Reenable an error code that was previously disabled

Author: srittau

tests/typecheck_typeshed.py

@@ -60,9 +60,8 @@ def run_mypy_as_subprocess(directory: str, platform: str, version: str) -> Retur
         "--no-error-summary",
         "--enable-error-code",
         "ignore-without-code",
-        # https://github.com/python/mypy/issues/14309
-        # "--enable-error-code",
-        # "possibly-undefined",
+        "--enable-error-code",
+        "possibly-undefined",
         "--enable-error-code",
         "redundant-expr",
         "--enable-error-code",