Regression: new <type alias> should compile

[japgolly]

Compiler version

3.0.0-RC1

Minimized code

object Ah {
  final case class Values[B, +A](a: A)

  trait Object[B] {
    final type Values[+A] = Ah.Values[B, A]
    object Values {
      def blah: Values[Unit] =
        new Values(())
    }
  }
}

Output

-- Error: as.scala:8:12 --------------------------------------------------------
8 |        new Values(())
  |            ^^^^^^
  |            Object.this.Values is not a class type
-- Error: as.scala:8:19 --------------------------------------------------------
8 |        new Values(())
  |                   ^^
  |     too many arguments for constructor Object in class Object: (): Object
2 errors found

Expectation

It should compile. It compiles with Scala 2.12 and 2.13.

[odersky]

Further minimization:

object Test:
  class C1[A](a: A)
  type D1[A] = C1[A]
  new D1(1)    // works

  class C2[B, A](a: A)
  type D2[A] = C2[Int, A]
  new D2(1)    // error

The problem arises if the type alias does not simply forward all its type parameters to the class.

[odersky]

As a workaround you can explicitly pass the type argument.

new D2[Int](1)   // works

The problem is that at the time we typecheck the new we do not have the inferred type. And just looking at D2 it's not true that this is an alias of C2. Hence the failure.