Strange type expected from inferred dict

[Garrett-R]

Bug Report

Strange inference with dict.

To Reproduce

The repro is found in this Mypy playground, but here it is for convenience:

import random

value: str|None = None

if random.random() > 0.5:
    my_dict = {'a': False, 'b': None}
else:
    my_dict = {'a': True, 'b': value}

Expected Behavior

No error.

Actual Behavior

Error: main.py:8: error: Dict entry 1 has incompatible type "str": "Optional[str]"; expected "str": "Optional[bool]" [dict-item

Your Environment

• Mypy version used: 0.911
• Python version used: 3.11

[gandhis1]

I believe this is the same issue as #2008.

[hauntsaninja]

This isn't related to #2008, which talks about how to narrow types of symbols that have a declared type.

There is no declared type here, adding one would remove the error, e.g.

my_dict: dict[str, bool | str | None]
if random.random() > 0.5:
    my_dict = {'a': False, 'b': None}
else:
    my_dict = {'a': True, 'b': value}

You may also want to use a TypedDict