Optional and union syntax give different output with subclass of generic class

[robjwells]

Bug Report

Using Optional and the new union syntax for the type argument to a class's generic superclass cause mypy to report different errors (none for Optional[T], two for T | None).

To Reproduce

The behaviour can be seen in this example, where A is the generic superclass, B uses Optional, and C uses the new type union syntax.

from __future__ import annotations
from typing import Generic, Optional, TypeVar

T = TypeVar("T")

class A(Generic[T]):
    ...

class B(A[Optional[int]]):
    ...

class C(A[int | None]):
    ...

Expected Behavior

My expectation is that and Optional[T] and T | None are equivalent.

Actual Behavior

mypy reports the following for only the class using the union syntax:

$ mypy repro.py
repro.py:12: error: Type expected within [...]
    class C(A[int | None]):
            ^
repro.py:12: error: Invalid base class "A"
    class C(A[int | None]):
            ^
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.800
• Mypy command-line flags: Occurs with --strict and without.
• Python version used: 3.9.0

[JelleZijlstra]

I believe this syntax is invalid at runtime in 3.9 (it will work in 3.10). Does it work if you run the file?

[robjwells]

Ah, no, it doesn't:

Traceback (most recent call last):
  File "/Users/robjwells/Dropbox/projects/adventofcode-solutions/2016/python/repro.py", line 12, in <module>
    class C(A[int | None]):
TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

[hauntsaninja]

Yeah, the PEP 604 support in mypy doesn't work well for runtime use cases yet (i.e. things that will only work on 3.10). Closing as a dupe of #9880