Confusing error from bidirectional inference?

Bug Report

When the return type from dict.pop is incompatible with existing variable type, mypy gives an arg-type error when an assignment error seems more intuitive.

(Originally noted in #10152)

To Reproduce

values_by_name = {"one": 1, "two": 2}
value: int
value = values_by_name.pop("one", None)

https://gist.github.com/mypy-play/972f4514ebe5620ece42583192907f62

Expected Behavior

example.py:3: error: Incompatible types in assignment (expression has type "int | None", variable has type "int")  [assignment]

Actual Behavior

example.py:3: error: Argument 2 to "pop" of "dict" has incompatible type "None"; expected "int"  [arg-type]

Note: this is a lot more surprising if value has this type because it was unintentionally reused from some code further up, e.g. if you're using value in a loop or something; it can be very difficult to spot what the real error is. See https://github.com/python/mypy/issues/10152#issuecomment-2538685996 for example.

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.12.2

Inizia riproducendo la diagnostica dell’issue in example.py con mypy 1.13.0, quindi traccia il modo in cui l’inferenza bidirezionale gestisce il valore predefinito di dict.pop e il tipo della variabile esistente. Il lavoro è completato quando l’esempio segnala un’assegnazione incompatibile con [assignment] anziché un errore di tipo dell’argomento.