Type aliases to PEP 604 unions using class from untyped package "not valid as a type"

[robertschweizer]

Bug Report

When I create a type alias to PEP604-style union type, using a type from a package without type annotations, mypy complains that it's "not valid as a type".

Similar to #11280, but using a class from an untyped package.

Interestingly, things work if I explicitly use str | Any.

To Reproduce

from shapely.geometry import BaseGeometry

b = str | BaseGeometry 
my_var: b

Expected Behavior

No error. Mypy should treat BaseGeometry like Any.

Actual Behavior

test.py:4: error: Variable "test.b" is not valid as a type  [valid-type]
test.py:4: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.6.1
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.10.10

Workaround

Explicitly annotate variable as TypeAlias.

[tmke8]

Workaround

Explicitly annotate variable as TypeAlias.

Isn't that a legitimate solution? This is the kind of thing TypeAlias was added for, no?

[robertschweizer]

Because str | Any and Union[str, BaseGeometry] work without annotating the variable with TypeAlias, this might be easily fixable to make using mypy more convenient. But in the end it's only that, convenience.

I marked this as a bug, because it felt like a regression to me running pyupgrade on our codebase and switching from Union to |.