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

Start by reproducing the diagnostic from the issue in example.py with mypy 1.13.0, then trace how bidirectional inference handles the dict.pop default and the existing variable type. Done means the example reports an incompatible assignment with [assignment] rather than an argument-type error.