Allow Final attributes declared in class body if initialized in ctor

[llchan]

There is currently a safety check around TypeVar-dependent Final attributes declared in the class body that I believe we can loosen when the variable is assigned in the ctor. This should work analogously to non-TypeVar-dependent final attributes declared in the class body, which also require an assignment in the ctor (otherwise they emit "Final name must be initialized with a value").

• Are you reporting a bug, or opening a feature request? feature request
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

  from typing import Final, Generic, TypeVar
  
  
  WidgetT = TypeVar('WidgetT', bound='Widget')
  
  
  class Widget:
      pass
  
  
  class WidgetHolder(Generic[WidgetT]):
  
      widget: Final[WidgetT]
  
      def __init__(self, widget: WidgetT) -> None:
          self.widget = widget

• What is the actual behavior/output?

  final.py: note: In class "WidgetHolder":
  final.py:13:5: error: Final name declared in class body cannot depend on type variables  [misc]
          widget: Final[WidgetT]
          ^
  

In case anyone comes across this issue, the workaround is currently to not declare in the class body, but declare in the ctor with self.widget: Final[WidgetT] = widget instead. But just because a workaround exists doesn't mean we shouldn't try to fix this.

[Kangaroux]

C# has this feature with a readonly keyword. It would be nice to have here. You can modify the field as much as you want within the constructor but after that it's considered const.

[llchan]

@Kangaroux yep that's the intent of Final as well, and for the most part it already works like const or readonly. This issue is more about an implementation restriction, where the declaration is not allowed to be generic even when the constructor initializes it with a compatible object.

[dtoniolo]

This is issue is over two years old, but i've just came across the exact same problem.