Can't bind TypeVar to Protocol and other TypeVars

[iddan]

• Are you reporting a bug, or opening a feature request? Bug
• 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.

import inspect
from typing import cast, Type, TypeVar, Callable
from typing_extensions import Protocol

ImplementedProtocol = TypeVar("ImplementedProtocol", bound=Protocol)
BaseClass = TypeVar("BaseClass", bound=Type)
Implementation = TypeVar("Implementation", bound=BaseClass)

def implement(protocol : ImplementedProtocol) -> Callable[[Implementation], Implementation]:
  def x(implementation: Implementation) -> Implementation:
    methods = inspect.getmembers(implementation, predicate=inspect.isfunction)
    mro = inspect.getmro(implementation)
    
    if len(mro) < 2:
        raise TypeError("implementation must extend a base class to enrich")

    base_class : BaseClass = inspect.getmro(implementation)[1]

    for name, function in methods:
      setattr(base_class, name, function)
    
    cast(ImplementedProtocol, base_class)
    
    return implementation

  return x

class Animal(Protocol):
  def name(self) -> str:
    pass

  def noise(self) -> str:
    pass

class Sheep:
  def __init__(self, name, noise):
    self._naked = False
    self._noise = noise
    self._name = name

  def is_naked(self) -> bool:
    return self._naked
  
  def sheer(self) -> None:
    self._naked = True

@implement(Animal)
class SheepAnimalExtension(Sheep):
    def name(self):
      return self._name

    def noise(self):
      return self._noise


sheep = Sheep(name="beee", noise="mee")
print(sheep.name())

• What is the actual behavior/output?

traits/trait.py:6: error: Invalid type "traits.trait.BaseClass"
traits/trait.py:16: error: Invalid type "traits.trait.BaseClass"
traits/trait.py:21: error: Invalid type "protocol"
traits/usage.py:33: error: "Sheep" has no attribute "name"; maybe "_name"?

• What is the behavior/output you expect? Sheep to be casted to SheepAnimalExtension
• What are the versions of mypy and Python you are using? 0.641
  Do you see the same issue after installing mypy from Git master? yes
• What are the mypy flags you are using? (For example --strict-optional) None
• If mypy crashed with a traceback, please paste
  the full traceback below. no crash

[JukkaL]

Mypy doesn't support type variables as bounds of type variables. It's possible that we'll support it in the future, but we now concrete plans at the moment.

Another issue is the use of Protocol as a type. It's not a real type -- it's only usable as a magic base class when defining a protocol class.

More generally, the code is too dynamic for mypy to understand, since mypy doesn't run the program, it only statically analyses it.

[iddan]

This would be possible in Flowtype, Typescript, OCaml

[JukkaL]

Can you give an example of how you'd do this in TypeScript (or another language)? Python is different from TypeScript and some things that are feasible in TypeScript may be too complicated (or not widely used enough) to implement in mypy.

[iddan]

Yes, I will implement later. But how can a feature not implmented be wildly used? And also dataclasses & attrs having custom bindings are two great examples why the type system is not expressive enough