Better narrowing

[hauntsaninja]

I have a stack of several commits that improve mypy's narrowing

I will post relevant PRs and notes in this thread

[hauntsaninja]

The first PR is:

• Rework narrowing logic for equality and identity #20492

It's been open for two weeks, so I will probably merge it in another week or so if there aren't comments

[hauntsaninja]

In case useful, for now I have several of my queued changes visible together on this branch: hauntsaninja#5

[hauntsaninja]

The second PR is pure refactoring (as a follow up to #20492 ):

• Refactor equality and identity narrowing for clarity #20595

I also opened a third PR with some more test cases:

• Add some additional narrowing test cases #20598

[hauntsaninja]

Note the first PR does introduce three regressions that I haven't yet solved:

• Regression with narrowing with multiple hasattr #20596
• Regression with narrowing with negative intersection #20597
• Regression with narrowing with TypeVar intersection #20628

[hauntsaninja]

A fourth PR is better narrowing for NotImplemented:

• Treat NotImplemented as a singleton type #20601

[hauntsaninja]

This next one is a useful one! Fifth PR is narrowing for collection containment:

• Narrow types based on collection containment #20602

[hauntsaninja]

Contributor PRs to follow up on as stack is merged: #20603 (done) #20277 (open) #20538 (done)

[hauntsaninja]

Sixth PR improves logic with enum subclasses:

• Improve narrowing logic for Enum int and str subclasses #20609

[hauntsaninja]

Add this test: #20602 (comment) (done in stack)

[hauntsaninja]

Seventh PR:

• Refactor and improve narrowing for type(x) == t checks #20634

[hauntsaninja]

Eighth PR (exploratory for now, do not review):

• [do not review] Avoid narrowing TypeType #20636

[hauntsaninja]

Ninth PR is trivial testing related changes:

• Move tests to check-narrowing , improve them slightly #20637