Type narrowing for Optional[T] fails inside an else branch inside a while cycle

Bug Report

from typing import Optional                                                      
                                                                                 
                                                                                 
class Node:                                                                      
    def __init__(self, value: int):                                      
        self.value = value                                                       
        self.next: Optional[Node] = None                                         
                                                                                 
                                                                                 
def foo(node: Node) -> None:                                                     
    while node.next is not None:                                                 
        if node.value != node.next.value:                                        
            node = node.next                                                     
        else:                                                                    
            node.next = node.next.next   # <-- HERE

Expected Behavior

No mypy errors. Mypy should know that node.next is not None in the else branch.

Actual Behavior

Mypy says error: Item "None" of "Optional[Node]" has no attribute "next" on the line marked by <-- HERE. (With column numbers turned on, the location is the beginning of node.next.next.)

Interestingly, everything is OK if the branches of if are reversed:

def foo(node: Node) -> None:                                                     
    while node.next is not None:                                                 
        if node.value == node.next.value:                                        
            node.next = node.next.next                                           
        else:                                                                    
            node = node.next                                                     

Writing assert node is not None before the problematic line also fixes the error (but seems quite redundant).

Your Environment

• Mypy version used: mypy 0.930
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): default, no config files
• Python version used: Python 3.10.1
• Operating system and version: Arch Linux

まず、issue の Optional[Node] の例に対して設定なしで mypy 0.930 を実行し、次に 2 つの分岐順序と assert 版を比較します。while 条件と代入の周辺での型の絞り込みを追跡します。元の例が冗長な assert なしでエラーを報告しなければ完了です。