Union[List[T], T] is not recognized by mypy

[arogozhnikov]

Bug Report

Union[List[T], T] is not recognized by mypy

To Reproduce

from typing import List, Union, TypeVar

T = TypeVar('T')


def first_or_only(x: Union[List[T], T]) -> T:
    pass

x1: int = first_or_only(1)
x2: int = first_or_only([1, 2, 3])
x3: tuple = first_or_only((1, 2, 3))

Expected Behavior

1. x1 should be inferenced as int
2. x2 should be inferenced as a) perfect case: int b) correct case int or list[int]
3. x3 should be inferenced as tuple[int]

Both vs code and pycharm inference as 1 + 2a + 3.

Actual Behavior

Mypy output for the example above makes no sense to me:

error: Argument 1 to "first_or_only" has incompatible type "List[int]"; expected "List[<nothing>]"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.971
• Mypy command-line flags: just file
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

[tmke8]

Relevant: #11149

[AmberAnsari89]

Can I check this issue..i am beginner in open source contribution however I can analyse it for further clarity .please input your suggestion ..

[EzR1d3r]

I guess the same issue with

from typing import TypeVar

StdPrimitiveType = TypeVar("StdPrimitiveType", int, float, str, bool)

def foo(val: StdPrimitiveType | set[StdPrimitiveType]):
    pass

foo({"a", "b"})

error: Argument 1 has incompatible type "Set[str]"; expected "Union[bool, Set[bool]]" [arg-type]