"Invalid type" error when creating a synonym for Optional[int]

[rwbarton]

from typing import Optional
MyType = Optional[int]
def f(x: bool) -> MyType:
    pass

produces an error

x/invalidtype.py: note: In function "f":
x/invalidtype.py:4: error: Invalid type "invalidtype.MyType"

A type synonym for Union[str, int] works fine.

[gvanrossum]

IIRC global assignments are only remembered as type aliases when they contain certain things. Maybe Optional isn't in the list of things it looks for, while Union is? (I've occasionally had a similar issue but always just worked around it.)

[rwbarton]

Yep that's exactly it. PR incoming...

[rwbarton]

Actually this turns out to be more subtle than expected because of the way that Optional[T] reduces to simply T. After strict optional checking lands this won't be an issue, so let's just defer this until then.

[rwbarton]

Wait, that plan only makes sense if we are getting rid of non-strict optional checking, which probably won't happen that soon.

I managed to get the tests passing again with some simple changes but I don't entirely understand the intent of the old code, so hopefully @JukkaL can take a quick look.

[gvanrossum]

Fixed by 2050d80.

[pgjones]

I don't think this is fixed, both the above and the snippet below fail with mypy 0.501 on Python 3.5.2, as discussed there is no error with --strict-optional enabled.

from typing import List, Optional

T = Optional[int]

Ts = []  # type: List[T]

mypy test.py gives test.py:5: error: Invalid type "test.T".

[ilevkivskyi]

I don't think this is fixed

Confirmed, although I am not sure this is the original issue. In fact this has nothing to do with --strict-optional, a simplest repro that fails both with and without --strict-optional:

A = Union[int]
x: A

The reason is how aliases are analyzed, there is a logical flaw in code, it naively expects that if alias is an IndexExpr then it should result in a parameterized type.

Sorry, I don't have time to fix this right now, although the fix should be quite simple.

[gvanrossum]

Please open a new issue since Ivan says it's not the same issue.