Special handling in the typer of scala.Singleton is missing

[soronpo]

  case class Foo[T <: Int with Singleton](t : T)
  val one = 1
  final val final_one = 1
  val a : 1 = Foo(1).t //Type Mismatch Error
  val b : Int = Foo(one).t //OK, but unexpected
  val c : 1 = Foo(final_one).t //Type Mismatch Error

Scastie link: https://scastie.scala-lang.org/DcDeG4eYQTG17exjb2FigQ

[smarter]

I don't think this has anything to do with class fields, it's just that Dotty doesn't know anything about theSingleton trait.

[soronpo]

Even without the Singleton the behavior differs between a function argument and a class field.

  object Test {
    val one = 1
    final val final_one = 1

    object OK {
      def foo[T <: Int](t : T) : T = t
      val a : 1 = foo(1)
      val b : Int = foo(one)
      val c : 1 = foo(final_one)
    }

    object Fail {
      case class Foo[T <: Int](t : T)
      val a : 1 = Foo(1).t //Error: Type mismatch
      val b : Int = Foo(one).t
      val c : 1 = Foo(final_one).t //Error: Type mismatch
    }
  }

[smarter]

Ah, that's interesting, still not specific to class fields though :). val a: 1 = Foo(1).t becomes val a: 1 = Foo.apply(1).t, when inferencing the type parameter of apply we first pick up >: 1 from the argument, we should pick up <: 1 from the return type of a but we don't for some reason, I'll open a separate issue for that.

[smarter]

See #2998

[soronpo]

OK, thanks!