Check well-formedness of @retains only under -Ycc

Author: odersky

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

@@ -2675,7 +2675,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val arg1 = typed(tree.arg, pt)
     if (ctx.mode is Mode.Type) {
       val cls = annot1.symbol.maybeOwner
-      if cls == defn.RetainsAnnot || cls == defn.RetainsByNameAnnot then
+      if ctx.settings.Ycc.value
+          && (cls == defn.RetainsAnnot || cls == defn.RetainsByNameAnnot)
+      then
         CheckCaptures.checkWellformed(annot1)
       if arg1.isType then
         assignType(cpy.Annotated(tree)(arg1, annot1), arg1, annot1)