Fix duplicate bases error (MROs) with GenericAlias

[cdce8p]

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Description

Fix duplicate bases error that occurred for Python <3.9 when using multiple inheritance from _GenericAlias. Starting with 3.9 this is parsed differently as _SpecialGenericAlias and _BaseGenericAlias.

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As this is a bug fix, it would be great if it could be included in the next release 2.5.1.

Type of Changes

	Type
✓	🐛 Bug fix
	✨ New feature
	🔨 Refactoring
	📜 Docs

Related Issue

Closes #905

[hippo91]

@cdce8p thanks for this review.
However i would prefer we wait a little bit to merge it because i think there is probably a better way to handle this.
I would like to dig in a bit more.

[hippo91]

I'm not found of this kind of solution. I think there is maybe something to handle through TypingBrain instead.
However i'm not sure, i'm still investigating this.
By the way on my machine the bug is also present with python 3.9.

[cdce8p]

I understand. Thanks for your work!

By the way on my machine the bug is also present with python 3.9.

Hmm, interesting. MacOS with Py 3.9.2 works. Please let me know if I can help test things, once you have found a better solution.

[cdce8p]

I took another look at it and even tried to modify brain_typing.py: de223ec
That worked. However I quickly noticed that we would have to add all generic aliases from the typing module to fix all issues which would be a lot of work.

After some more investigation, I found that pylint normally would raise a duplicate-bases (E0241) error in such cases. There is even an existing false-positive issue pylint-dev/pylint#2717. Since this wasn't the case, it meant that the astroid.DuplicateBasesError was raised unexpectedly.

That's when I though about adding a flag to guard the exception during the ast parsing which I've done in the last commit.
This resolves the issue and only leaves the false-positive for E0241 which can be ignored by pylint.

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A good test example is:

from typing import Sized, Hashable
class Derived(Sized, Hashable):
    def __init__(self):
        self.var = 1

[hippo91]

@cdce8p i opened #913 that try to fix the same issue.
IMHO it is more generic than what you proposes here.
I would greatly appreciate your opinion on it, as the one of @AWhetter and @Pierre-Sassoulas.
It still failing on pypy and python3.6. I will investigate it ASAP.

[cdce8p]

@cdce8p i opened #913 that try to fix the same issue.
IMHO it is more generic than what you proposes here.
I would greatly appreciate your opinion on it, as the one of @AWhetter and @Pierre-Sassoulas.
It still failing on pypy and python3.6. I will investigate it ASAP.

@hippo91 I just saw it. On first glance, your solution does look quite elegant (and way better than mine). I'll run some tests on my code to see if the issue is completely fixed and will get back to you.

[cdce8p]

Replaced by #913