Summary

Here's some Python code extracted from a project I'm working on:

class TypedFieldValue[T]:
    pass

class CodecBase[T]:
    typed_field_value_cls: type[TypedFieldValue[T]]

    def __init_subclass__(cls) -> None:
        class ConcreteTypedFieldValue[U](TypedFieldValue[U]):
            pass
            
        cls.typed_field_value_cls = ConcreteTypedFieldValue[T]

Mypy accepts it and ty does not. While, in general, I tend to trust ty more than mypy, the error message and, in particular, its details, make me highly suspicious that ty is very confused:

error[invalid-generic-class]: Generic class `ConcreteTypedFieldValue` must not reference type variables bound in an enclosing scope
 --> ty_is_sad.py:8:15
  |
7 |     def __init_subclass__(cls) -> None:
8 |         class ConcreteTypedFieldValue[U](TypedFieldValue[U]):
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `T` referenced in class definition here
9 |             pass
  |
 ::: ty_is_sad.py:4:7
  |
2 |     pass
3 |
4 | class CodecBase[T]:
  |       --------- Type variable `T` is bound in this enclosing scope
5 |     typed_field_value_cls: type[TypedFieldValue[T]]
  |
info: rule `invalid-generic-class` is enabled by default

Found 1 diagnostic

I don't think I'm referencing T from the class definition. Certainly the line that ty indicates does not contain [T] or any other explicit reference to the type variale T. The variable U is not T! (I can omit the [U] in both places in that line, and ty accepts the entire file, but I find that rather surprising, too.)

Maybe I'm missing something and, if so, I apologize for the noise, but at the very least the error message could be less bizarre.

Version

0.0.16