subscriptable dicts not recognized as Python 3.9

[barsnick]

Describe the bug
In Python < 3.9, subscripting a dict results in a type error. This was apparently changed with PEP 585. vermin does not recognize this version dependency.

To Reproduce
Use this file testcase.py:

import typing

bla: dict[int, str] = { 2, "foo", 3, "bar" }
bla2: typing.Dict[int, str] = { 2, "foo", 3, "bar" }

Run as such:

$ vermin.py -vv testcase.py
Detecting python files..
Analyzing using 2 processes..
!2, 3.6      /path/to/testcase.py
  'typing' module requires !2, 3.5
  variable annotations requires !2, 3.6

Tips:
- You're using potentially backported modules: typing
  If so, try using the following for better results: --backport typing
- Since '# novm' or '# novermin' weren't used, a speedup can be achieved using: --no-parse-comments
(disable using: --no-tips)

Minimum required versions: 3.6
Incompatible versions:     2

Expected behavior
Something like this:

!2, 3.9      /path/to/testcase.py
  subscripting dicts requires !2, 3.9

Environment (please complete the following information):
vermin 1.4.1, git ec1e551

Additional context
Note: In order to restore compatibility with Python < 3.9, the subscripted dict can be replaced with typing.Dict, perhaps with an explicit typecast to a list, as used in the example above. That's not what vermin is concerned with, but what I had to do, to get rid of the surprising (as unreported) dependency.

[netromdk]

Thanks for reporting this! I'll have a look soon.

[netromdk]

As per the caveats section of the README:

Function and variable annotations aren't evaluated at definition time when from __future__ import annotations is used (PEP 563). This is why --no-eval-annotations is on by default (since v1.1.1, #66). If annotations are being evaluated at runtime, like using typing.get_type_hints or evaluating __annotations__ of an object, --eval-annotations should be used for best results.

If you use --eval-annotations then it gives the desired result:

!2, 3.9      issue-86.py
  'typing' module requires !2, 3.5
  builtin generic type annotation (dict[..]) requires !2, 3.9
  set literals requires 2.7, 3.0
  variable annotations requires !2, 3.6

Minimum required versions: 3.9
Incompatible versions:     2

As a side note, I've added heuristics for displaying when generic or literal annotations might be in use, which would display the following tip:

Tips:
- Generic or literal annotations might be in use. If so, try using: --eval-annotations
  But check the caveat section: https://github.com/netromdk/vermin#caveats
(disable using: --no-tips)

But it does not show in this specific case. I'll see if that can happen without ruining things/tests.

[netromdk]

I've improved the heuristics such that it will show the above tip in your case: v1.4.2

[barsnick]

Great, thanks for the hint! I obviously didn't read the caveats section correctly, sorry.

The added tip is indeed helpful.

[netromdk]

Don't mention it. Glad it helped. :-)