False-positive error when using Callable in a PEP 604 type alias

[AlexWaygood]

Bug Report

The following snippet works fine at runtime on 3.10+. It should be accepted as a valid type alias definition on Python 3.10+ (and should be accepted in stub files regardless of Python version). However, mypy raises a false-positive error. The error does not go away if you use typing.TypeAlias:

from typing import Callable, TypeAlias

A = Callable[[], str] | int  # error: Unsupported left operand type for | ("object")
B: TypeAlias = Callable[[], str] | int  # error: Unsupported left operand type for | ("object")

Mypy does accept the following (with the order of types reversed) as a valid type alias on Python 3.10+ and in stub files:

C = int | Callable[[], str]  # No error using Python 3.10 (in a `.py file), or in a stub file on any Python version.

Much like with #12392, this error is only reproduceable if Callable is the outermost type. The following produces no errors:

D = list[Callable[[], str] | int]

There is also an additional bug that only exists for stub files running with --python-version set to 3.9 or lower, when using a Callable that has a PEP 604 union as its return type or a PEP 604 union in its parameter specification, and when the Callable is itself in a PEP 604 union:

X = int | Callable[[], str | bool]  # error: Unsupported left operand type for | ("Type[str]")
Y = int | Callable[[str | bool], str]  # error: Unsupported left operand type for | ("Type[str]")

To Reproduce

• Mypy playground link here for the errors which affect stubs and .py files.
• For the stub-specific errors, paste the code samples into a .pyi file, then run mypy on the file with --python-version=3.9.

Expected Behavior

No errors should be reported.

Cc. @JelleZijlstra

[gvanrossum]

I hit this with T = Callable[[], int] | None. No PEP 604 TypeAlias needed in this repro.

[AlexWaygood]

I hit this with T = Callable[[], int] | None. No PEP 604 TypeAlias needed in this repro.

That is a PEP 604 type alias, and it's the same as my first example :-)

If you were to write that type alias without PEP 604, it would be T = Union[Callable[[], int], None] or T = Optional[Callable[[], int]]

[gvanrossum]

Oh duh, I misremembered what PEP 604 was. :-(

…

-- --Guido (mobile)

[ilevkivskyi]

The original example works correctly on master (assuming you use 3.10+).

[AlexWaygood]

The original example works correctly on master (assuming you use 3.10+).

Not in a stub file on 3.9, I don't think?

[ilevkivskyi]

Hm, I just tried and first example works in a stub on 3.9, but the last example actually fails in stub on 3.9

X = int | Callable[[], str | bool]  # error: Unsupported left operand type for | ("Type[str]")
Y = int | Callable[[str | bool], str]  # error: Unsupported left operand type for | ("Type[str]")

Maybe open a separate issue to track specifically this case?

[AlexWaygood]

Maybe open a separate issue to track specifically this case?

Okay, I will open a separate issue. On mypy master these all still cause mypy to emit false positives in a stub file with --python-version 3.9:

# test.pyi

from typing import Callable
from typing_extensions import TypeAlias

X = int | Callable[[], str | bool]
XX: TypeAlias = int | Callable[[], str | bool]
Y = int | Callable[[str | bool], str]
YY: TypeAlias = int | Callable[[str | bool], str]

[AlexWaygood]

Okay, I will open a separate issue.

(#14158 is the follow-up issue for stubs, for those following along.)