Example
Given the file ./mylib/minimalistic_repro.py

class Foo:
    def __init__(self, param):
        self.param = param

class Bar:
    def __init__(self, param):
        self.param = param

def foobar(par1, par2):
    return par1.param + par2.param

and its stub ./mylib/minimalistic_repro.pyi

from typing import TypeAlias

FooBar: TypeAlias = Foo | Bar

class Foo:
    param: int
    def __init__(self, param: int) -> None: ...

class Bar:
    param: int
    def __init__(self, param: int) -> None: ...

def foobar(par1: FooBar, par2: FooBar) -> int: ...

Note the type alias on the third line: FooBar: TypeAlias = Foo | Bar

mypy --strict mylib, flake8 mylib and ruff check --select E,F,B,SIM all pass.

When running stubtest however:
python -m mypy.stubtest mylib

I get the following error:
error: mylib.minimalistic_repro.FooBar is not present at runtime …

My current workaround is to use an allowlist (stubtest --generate-allowlist).

Question(s)
● Is there a “better” way to avoid this “error”? / …
● … Am I doing something fundamentally wrong? …
● … and if not: Might this be worth a feature request?

Other approaches
● Of course I could declare
 def foobar(par1: Foo | Bar, par2: Foo | Bar),
 but my actual task (writing type hints for a third party pypi package) requires a union of up to 18 types.

● I got the above example to run with stubtest by placing the FooBar type alias definition in a .py file (tp_aliases.py) and then reimporting. This approach failed to work in the case of with my actual pypi package type hinting task (tp_aliases.py is not part of the pypi package).