> 2. JSON = RecTypeVar('JSON', lambda Self: Union[None, bool, int, float, str, List[Self], Dict[str, Self]])

You don't actually need this workaround—the problem is typechecker support, not syntax. You can use strings for forward definitions, like this:

  JSON = Union[str, int, None, bool, List["JSON"], Dict[str, "JSON"]]

What happens next depends on your typechecker. Mypy says this:

  error: Cannot resolve name "JSON" (possible cyclic definition)

Pytype says this:

  Recursive type annotations not supported yet

But Pyre accepts it and correctly uses it for typechecking.

In fairness I find (1) to even less readable than the existing Callable syntax, but yeah, it would be cool if Python could express callables with kwargs and recursive types.