Inheritance through a parameterized type alias fails

[OlivierBlanvillain]

class Parser[T, Elem]

object Test {
  type P[T] = Parser[T, Char]
  class CustomParser extends P[Unit]
}

Dotty wrongly complains that Test.P is not a class type

[Blaisorblade]

So apparently the problem is only with applied types — contrast working:

object Test {
  class Parser[T, Elem]
  type P = Parser[Unit, Char]
  class CustomParser extends P
}

with failing:

object Test {
  class Parser[T, Elem]
  type P[T] = Parser[T, Char]
  class CustomParser extends P[Unit]
}

Also note how it complains on Test.P not Test.P[Unit], and that's crucial.
Modifying the error in checkClassType to be

      case tp1 =>
        ctx.error(ex"$tp is not a class type, it's ${tp.dealias} hence $tp1", pos)
        defn.ObjectType

gives

Test.P is not a class type, it's [T] => Test.Parser[T, Char] hence <nonsensical><notype></nonsensical>