More specific inline dict type inference can fail on 1.17.0

[markhobson]

Bug Report

More specific inline dict type inference can fail on 1.17.0.

To Reproduce

The following fails on 1.17.0 but is fine on 1.16.1 (the playground link uses master as 1.17.0 isn't available there):
https://mypy-play.net/?mypy=master&python=3.13&gist=7e77b89fe5f9cb7e548c4ef83b3783c1

Revealing the type for the inline dict gives:

• 1.16.1: builtins.dict[builtins.str, Union[typing.Collection[builtins.str], None]]
• 1.17.0: builtins.dict[builtins.str, builtins.dict[builtins.str, builtins.list[Any] | None] | builtins.dict[builtins.str, builtins.str] | builtins.str | None]

Expected Behavior

No mypy errors.

Actual Behavior

main.py:11: error: Dict entry 0 has incompatible type "str": "None"; expected "str": "str"  [dict-item]
main.py:12: error: Dict entry 1 has incompatible type "str": "list[Never]"; expected "str": "str"  [dict-item]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.17.0
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.13.5

[sterliakov]

Bisects to #19158; it might have only uncovered an existing issue and not be the direct cause.

[hauntsaninja]

Yeah, it just exposed existing issue. The following will repro on 1.16 as well:

from typing import Any


json: Any
if json == {
    "x": None,
    "x": "y",
    "x": {
        "x": "y",
    },
    "x": {
        "x": None,
        "x": [],
    },
}:
    pass