Poor handling of `attr`s converters.

[alanhdu]

When running mypy over the following file:

from typing import Sequence
import attr

@attr.s
class A:
    a: Sequence[str] = attr.ib(converter=list)
A(["a"])

I get:

$ mypy a.py
a.py:7: error: List item 0 has incompatible type "str"; expected "_T"

which is a little confusing, because str should work fine here. This is with mypy 0.782 and on Python 3.6.

[Akuli]

Does it work if you use converter=(lambda thing: list(thing)) instead of converter=list? If it works, then this might be related to #9253

[alanhdu]

@Akuli When I use a lambda, mypy complains with

a.py:6: error: Unsupported converter, only named functions and types are currently supported

When I try to use a named generic:

from typing import Sequence, List, TypeVar
import attr

T = TypeVar("T")
def convert(x: Sequence[T]) -> List[T]:
    return list(x)

@attr.s
class A:
    a: Sequence[str] = attr.ib(converter=convert)
A(["a"])

then mypy errors with:

a.py:5: error: Type variable "a.T" is unbound
a.py:5: note: (Hint: Use "Generic[T]" or "Protocol[T]" base class to bind "T" inside a class)
a.py:5: note: (Hint: Use "T" in function signature to bind "T" inside a function)
Found 1 error in 1 file (checked 1 source file)

This does work if I "monomorphize" convert to go from Sequence[str] to List[str] though.