"Cannot infer type argument 1 of shutil.copyfileobj" error with mypy 0.780

[deveshks]

(This was originally filed as python/typeshed#4201, but it was suggested
to file here instead)

The following code used to pass mypy type check with mypy 0.770 but doesn't work for mypy 0.780

import shutil

filename1 = '/foo/bar'
filename2 = '/foo/baz'

srcfp =  open(filename1, 'rb')
destfp = open(filename2, 'wb')

shutil.copyfileobj(srcfp, destfp)

> mypy 0.770

$ mypy --version
mypy 0.770
$  mypy scratch.py
Success: no issues found in 1 source file

> mypy 0.780

$ mypy --version
mypy 0.780

$ scratch.py  
scratch.py:9: error: Cannot infer type argument 1 of "copyfileobj"

[srittau]

Something seems to have changed between mypy 0.770 and 0.780 regarding how overloads are handled. BinaryIO.write() has two overloads: bytearray and bytes in that order (since 2018). When removing the superclass IO[bytes] from BinaryIO, the error message changes to:

error: Argument 1 to "copyfileobj" has incompatible type "BinaryIO"; expected "_Reader[bytearray]"

Here is an example that doesn't rely on typeshed types (assuming that IO is special-cased):

from typing import TypeVar, Protocol, Any, overload, IO

_S_co = TypeVar("_S_co", covariant=True)
_S_contra = TypeVar("_S_contra", contravariant=True)
_T = TypeVar("_T")

class BinaryIO(IO[bytes]):
    @overload
    def write(self, s: bytearray) -> int: ...
    @overload
    def write(self, s: bytes) -> int: ...
    def write(self, s) -> int:
        return 0

class _Reader(Protocol[_S_co]):
    def read(self, n: int) -> _S_co: ...

class _Writer(Protocol[_S_contra]):
    def write(self, s: _S_contra) -> Any: ...

def foo(fsrc: _Reader[_T], fdst: _Writer[_T]) -> None:
    pass

srcfp: BinaryIO
destfp: BinaryIO

foo(srcfp, destfp)

[McSinyx]

This seems to be fixed by 0.790.

[hauntsaninja]

Thanks for reporting the fix! Used mypy_primer to bisect it to #9064 , so probably the result of python/typeshed#4203

While I'm here, I also bisected the original regression to #8845 . Since this issue is "fixed" by the typeshed change, I'm going to close — but maybe @msullivan is curious enough to take a closer look.