--possibly-undefined misses for-loop conditional definitions

Bug Report

--enable-error-code possibly-undefined does not detect when a local variable is conditionally set by a for loop.

To Reproduce

https://gist.github.com/mypy-play/25a51361be6d697e25b32587e4cba351
https://mypy-play.net/?mypy=latest&python=3.11&gist=25a51361be6d697e25b32587e4cba351

Expected Behavior

If a variable is set by a for loop over an iterable (i.e. for x in l), and

• that variable was not previously initialized
• the iterable may be empty

then mypy should warn against a UnboundLocalError that would be raised by accessing that variable after the loop, were that iterable to be empty.

Actual Behavior

mypy reports no issues.

A workaround is to wrap the for loop with an if condition to check if the iterable being looped over is not empty.

For example:

if l:
  for x in l:
    # ...

Your Environment

• Mypy version used: 1.5.1
• Mypy command-line flags: --enable-error-code possibly-undefined
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.4

Start with the linked mypy-play reproduction and the --enable-error-code possibly-undefined flag to confirm the missing warning for an empty iterable. Trace the relevant for-loop flow analysis, add a regression test for the conditional definition case, and verify that mypy reports the expected possibly-undefined error.