Memoize should produce constant DefDefs for constant final vals.

It produced just the right hand side literal before.

Author: odersky

src/dotty/tools/dotc/transform/Memoize.scala

@@ -71,20 +71,19 @@ import Decorators._
     if (sym.is(Accessor, butNot = NoFieldNeeded))
       if (sym.isGetter) {
         def skipBlocks(t: Tree): Tree = t match {
-          case Block(a, b) if a.forall(isIdempotentExpr) => skipBlocks(b)
+          case Block(_, t1) => skipBlocks(t1)
           case _ => t
         }
-        if (sym.is(Flags.Final) && skipBlocks(tree.rhs).isInstanceOf[Literal])
-          // duplicating scalac behavior: for final vals that have rhs as constant, we do not create a field
-          // and instead return the value. This seemingly minor optimization has huge effect on initialization
-          // order and the values that can be observed during superconstructor call
-          tree
-        else {
-          var rhs = tree.rhs.changeOwnerAfter(sym, field, thisTransform)
-          if (isWildcardArg(rhs)) rhs = EmptyTree
-          val fieldDef = transformFollowing(ValDef(field, rhs))
-          val getterDef = cpy.DefDef(tree)(rhs = transformFollowingDeep(ref(field)))
-          Thicket(fieldDef, getterDef)
+        skipBlocks(tree.rhs) match {
+          case lit: Literal if sym.is(Final) && isIdempotentExpr(tree.rhs) =>
+            // see remark about idempotency in PostTyper#normalizeTree
+            cpy.DefDef(tree)(rhs = lit)
+          case _ =>
+            var rhs = tree.rhs.changeOwnerAfter(sym, field, thisTransform)
+            if (isWildcardArg(rhs)) rhs = EmptyTree
+            val fieldDef = transformFollowing(ValDef(field, rhs))
+            val getterDef = cpy.DefDef(tree)(rhs = transformFollowingDeep(ref(field)))
+            Thicket(fieldDef, getterDef)
         }
       }
       else if (sym.isSetter) {