False-positive cannot assign expression to yield type (with TypeVar, overload, and Union)

[cdce8p]

Environment data

• Language Server version: 2021.9.1
• OS and version: darwin x64
• Python version: 3.10.0rc2
• python.analysis.indexing: true
• python.analysis.typeCheckingMode: basic

Expected behaviour

no error

Actual behaviour

Expression of type "X" cannot be assigned to yield type "T1@return_iter | T2@return_iter"
  Type "X" cannot be assigned to type "T1@return_iter | T2@return_iter"
    Type "X" cannot be assigned to type "T1@return_iter"
    Type "X" cannot be assigned to type "T2@return_iter"

Logs

--

Code Snippet / Additional information

from typing import TypeVar, overload, Iterator

T1 = TypeVar("T1", bound="X")
T2 = TypeVar("T2", bound="X")

class X:
    element_list: list["X"]

    @overload
    def return_iter(self, klass: type[T1]) -> Iterator[T1]:
        ...

    @overload
    def return_iter(self, klass: tuple[type[T1], type[T2]]) -> Iterator[T1] | Iterator[T2]:
        ...

    def return_iter(self, klass: type[T1] | tuple[type[T1], type[T2]]) -> Iterator[T1] | Iterator[T2]:
        for item in self.element_list:
            if isinstance(item, klass):
                yield item  # error

Mypy output
no error

[judej]

Thanks for the report.

[erictraut]

Thanks, this will be fixed in the next release.

The code that handles isinstance and issubclass type narrowing was handling the case where the second argument was a union of classes or a tuple of classes, but it didn't properly handle a union that included both classes and tuples of classes. That's now fixed.

[jakebailey]

This issue has been fixed in version 2021.9.2, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202192-16-september-2021

[cdce8p]

This is still an issue when modified slightly:

from typing import TypeVar, Iterator

T1 = TypeVar("T1", bound="X")
T2 = TypeVar("T2", bound="X")


class X:
    element_list: list["X"]

    def return_iter(
        self, klass: type[T1] | tuple[type[T1], type[T2]] | tuple[type[T1], ...]
    ) -> Iterator[T1] | Iterator[T2]:
        for item in self.element_list:
            if isinstance(item, klass):
                yield item  # error

Added tuple[type[T1], ...] as fallback for tuples with len > 2. No error when tested with mypy.

[erictraut]

You're right — I didn't handle the required special casing for open-ended tuples (the ... form) in this code path. It will be fixed in the next release.

[cdce8p]

You're right — I didn't handle the required special casing for open-ended tuples (the ... form) in this code path. It will be fixed in the next release.

Thank you @erictraut! I appreciate how fast you and the team address these issues. It's amazing to see pylance and pyright getting better.