Calling next over generic iterator inside list comprehension inside function call gives not callable error

[juanpablos]

Environment data

• Language Server version: 2020.9.6
• OS and version: Windows 10
• Python version (& distribution if applicable, e.g. Anaconda): Python 3.8.5

Expected behaviour

No error when calling next.

Actual behaviour

I get an error that next is not callable.

Code Snippet / Additional information

Pylance 2020.9.5 and older does not have this issue, just encountered it in 2020.9.6.

Maybe this is too specific of an issue, but just encountered this (the builtin next function gives error that it is not callable):

T = TypeVar("T")

class SomeProtocol(Protocol[T]): ...

def some_function(arg: SomeProtocol[T]): ...

class SomeClass(Generic[T]):
	def __init__(self, it: Iterator[T]):
		# for some reason, here I get an error:
		# "next" has type "Overload[(__i: Iterator[_T]) -> _T, (__i: Iterator[_T], default: _VT) -> (_T | _VT)]" and is not callable
		some_function([next(it) for _ in range(10)])

	def my_function(self, it: Iterator[T]):
		# error:
		# "next" has type "Overload[(__i: Iterator[_T]) -> _T, (__i: Iterator[_T], default: _VT) -> (_T | _VT)]" and is not callable
		some_function([next(it) for _ in range(10)])

On a simpler example (but not realistic, just to show the error):

# I know T has no meaning here but just for the example
# by the way Pylance also does not raise error for the no meaning of T in this context
def iterator() -> Iterator[T]: ...

it = iterator()
# type T
value = next(it) # no problem here
# type List[T]
ten_values = [next(it) for _ in range(10)] # no problem here
# no error if pass the generated list
some_function(ten_values) # no problem

# error:
# "next" has type "Overload[(__i: Iterator[_T]) -> _T, (__i: Iterator[_T], default: _VT) -> (_T | _VT)]" and is not callable
some_function([next(it) for _ in range(10)])

# no error if I just unpack the iterator
some_function([*it])

The error is the same (that next is not callable) if I change SomeProtocol from a Protocol to just some Generic class.
If I fix the iterator type as e.g. Iterator[int] the problem disappears. If I remove the type of arg the error also goes away.

Here is what I just said with Iterator[int]:

T = TypeVar("T")
class SomeProtocol(Protocol[T]): ...
def some_function(arg: SomeProtocol[T]): ...
# change here Iterator[T] to Iterator[int]
def iterator() -> Iterator[int]: ...

it = iterator()
some_function([next(it) for _ in range(10)]) # no errors

It also disappears if T is resolved to some type:

def iterator(var: T) -> Iterator[T]: ...

it = iterator(1)
some_function([next(it) for _ in range(10)]) # no errors

[erictraut]

Thanks for the detailed bug report. I'll look into it.

[erictraut]

This will be fixed in the next release.

[jakebailey]

This issue has been fixed in version 2020.9.7, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/master/CHANGELOG.md#202097-30-september-2020