Wrong type inferred(?) in union type and implicit decorators part 2

[felixmulder]

The following does not compile:

object Test {
  type Result[A] = Errors | A
  final case class Errors(msgs: Seq[String])

  implicit class RichRes[A](val res: Result[A]) extends AnyVal {
    def map[B](f: A => B): Result[B] = res match {
      case xs: Errors => xs
      case a: A => f(a)
    }
  }

  val foo: Result[String] = ???
  def f(str: String): Int = ???

  foo.map(f(_)) // error
}

[error] -- [E007] Type Mismatch Error: /Users/fixel/Projects/dotty/repl/src/dotty/tools/repl/Repl.scala:67:13
[error] 67 |    x1.map(f(_))
[error]    |             ^
[error]    |found:    (dotty.tools.repl.results.Errors | dotty.tools.repl.ReplTyper.Success)(_$1)
[error]    |required: dotty.tools.repl.ReplTyper.Success
[error]    |
[error] one error found
[error] (dotty-repl/compile:compileIncremental) Compilation failed

ping @odersky

[sjrd]

That would actually be a show-stopper for Scala.js to use type js.UndefOr[+A] = A | js.Undefined, which is something we'd like to do.