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.

Inizia eseguendo il riproduttore Python dell’esempio mypy-play collegato e traccia come viene gestito il restringimento dei tipi attraverso l’assegnazione await e il ramo except. L’issue non indica alcun file del repository né alcun test; il lavoro è completo quando l’esempio viene accettato senza l’errore di assegnazione segnalato e il comportamento ha una copertura di regressione.