`attr.ib(converter=)` appears to cause mypy to lose generic types for `__init__` function.

$ mypy --version
mypy 0.760

attr_test.py:

import attr
from typing import Sequence

@attr.s(auto_attribs=True)
class Foo:
    things: Sequence[str]

@attr.s(auto_attribs=True)
class Bar:
    things: Sequence[str] = attr.ib(converter=tuple)

@attr.s(auto_attribs=True)
class Raz:
    thing: str = attr.ib(converter=str) 

Foo(['hello', 'world'])
Raz('hello')
Bar(['hello', 'world'])
reveal_type(Foo.__init__)
reveal_type(Bar.__init__)
reveal_type(Raz.__init__)

Output:

$ mypy attr_test.py
attr_test.py:18: error: List item 0 has incompatible type "str"; expected "_T_co"
attr_test.py:18: error: List item 1 has incompatible type "str"; expected "_T_co"
attr_test.py:19: note: Revealed type is 'def (self: attr_test.Foo, things: typing.Sequence[builtins.str])'
attr_test.py:20: note: Revealed type is 'def (self: attr_test.Bar, things: typing.Iterable[_T_co`1])'
attr_test.py:21: note: Revealed type is 'def (self: attr_test.Raz, thing: builtins.object)'

I would expect attr_test.py to typecheck cleanly and for the signature of Bar.__init__ to be def (self: attr_test.Bar, things: typing.Iterable[str]) . I'm guessing mypy created a new type _T_co`1 that's something like TypeVar('T', bound=str, covariant=True) but it doesn't seem like that's being used later.

Start by running the supplied attr_test.py example with mypy 0.760 and compare the revealed init types for Foo, Bar, and Raz. Trace why converter=tuple produces Iterable[_T_co] and converter=str produces object. Done means the example typechecks cleanly and Bar.init is revealed as accepting Iterable[str].