but not for generics. Please let me know whether the issue is worded more clearly (and correctly) now.
AlexWaygood commentedon Jul 7, 2022
Oh yikes — yup, your revised first example definitely looks like a bug to me!
AlexWaygood commentedon Jul 7, 2022
... But it's a duplicate of #10445. Your example using the constrained
TypeVaris new, but I'm closing this so that we can keep all the discussion in one place, since it's the same bug :)harahu commentedon Jul 7, 2022
Thank you for noticing! Not sure how that issue slipped past me.