False negative: potentially unbound instance attribute doesn't trigger `possibly-undefined` error

Bug Report

When enabling the possibly-undefined error code, a potentially undefined instance attribute doesn't trigger the error, while a potentially unbound variable does.

To Reproduce

def test_possibly_undefined(flag: bool):
    if flag:
        a = 1
    print(a)  # error: Name "a" may be undefined  [possibly-undefined]


class Foo:
    def test_possibly_undefined(self, flag: bool):
        if flag:
            self.a = 1
        print(self.a)  # false negative, should also be a possibly-undefined error


try:
    test_possibly_undefined(False)
except Exception as e:
    print(f"{type(e).__name__}: {e}")  # UnboundLocalError: cannot access local variable 'a' where it is not associated with a value

f = Foo()
try:
    f.test_possibly_undefined(False)
except Exception as e:
    print(f"{type(e).__name__}: {e}")  # AttributeError: 'Foo' object has no attribute 'a'

Also available as mypy Playground, but cannot enable enable_error_code = possibly-undefined there.

Expected Behavior

A possible-undefined error should be reported for line 11 as well, e.g.:

test.py:4: error: Name "a" may be undefined  [possibly-undefined]
test.py:11: error: Attribute "a" may be undefined  [possibly-undefined]
Found 2 errors in 1 file (checked 1 source file)

Actual Behavior

No error reported for line 11.

test.py:4: error: Name "a" may be undefined  [possibly-undefined]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.11.2
• Mypy command-line flags: --enable-error-code=possibly-undefined
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.12.6

Commencez par reproduire l’exemple avec mypy 1.11.2 et le code d’erreur possibly-undefined, puis localisez la logique du checker et les tests de régression concernant les variables locales potentiellement non définies. Comparez le traitement des noms locaux avec celui des attributs d’instance et ajoutez une couverture pour l’exemple Foo ; le travail est terminé lorsque mypy signale l’erreur attendue Attribute "a" may be undefined.