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

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 https://github.com/python/mypy/issues/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.

Start by reproducing the reported case in test.py with mypy 1.6.1 and Python 3.10.10, comparing the failing alias str | BaseGeometry with the working str | Any case. Trace the type-alias validation path for classes imported from untyped packages; done means the reproduction reports no valid-type error while preserving the existing workaround behavior.