Are you reporting a bug, or opening a feature request?

Reporting a bug.

Please insert below the code you are checking with mypy

import abc

class A(abc.ABC):

    @abc.abstractmethod
    def foo(self):
        pass

class B(A):

    def foo(self):
        pass

class C(A):

    def foo(self):
        pass

cls_b = {1: B}[1]
cls_b()

cls_c = {2: C}[2]
cls_c()

cls_b_2 = {1: B, 2: C}[1]
# mypy error here:
# example.py:26: error: Cannot instantiate abstract class 'A' with abstract attribute 'foo'
cls_b_2()

What is the actual behavior/output?

# example.py:26: error: Cannot instantiate abstract class 'A' with abstract attribute 'foo'

What is the behavior/output you expect?

No error.

What are the versions of mypy and Python you are using?

mypy 0.600
Python 3.6.5

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)

No flags.