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"

Python 3.7.0 と mypy 0.620 を使用して mypy-test.py の例を再現し、制約付きおよび制約なしのジェネリッククラスメソッドから始めます。クラスメソッド、インスタンス属性、継承されたコンストラクターについて診断結果を比較し、その後、実装の変更またはリグレッションテストが必要かどうかを判断する前に、意図された動作を特定します。