Platform check is not remembered after `return`

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers

Research direction

Start with the provided Python reproducer and mypy-play link, then trace how platform checks and early returns affect type narrowing. Compare the if/else and if ... return cases; done means the second form no longer reports a false winreg.REG_SZ attribute error.

Written by the indexing model from the issue text.

Description

feature topic-type-narrowing

Bug Report

A platform check that is working fine in if and else is not working after if ... return.

To Reproduce

import sys

def works() -> None:
    if sys.platform != "win32":
        return
    else:
        from winreg import REG_SZ

def fails() -> None:
    if sys.platform != "win32":
        return
    from winreg import REG_SZ

https://mypy-play.net/?mypy=latest&python=3.11&gist=eddad091a6082bd85c5c7524839726b1

Expected Behavior

No error.

Actual Behavior

main.py:12: error: Module "winreg" has no attribute "REG_SZ" [attr-defined]
Found 1 error in 1 file (checked 1 source file)

Your Environment

  • Failure is on Linux
  • Mypy version used: 1.1.1
  • Mypy command-line flags: none
  • Mypy configuration options from mypy.ini (and other config files): none
  • Python version used: 3.11.2
Dominant language
Python
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Avg merge
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Merged PRs (30d)
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