@jay I made a PR on the ERC6551 reference implementation.
The examples in the repo, despite being basic, are correct, audited and safe. They can be used as a solid base to build more advanced contracts. In order to allow so, the functions must be virtual, so that they can be overridden, and public to be called as super (like supportsInterfaceId).
This PR simply add virtual to the functions’ signature.
PS > I also made a version of the reference implementation for the npm registry at
published as erc6551 on npm, which has already virtualized the functions, since we use the upgradeable example in one of our projects and needed it.