Associated types on generics

• Request Type: Feature Request

• Mock-up repro (more detailed mock-up here):

  import abc
  import typing
  
  
  # Define two base types `ABase` & `BBase` with a similar interface,
  # that each return a different object type `AValue` & `BValue` from
  # some method
  #
  # The object type is also reexposed as a class attribute on the
  # respective base type for easier access.
  class AValue:
  	value: bytes
  	
  	def __init__(self, value: bytes):
  		self.value = value
  
  class ABase(metaclass=abc.ABCMeta):
  	VALUE_T = AValue
  	
  	@abc.abstractmethod
  	def get_some_value(self) -> AValue: ...
  
  
  class BValue:
  	value: str
  	
  	def __init__(self, value: str):
  		self.value = value
  
  class BBase(metaclass=abc.ABCMeta):
  	VALUE_T = BValue
  	
  	@abc.abstractmethod
  	def get_some_value(self) -> BValue: ...
  
  
  # Define generic type over the two defined above
  AnyBase = typing.TypeVar("AnyBase", ABase, BBase)
  
  #
  # HOW WOULD I NAME THE RETURN TYPE IN THE NEXT LINE?
  #
  # In particular note that the return type could not be just `AnyBase`
  # (over which we are generic) since we aren't returning *that* type,
  # but some other type determined by which type `AnyBase` is.
  def magic_convert(input: AnyBase) -> AnyBase.VALUE_T: #???
  	return input.VALUE_T(self.get_some_value().value * 2)```
  
  

• Actual behaviour: There does seem to be anything like AnyBase.VALUE_T available for use in mypy and the given syntax results in both type-time and runtime errors.

• Expected behaviour: Some way to do this.

• mypy version: 0.730

• Python version: 3.7 (Debian)

• mypy flags: None

I used some title that probably is close to the real (type theory) name of this feature, feel free to change it to something more fitting. 🙂

Start with the Python 3.7 reproduction in the issue and read the linked detailed mock-up to understand the desired associated-type relationship. Define what supported syntax and type-checking behavior should look like for the generic return type, including the shown magic_convert example; the issue is done when mypy can express and validate that relationship without the reported type-time or runtime errors.