Revert "Add post-condidtion to CheckPhantomCast"

This reverts commit 4b5bd1f.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/CheckPhantomCast.scala

@@ -16,14 +16,6 @@ class CheckPhantomCast extends MiniPhaseTransform { thisTransformer =>
 
   override def phaseName = "checkPhantomCast"
 
-  override def checkPostCondition(tree: tpd.Tree)(implicit ctx: Context): Unit = {
-    tree match {
-      case TypeApply(fun, targs) if fun.symbol eq defn.Any_asInstanceOf => assert(!containsPhantom(targs.head.tpe))
-      case Bind(_, Typed(_, tpt)) => assert(!containsPhantom(tpt.tpe))
-      case _ =>
-    }
-  }
-
   override def transformTypeApply(tree: tpd.TypeApply)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
     if (tree.fun.symbol eq defn.Any_asInstanceOf)
       checkNoPhantoms(tree.args.head)
@@ -39,12 +31,13 @@ class CheckPhantomCast extends MiniPhaseTransform { thisTransformer =>
   }
 
   private def checkNoPhantoms(tpTree: tpd.Tree)(implicit ctx: Context): Unit = {
-    if (containsPhantom(tpTree.tpe))
-      ctx.error("Cannot cast type containing a phantom type", tpTree.pos)
+    val checker = new TypeTraverser() {
+      override def traverse(tp: Type): Unit = {
+        if (tp.isPhantom) ctx.error("Cannot cast type containing a phantom type", tpTree.pos)
+        else traverseChildren(tp)
+      }
+    }
+    checker.traverse(tpTree.tpe)
   }
 
-  private def containsPhantom(tp: Type)(implicit ctx: Context): Boolean = new TypeAccumulator[Boolean] {
-    override def apply(x: Boolean, tp: Type): Boolean = x || tp.isPhantom || foldOver(false, tp)
-  }.apply(x = false, tp)
-
 }