Bug Report

I've noticed that overloading a function can hide a mismatch between the runtime type and the stub type. I noticed this as I was going over typeshed stubtest exceptions, specifically for itertools.accumulate.
Tests for python 3.6 and 3.7 failed because the stubs declared accumulate as a function while it was actually a class. However for 3.8 it is defined as two functions with overload and the tests passed successfully.

To Reproduce

(Write your steps here:)

1. Run stubtest for typeshed in python 3.8.
2. Test passes.
3. Remove one of the overloads in itertools.pyi
4. run again and see it fails.

Expected Behavior

I t should fail on a type mismatch like it does for no overloads.

Actual Behavior

passed.

Your Environment

Ran from latest python 3.8 and latest mypy from github.