Generic class with constrained type vars

On Python 3.7.0 and mypy 0.620, I'm getting some error messages I don't understand. The Mypy documentation and PEP 484 didn't clear it up for me. Maybe it's a bug in Mypy. Maybe I don't understand type erasure. In case it's the former, here's a minimal example. In case it's the latter, please help.

In the following examples, the fact that m does not return is not critical. In the real code that gave me these errors, m is an abstract method, which also doesn't change anything important.

Put the following examples in mypy-test.py to get the corresponding output from running mypyt mypy-test.py. The first and the last examples are the ones that are baffling me.

classmethod with constrained type variable

from typing import TypeVar, Generic
T = TypeVar('T', int, str)
class K(Generic[T]):
	@classmethod
	def m(cls) -> T:
		raise NotImplementedError

Mypy output:

mypy-test.py:5: error: The erased type of self 'def [T in (builtins.int, builtins.str)] () -> mypy-test.K[builtins.int*]' is not a supertype of its class 'Type[mypy-test.K[T`1]]'
mypy-test.py:5: error: The erased type of self 'def [T in (builtins.int, builtins.str)] () -> mypy-test.K[builtins.str*]' is not a supertype of its class 'Type[mypy-test.K[T`1]]'

classmethod with generic self

from typing import TypeVar, Generic
S = TypeVar('S')
T = TypeVar('T', int, str)
class K(Generic[T]):
	@classmethod
	def m(cls: S) -> T:
		raise NotImplementedError

produces no error.

classmethod with unconstrained type variable

from typing import TypeVar, Generic
T = TypeVar('T')
class K(Generic[T]):
	@classmethod
	def m(cls) -> T:
		raise NotImplementedError

produces no error.

instance method with constrained type

from typing import TypeVar, Generic
T = TypeVar('T', int, str)
class K(Generic[T]):
	def __init__(self, t: T) -> None:
		self.t = t

produces

mypy-test.py:5: error: Need type annotation for 't'

instance method with unconstrained type

from typing import TypeVar, Generic
T = TypeVar('T')
class K(Generic[T]):
	def __init__(self, t: T) -> None:
		self.t = t

produces no error.

instance method with constrained type and inheritance

from typing import TypeVar, Generic
T = TypeVar('T', int, str)
class K(Generic[T]):
	def __init__(self, t: T) -> None:
		self.t: T = t

class L(K[T]):
	def __init__(self, t: T) -> None:
		self.u = 1
		super().__init__(t=t)

produces

mypy-test.py:10: error: Argument "t" to "__init__" of "K" has incompatible type "int"; expected "T"
mypy-test.py:10: error: Argument "t" to "__init__" of "K" has incompatible type "str"; expected "T"

Riproduci gli esempi in mypy-test.py con Python 3.7.0 e mypy 0.620, iniziando dai metodi di classe generici vincolati e non vincolati. Confronta le diagnosi per i metodi di classe, gli attributi di istanza e i costruttori ereditati, quindi determina il comportamento previsto prima di decidere se sono necessarie modifiche all’implementazione o test di regressione.