Cannot write type members in phantom types

[nicolasstucki]

As found in #2040

object MyPhantom extends Phantom {
  type Foo = this.Any { type Member = Int }
}

will fail with

  type Foo = this.Any { type Member = Int }
             ^^^^^^^^
illegal trait inheritance: superclass Object does not derive from trait Any's super<nonsensical><none></nonsensical>

The issue probably that there is an Object added to the type of this.Any { type Member = Int } when desugaring.

[nicolasstucki]

A quick patch in Checking.checkTraitInheritance to avoid the error showed that the following code works.

object MyPhantom extends Phantom {
  type Foo = this.Any { type M }

  def a: Foo { type M = Int } = assume
  def b: Foo { type M = Boolean } = assume

  def foo(a: Foo { type M = Int }): Unit = ()

  foo(a)
  foo(b) // fails as expected
}

[nicolasstucki]

Phantom will be removed in #3410