Support TypeAliasType in a class scope
#17038
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There is also the question of whether to explicitly disallow aliases in a function scope but mypy currently allows PEP 613 |
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
JelleZijlstra
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Mar 28, 2024
Follow up to #17038 When a type variable is bound to a class, it cannot be reused in a type alias. Previously in `TypeAliasType`, this error was reported as "not included in type_params". However in the following example, the error is misleading: ```python from typing import Dict, Generic, TypeVar from typing_extensions import TypeAliasType T = TypeVar("T") class A(Generic[T]): Ta11 = TypeAliasType("Ta11", Dict[str, T], type_params=(T,)) x: A.Ta11 = {"a": 1} reveal_type(x) ``` On the master branch: ``` main.py:8: error: Type variable "T" is not included in type_params [valid-type] main.py:8: error: "T" is a type variable and only valid in type context [misc] main.py:8: error: Free type variable expected in type_params argument to TypeAliasType [type-var] main.py:12: note: Revealed type is "builtins.dict[builtins.str, Any]" Found 3 errors in 1 file (checked 1 source file) ``` With this PR: ``` typealiastype.py:8: error: Can't use bound type variable "T" to define generic alias [valid-type] typealiastype.py:8: error: "T" is a type variable and only valid in type context [misc] typealiastype.py:12: note: Revealed type is "builtins.dict[builtins.str, Any]" Found 2 errors in 1 file (checked 1 source file) ``` This is possible by storing the names of all the declared type_params, even those that are invalid, and checking if the offending type variables are in the list.
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Fixes #16614 (comment)