I don't believe this is a regression. The error displays because the glob match src/*.mm did not resolve to any files.

Ok, so a behaviour change I suppose.
it still accepts non existent glob matches like this

        - src/**/*.cpp

this is why this error was not clear for me at first.

I was using this ot have an "one size fits all" kind of project.
so I suppose I have to directory list recursively and add files separately as needed?

I tried with src/**/*.mm where no .mm files exist in src, and it does the correct thing - it errors.

As long as a glob match for source includes resolves to at least one file, you should be fine.