no error when `Any` type in narrowed generic when the no `Any` flags are enabled

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#12,015 opened on Jan 19, 2022 3 comments 2 reactions 0 assignees View on GitHub

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bug

Dominant language: Python
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Description

foo: object
if isinstance(foo, list):
    a = foo[1]
    reveal_type(a) # Any
    a.asdfasdfasdf # no error

https://mypy-play.net/?mypy=master&python=3.10&flags=show-error-codes%2Callow-redefinition%2Cstrict%2Ccheck-untyped-defs%2Cdisallow-any-decorated%2Cdisallow-any-expr%2Cdisallow-any-explicit%2Cdisallow-any-generics%2Cdisallow-any-unimported%2Cdisallow-incomplete-defs%2Cdisallow-subclassing-any%2Cdisallow-untyped-calls%2Cdisallow-untyped-decorators%2Cdisallow-untyped-defs%2Cno-implicit-optional%2Cno-implicit-reexport%2Cstrict-equality%2Cwarn-incomplete-stub%2Cwarn-redundant-casts%2Cwarn-return-any%2Cwarn-unreachable%2Cwarn-unused-configs%2Cwarn-unused-ignores&gist=21c3e4f59ece68483eee16c5dc6d880b

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Research direction

Reproduce the Python snippet in the linked mypy-play example with the listed no-Any flags enabled, and confirm the revealed type and missing attribute error. Trace the type narrowing and generic list-item inference involved, then verify that the invalid attribute access is rejected without introducing unrelated diagnostics.

Written by the indexing model from the issue text.

Assessment

Tech stack: python
Domain: devtools
Issue type: Bug
Difficulty: 4/5
Estimated time: 3-5 days
Activity status: Stale
Clarity: Mostly clear
Newbie friendliness: 45/100