==-based narrowing of Optional

[tyralla]

1. Mypy's current behaviour:

x: int | None
if x == None:
    x  # "Union[builtins.int, None]"
else:
    x  # "Union[builtins.int, None]"

2. Pyright's current behaviour:

x: int | None
if x == None:
    x  # x is None
else:
    x  # x is int

3. I think the ideal solution would be:

x: int | None
if x == None:
    x  # "Union[builtins.int, None]"
else:
    x  # "builtins.int"

This Mypy code comment seems to favour solution 2.

I am asking because if Mypy would follow solution 2 or 3, in-based narrowing of optional types could be implemented more consistently.

Related pull requests: #15760 #17154 #17044.