False positive on re.compile as attrs converter

I've found what I think is a bug in mypy's attrs support.

# test.py
import re
import attr

@attr.s
class Test:
    foo = attr.ib(converter=re.compile)

test = Test("foo")

$ mypy test.py
test.py:8: error: Argument 1 to "Test" has incompatible type "str"; expected "Union[AnyStr, Pattern[AnyStr]]"

Unless I've completely misunderstood what AnyStr is for, I don't think that signature should throw on receiving a string.

I can't quite figure out if attrs or mypy is at fault here, but I can't get the same error to appear on re.compile when it's not being applied as a converter by attrs. Apologies if I've missed an obvious issue here.

Python v3.6.5
Mypy v0.610
Attrs v18.1.0

Inizia riproducendo il report con il test.py mostrato e confronta la gestione di re.compile da solo da parte di mypy con la sua gestione come converter di attrs. Traccia l'inferenza del tipo del converter di attrs e il conseguente controllo dell'argomento del costruttore. Il lavoro è completato quando Test("foo") viene accettato senza indebolire la tipizzazione valida del converter.