Drop requirement that traits cannot access protected members of superclasses

Superaccessors handles this already correctly by generating a super accessor in
this case.

Author: odersky

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

@@ -711,11 +711,6 @@ object SymDenotations {
             i"""
                | Access to protected $this not permitted because enclosing ${ctx.owner.enclosingClass.showLocated}
                | is not a subclass of ${owner.showLocated} where target is defined""")
-        else if (cls.is(Trait) && !this.owner.is(Trait))
-          fail(
-            i"""
-               | Access to protected $this not permitted from $cls,
-               | since traits cannot access protected members of superclasses""")
         else if (
           !(  isType // allow accesses to types from arbitrary subclasses fixes #4737
            || pre.derivesFrom(cls)

tests/neg/protectedacc.scala

@@ -143,7 +143,7 @@ package p {
 
         trait InnerInner {
           val g = getB
-          //g.tie(self2.asInstanceOf[g.Node]) -- this would be a static error
+          g.tie(self2.asInstanceOf[g.Node])
         }
       }
     }