When having a composer.local.json like follows:

{ "extra": { "merge-plugin": { "include": [ "foo/bar/composer.json" ] } } }

Where foo/bar does not exist, there is no error from running composer update.

For MediaWiki core this is convenient since existence of composer.local.json itself is optional. And things like * matches for subdirectories may also legitimately yield no matches. But when specifying an exact file it should report an error to catch regressions, typos etc.

This is a known problem in many libraries that pull everything through pattern-matchers (same in Grunt often), but should be fixable hopefully.