Provide more context about why incompatible with supertype is an error

[JukkaL]

Users often don't realize that overriding a method with an incompatible signature is unsafe from a type checking perspective; see #1237 for an example. We could generate some more descriptive notes with the error message, perhaps with a link to mypy documentation what contains a detailed discussion of Liskov Substitution Principle. We could also discuss how to work around the issue.

#5704 and #5025 are related issues.

[ckarnell]

@JukkaL I would be happy to take a stab at this if I get a couple extra cycles soon, if were a little more clear exactly what we want here. What should the notes be exactly? Or should I just wing it?

[djb4ai]

Hey @ckarnell are you still working on it?

[JukkaL]

Here's a proposal for the notes that could be added:

prog.py:7: error: Argument 1 of "f" is incompatible with supertype "C"; supertype defines the argument type as "Optional[int]"
prog.py:7: note: This violates the Liskov substitution principle
prog.py:7: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides

[ckarnell]

Hi, just want to drop a note that I haven't gotten a chance to work on this.

[ChetanKhanna]

Hi, if no one's working on it, maybe I can?
And jump-starting to it, can someone let me know which file should define these new error/note messages? Is it one of the functions in mypy/messages.py that needs to be edited?

[msullivan]

Yeah, feel free to take a shot. Probably the messages are in mypy/messages.py, yeah. It is possible that you'll need to gather more information from the call sites (probably in checker.py) to produce a good message.

[ChetanKhanna]

Here's a proposal for the notes that could be added:

prog.py:7: error: Argument 1 of "f" is incompatible with supertype "C"; supertype defines the argument type as "Optional[int]"
prog.py:7: note: This violates the Liskov substitution principle
prog.py:7: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides

I made a PR with the notes suggested here