Don't allow accesses to <into> from source code

Author: odersky

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

@@ -2094,6 +2094,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           && checkedArgs(1).tpe.derivesFrom(defn.RuntimeExceptionClass)
       then
         report.error(em"throws clause cannot be defined for RuntimeException", checkedArgs(1).srcPos)
+      else if tycon == defn.IntoType then
+        report.error(em"not found: <into>", tpt1.srcPos)
       else if (ctx.isJava)
         if tycon eq defn.ArrayClass then
           checkedArgs match {