Emit errors with `--disallow-any-generics` if `type[T]` or `TypeForm[T]` receives a generic `T` with missing type arguments
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Description
Feature
If --disallow-any-generics or --strict is on, I'd like mypy to emit E: Missing type parameters for generic type "list" [type-arg] when calling f(list) for the following function definitions:
def f[T](arg: type[T], /) -> T: ...def f[T](arg: TypeForm[T], /) -> T: ...
Currently, there's no error report, and reveal_type(f(list)) shows Any.
Pitch
Due to the lack of error reporting in the above situation, implicit Anys currently easily leak when using type[T] or TypeForm[T], creating an implicit source of unsafety, which is surprising under --strict mode. Adapting the examples from PEP 747: Motivation:
# mypy: enable-incomplete-feature=TypeForm, disable-error-code=empty-body
from collections.abc import Callable
from typing_extensions import TypeForm, TypeIs
def trycast_list_item[T](typx: TypeForm[list[T]], value: object) -> T | None: ...
reveal_type(trycast_list_item(list, [1])) # N: Revealed type is "Any | None"
def isassignable_list_item[T](value: object, typx: TypeForm[list[T]]) -> TypeIs[T]: ...
a: object
if isassignable_list_item(a, list):
reveal_type(a) # N: Revealed type is "Any"
else:
reveal_type(a) # N: Revealed type is "builtins.object"
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 reproducing the two generic function examples with --disallow-any-generics or --strict, including the reveal_type calls and TypeForm incomplete-feature setting. Trace how calls using list are analyzed for type[T] and TypeForm[T]; done means the missing type-argument error is emitted and the implicit Any no longer leaks in the shown reveals.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- tooling
- Issue type
- Feature
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 48/100