Change flags for Any_typeCast

 - it should not be Erased since that will remove the complete call, including the expression that
   is casted. It should be StableRealizable, so that a pure expression followed by a synthesized cast
   is still recognized as pure.

 - to make up for the loss of Erased, we take the method into account specifically when deciding
   whether a tree is erased.

Author: odersky

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -277,7 +277,7 @@ class Definitions {
     lazy val Any_isInstanceOf: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.isInstanceOf_, _ => BooleanType, Final)
     lazy val Any_asInstanceOf: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.asInstanceOf_, _.paramRefs(0), Final)
     lazy val Any_typeTest: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.isInstanceOfPM, _ => BooleanType, Final | Synthetic | Artifact)
-    lazy val Any_typeCast: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.asInstanceOfPM, _.paramRefs(0), Final | Synthetic | Artifact | Erased)
+    lazy val Any_typeCast: TermSymbol = enterT1ParameterlessMethod(AnyClass, nme.asInstanceOfPM, _.paramRefs(0), Final | Synthetic | Artifact | StableRealizable)
       // generated by pattern matcher, eliminated by erasure
 
     def AnyMethods: List[TermSymbol] = List(Any_==, Any_!=, Any_equals, Any_hashCode,

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

@@ -319,8 +319,14 @@ object Erasure {
   class Typer(erasurePhase: DenotTransformer) extends typer.ReTyper with NoChecking {
     import Boxing._
 
+    def isErased(tree: Tree)(implicit ctx: Context): Boolean = tree match {
+      case TypeApply(Select(qual, _), _) if tree.symbol == defn.Any_typeCast =>
+        isErased(qual)
+      case _ => tree.symbol.isEffectivelyErased
+    }
+
     private def checkNotErased(tree: Tree)(implicit ctx: Context): tree.type = {
-      if (tree.symbol.isEffectivelyErased && !ctx.mode.is(Mode.Type))
+      if (isErased(tree) && !ctx.mode.is(Mode.Type))
         ctx.error(em"${tree.symbol} is declared as erased, but is in fact used", tree.sourcePos)
       tree
     }