Workaround for scala#3121

Author: nicolasstucki

Foo.scala

@@ -686,6 +686,7 @@ trait Checking {
         val psuper = parent.superClass
         val csuper = cls.superClass
         val ok = csuper.derivesFrom(psuper) ||
+          parent == defn.Phantom_AnyClass ||
           parent.is(JavaDefined) && csuper == defn.AnyClass &&
           (parent == defn.JavaSerializableClass || parent == defn.ComparableClass)
         if (!ok)

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -0,0 +1,6 @@
+object MyPhantom extends Phantom {
+  type Foo = this.Any { type Member = Int }
+
+  type Bar <: this.Any
+  type Foo2 = Bar { type Member = Int }
+}