How are updates.fileExtensions merged when multiple config files are in scope?

[bpholt]

How are updates.fileExtensions merged when you specify a default config with --repo-config and a repo also has its own .scala-steward.conf file? I'm trying to write some tests to validate our config files and I want to confirm that I understand the existing behavior (and that the existing behavior is the desired behavior).

e.g. the --repo-config file has

updates.fileExtensions = [".foo", ".bar"]

and the repo .scala-steward.conf file has

updates.fileExtensions = [".bar", ".baz"]

I assumed the repo's config file would override the defaults as set by --repo-config, but it looks like RepoConfigAlg#mergeWithGlobal eventually (via the Monoid instance definitions) uses UpdatesConfig$mergeFileExtensions:

scala-steward/modules/core/src/main/scala/org/scalasteward/core/repoconfig/UpdatesConfig.scala

Lines 227 to 231 in 15ff290

	private[repoconfig] def mergeFileExtensions(
	x: Option[List[String]],
	y: Option[List[String]]
	): Option[List[String]] =
	combineOptions(x, y)(_.intersect(_))

This calcuates the intersection of the two lists of extensions. Is the intention that repos can only narrow the set of updatable options?

Ultimately this means the combination of those two config files would result in the equivalent of

updates.fileExtensions = [".bar"]

which I thought was a surprising result. I was expecting either that the repo-specific config file would override the default entirely, or the union of the two lists would be used.

This came up as I tried to write tests validating that our Scala Steward instance's config files are valid, e.g.

private given Logger[IO] = Slf4jLogger.getLogger[IO]
private given FileAlg[IO] = FileAlg.create
private given WorkspaceAlg[IO] = … stub instance …

test("validate this repo's .scala-steward.conf"):
  val file = File(".scala-steward.conf")
  for
    thisRepoResult        <- RepoConfigAlg.readRepoConfigFromFile(file)
    globalConfig          <- RepoConfigAlg.readRepoConfigFromFile(File("default.scala-steward.conf"))
    repoConfigAlgInstance =  new RepoConfigAlg(globalConfig.maybeRepoConfig)
    mergedResult          =  repoConfigAlgInstance.mergeWithGlobal(thisRepoResult.maybeRepoConfig)
    _ <- IO.println(mergedResult)
  yield
    thisRepoResult.fold(
      fail(s"$file does not exist"),
      e => fail(s"Config is invalid: $e"),
      _ => () // TODO assert exceptions on mergedResult
    )