When a type variable is bound to a class, it cannot be reused in a type alias. Previously inTypeAliasType, this error was reported as "not included in type_params". However in the following example, the error is misleading:

fromtypingimportDict,Generic,TypeVarfromtyping_extensionsimportTypeAliasTypeT=TypeVar("T")classA(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.

Improve error message for bound typevar in TypeAliasType

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hamdanal commentedMar 20, 2024

It would be nice to also get rid of the second remaining error, but this will require adding a special-cased context only for analyzingtype_params and I am not comfortable doing this change without first consulting someone with more experience than me.