Spurious type error in `except` branch after an assignment from an `await` that throws

Bug Report

If a loop is guarded with an x is not None condition, and the body of the loop contains a try...except that assigns to x, mypy erroneously concludes that x might be None in the except branch.

To Reproduce

(https://mypy-play.net/?mypy=latest&python=3.12&gist=000b86de168d7bca72f3ff58de4121cb)

import asyncio
from collections.abc import Awaitable
from typing import Optional

R = Awaitable[Optional['R']]

async def boom() -> R:
	raise ValueError("Boom!")

async def main() -> None:
	x: Optional[R] = boom()
	y: R
	while x is not None:
		try:
			x = await x
		except:
			y = x
			break

asyncio.run(main())

Expected Behavior

The program is well-typed.

Actual Behavior

main.py:17: error: Incompatible types in assignment (expression has type "R | None", variable has type "R")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

See Playground details.

まず、リンクされた mypy-play の例から Python reproducer を実行し、await での代入と except 分岐にまたがって型の絞り込みがどのように扱われるかを追跡します。Issue ではリポジトリ内のファイルもテストも指定されていません。完了条件は、報告された代入エラーなしで例が受け入れられ、その挙動に回帰テストのカバレッジがあることです。