Incorrect inferred type

[epenet]

Bug Report

Whilst working on HomeAssistant PR, found this weird behavior where the type inferred from a dict is completely wrong.
home-assistant/core#82853

To Reproduce

ZHA_HW_DISCOVERY_DATA = {
    "name": "Yellow",
    "port": {
        "path": "/dev/ttyAMA1",
        "baudrate": 115200,
        "flow_control": "hardware",
    },
    "radio_type": "efr32",
}
reveal_type(ZHA_HW_DISCOVERY_DATA)

Expected Behavior

I would have expected the value type to be dict[str, str | dict], or maybe dict[str, Any]

Actual Behavior

Revealed type is builtins.dict[builtins.str, typing.Collection[builtins.str]]

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.9.15

[AlexWaygood]

This is another one of the many "join-vs-union" behavioural peculiarities that mypy has: in many situations, mypy prefers to infer a type as being a common supertype rather than a union. Here, it sees that some values in the dictionary are str, and others are dict[str, object], so infers Collection[str], since this is indeed a common supertype of str and dict[str, object] according to the definitions in typeshed.

We already have many join-vs-union issues open, and I don't really think we need another, so I'm going to close this out. Follow #12056 for discussion on possibly changing mypy's behaviour here :)