Attribute of type `A | B | None` becomes `object` after narrowing checks

[bluetech]

Bug Report

When doing narrowing of a class attribute with a particular type shape A | B | None, the type of the attribute becomes object after the narrowing condition chain.

To Reproduce

The following is the most minimal reproduction I was able to create.

class A: pass
class B: pass
class C:
    attr: A | B | None

c = C()
if c.attr is None: pass
elif isinstance(c.attr, A): pass
elif isinstance(c.attr, B): pass  # Optional line - no difference if removed
reveal_type(c.attr)

Expected Behavior

Revealed type is A | B | None.

Actual Behavior

Revealed type is object.

I made some additional tests:

• pyright gives A | B | None (good).
• Doing x = c.x and then doing the narrowing and reveal_type on attr gives A | B | None (good).
• Replacing the None with A | B | D gives A | B | D (good). Also if I replace if isinstance(c.attr, D) with if type(c.attr) is D (good). So the None seems essential.
• With just attr: A | None, gives A | None (good). So more than one type besides the None seems essential.
• Changing the elif -> if gives A | B | None (good). So the elif seems essential.

Your Environment

• Mypy version used: master, 0.931, 0.900 (didn't try earlier versions).
• Mypy command-line flags: --strict.
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.9, 3.10
• Operating system and version: Arch Linux

[erictraut]

This is yet another case where mypy's use of join rather than union produces undesirable results.

Here are some of the other bugs that would be fixed if this change were made:

#11934
#11440
#11618
#10740
#10442
#7884
#7835
#7616
#6968
#6079
#5512
#5128
#4134
#3339

I'm sorry to sound like a broken record, but I really think there should be a serious discussion about moving from join to union. Pyright uses only union, never join, and it avoids all of these issues.

[JelleZijlstra]

I agree with Eric that we should strongly consider moving away from join. I am going to open a new issue to propose that.

[brianschubert]

Fixed by #18138, which should be released with mypy v1.14.0. The revealed type is now Union[SCRATCH.A, SCRATCH.B, None] as expected.