Fix #2378: Better error message for extractors

[odersky]

No description provided.

[liufengyun]

Now it produces the following error message, which is incorrect:

-- Error: examples/abstract.scala:11:9 -----------------------------------------
11 |    case Apply(fun, args) => 3
   |         ^^^^^
   |Apply cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method

one error found

BTW, it seems you accidentally added scala-scala.

For the overloaded extractors, following test case is preferred:

trait Cap
trait Tree

object Apply {
  def unapply(tree: Tree): Option[(Tree, Seq[Tree])] = ???
  def unapply(tree: Tree)(implicit c: Cap): Option[(Tree, Seq[Tree])] = ???
}

class Test {
  def foo(tree: Tree): Int = tree match {
    case Apply(fun, args) => 3
  }
}

[odersky]

Now it produces the following error message, which is incorrect:

I know... But it's better then what we had, no? It's surprisingly difficult to do better. I tried, but had to postpone due to lack of time.

BTW, it seems you accidentally added scala-scala.

Ah, that seems to be the source of my problems. Git claims it's a modified file that needs to be added. But if I do that CI breaks. It would be good if someone told me what the issue here is.

[liufengyun]

Git claims it's a modified file that needs to be added. But if I do that CI breaks. It would be good if someone told me what the issue here is.

The simple rule works for me is: if there's no reason to forward a submodule to a specific new version, never add changes in the submodule folder. The changes might be some generated files that are not added to .gitignore of the submodule.

It's surprisingly difficult to do better. I tried, but had to postpone due to lack of time.

I see it's difficult to do given our current tryEither, typeIdent, typeSelect structure. Later we can consider refactor parts of typeIdent and typeSelect to a standalone NameResolution, which may help. I agree we can address this later.

BTW, you need to using the following test case, as the original test case is already fixed in #2358 .

trait Cap
trait Tree

object Apply {
  def unapply(tree: Tree): Option[(Tree, Seq[Tree])] = ???
  def unapply(tree: Tree)(implicit c: Cap): Option[(Tree, Seq[Tree])] = ???
}

class Test {
  def foo(tree: Tree): Int = tree match {
    case Apply(fun, args) => 3
  }
}