Disallow @assumeSafe under safe mode

Author: odersky

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

@@ -1537,6 +1537,8 @@ trait Checking {
       else tree
     else if !cls.derivesFrom(defn.AnnotationClass) then
       errorTree(tree, em"$cls is not a valid Scala annotation: it does not extend `scala.annotation.Annotation`")
+    else if cls == defn.AssumeSafeAnnot && Feature.safeEnabled then
+      errorTree(tree, em"@assumeSafe cannot be used in safe mode")
     else tree
 
   /** Check arguments of compiler-defined annotations */

tests/neg-custom-args/captures/i25459.scala

@@ -0,0 +1,15 @@
+import language.experimental.safe
+import caps.unsafe.untrackedCaptures
+import caps.assumeSafe
+import scala.collection.mutable.HashMap
+
+@assumeSafe // error
+class Memoized[A, B](f: A -> B) {
+
+  @untrackedCaptures
+  private val cached = HashMap[A, B]()
+
+  val x: A => B = f.asInstanceOf[A => B]
+
+  def apply(x: A) = cached.getOrElseUpdate(x, f(x))
+}