But diff does not exists in a vacuum. It would need to be integrated to IDEs, editor, merge tools, PR tools. You now have to have `patch` understand and depend on the details of the syntax of your language. In a way that may break between different versions of diff and patch. Not even starting with variants of a language, different interpretations of how to handle the preprocessor/macros system, different editions of the same language.

All that, just to not have to add a trailing comma ?

Most things like patch would still use line diff. I think I would want this semantic diff in the merge request review and the logs. It could have definitions distributed with the syntax highlighters in language plugins in an editor/IDE, git CLI, and git forge.

So GitHub, git lab, gutbucket, gittea (and its 50 forks), fossil, and all the other tools should change, not the formatting of the code?

No, having a concise separator is still preferable to saving a line in a diff.