Having to add type hint 2nd time on AnyStr when assigning as instance attribute

I am reporting a potential bug/improvement in mypy.

I assign a class's instance attribute to be an argument from the __init__ signature.

I noticed that when the argument is type hinted as a AnyStr, mypy raises an error unless the instance attribute receives a second type hint.

from typing import AnyStr, Generic


class A(Generic[AnyStr]):
    def __init__(self, arg: AnyStr):
        # Case A... error: Need type annotation for 'attr'
        self.attr = arg

        reveal_type(arg)
        # note: Revealed type is 'builtins.str*'
        # note: Revealed type is 'builtins.bytes*'

        reveal_type(self.attr)
        # note: Revealed type is 'builtins.str'
        # note: Revealed type is 'Any'
        # Observation: 'builtins.bytes*' doesn't seem to propagate


class B(Generic[AnyStr]):
    def __init__(self, arg: AnyStr):
        # Case B... Success
        self.attr = arg  # type: AnyStr

The desirable behavior is not to have to supply a second type hint.

What's going on here? Is there some way I can not supply a second type hint?

I am invoking mypy from the command line with no flags. Here are my versions:

python==3.6.5
mypy==0.770

Aside

I am coming here from Stack Overflow, where a community member thought it was a bug: Python mypy why does class instance attribute require type hint if expression already has type hint

Comienza reproduciendo el problema del fragmento de Python incluido con mypy 0.770, comparando Case A y Case B y sus tipos revelados. Rastrea la inferencia de atributos de instancia para Generic[AnyStr] y determina si la variable de tipo debería propagarse sin la anotación inline. Se considera terminado cuando Case A ya no requiere una segunda indicación de tipo y existe cobertura de regresión para el comportamiento demostrado.