Fix #134 by marking Enum::__callStatic as @psalm-pure

[Ocramius]

To achieve proper static analysis coverage:

• vimeo/psalm has been updated
• a new static-analysis dir has been added (to test static analysis properties only)
• impure $this usages in Enum have been ignored in static analysis checks
• PHP version support has been upgraded from >=7.1 to ^7.3 || ^8.0
• PHPUnit version has been upgraded to its latest and greatest

Fixes #134

[Ocramius]

This was changed because:

1. psalm does not understand this union type
2. new MyEnum(new MyEnum('foo')) makes no sense anyway

[mnapoli]

new MyEnum(new MyEnum('foo')) is a supported use case though 🤔

But your change only prevents that from happening explicitly, through static analysis. So after all this change makes sense I think.

[drealecs]

I thought about it in my other PR: #136 and, based on https://psalm.dev/articles/immutability-and-beyond#mutation-free-methods article, I think we should replace some @psalm-pure with @psalm-mutation-free so we don't suppress ImpureVariable

[Ocramius]

@drealecs nack: @psalm-immutable is correct here, as ENUMs also do not change internally post-instantiation, it's just that the implementation is quite convoluted due to the magic subclass constant detection logic

[drealecs]

@Ocramius , yes, totally agree.
And @psalm-immutable on class is basically @psalm-mutation-free on all instance methods, not @psalm-pure.
For static methods, I agree we can keep @psalm-pure.

[mnapoli]

Looking good, I'm not able to have an opinion on the Psalm thing. Are you both saying you're happy with the current state of the PR?

Or is there still a question to resolve?

[mnapoli]

new MyEnum(new MyEnum('foo')) is a supported use case though 🤔

But your change only prevents that from happening explicitly, through static analysis. So after all this change makes sense I think.

[Ocramius]

new MyEnum(new MyEnum('foo')) is a supported use case though thinking

Yes, but at type level, I don't expect anyone to actually use it anyway, so people that use vimeo/psalm are going to be affected by the change, but that's also not a big deal either.

EDIT: this was also discussed in private with @michaelpetri, who worked on #135.

This also fixes #137

[mnapoli]

Perfect thanks!