"Missing type parameters for generic type" gives incorrect results for type aliases

[mgilson]

I'm trying mypy==0.650 on my codebase (using --strict) and I'm hitting some new positives associated with --disallow-any-generics. Some of these are legit warnings that I should fix, but a couple have me flummoxed.

For example, the following is an attempt to describe the type that you can pass to requests as the json= argument (it'll take a file-like object, anything that you can shove into json.dumps a string or bytes object):

from typing import Union, Sequence, Any, IO, AnyStr, Mapping

# This is a pretty lousy approximate way to say "an object that can be made into JSON"
_JSON_TYPE_ISH = Union[
    Mapping[str, Any],
    Sequence[Any],
    str, int, None]
_REQUEST_BYTES = Union[
    IO[AnyStr],
    AnyStr, None]
_REQUEST_TYPE = Union[
    _JSON_TYPE_ISH,
    _REQUEST_BYTES,
    None
]

Running this gets me:

$ mypy --strict sandbox/foo.py
sandbox/foo.py:13: error: Missing type parameters for generic type

However, if I inline the _REQUEST_BYTES type alias, it seems to work OK:

from typing import Union, Sequence, Any, IO, AnyStr, Mapping

# This is a pretty lousy approximate way to say "an object that can be made into JSON"
_JSON_TYPE_ISH = Union[
    Mapping[str, Any],
    Sequence[Any],
    str, int, None]
# _REQUEST_BYTES = Union[
#     IO[AnyStr],
#     AnyStr, None]
_REQUEST_TYPE = Union[
    _JSON_TYPE_ISH,
    # _REQUEST_BYTES,
    Union[IO[AnyStr], AnyStr, None],
    None
]

It's possible that this issue is related to #6112 -- though I tried to read though that issue and I'm not entirely sure if it's just another symptom of the same problem or not.

[AvdN]

If I read ilevkivskyi comment on issue #6112 correctly, you should be able to get rid of the error by changing line 13 to read: _REQUEST_BYTES[AnyStr]. I am not sure if that is the correct solution, but

# This is a pretty lousy approximate way to say "an object that can be made into JSON"
_JSON_TYPE_ISH = Union[
    Mapping[str, Any],
    Sequence[Any],
    str, int, None]
_REQUEST_BYTES = Union[
    IO[AnyStr],
    AnyStr, None]
_REQUEST_TYPE = Union[
    _JSON_TYPE_ISH,
    _REQUEST_BYTES[AnyStr],
    None
]

doesn't give an error.

[ilevkivskyi]

@mgilson This is not a bug. You define a generic alias and then use it without a type parameter (FYI AnyStr is a type variable).