Memoize: duplicate scala2 behaviour: don't create fields for final vals.

This affect initialisation order and we rely on artifacts of initialisation order in Dotty.

Author: DarkDimius

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

@@ -70,11 +70,22 @@ import Decorators._
     lazy val field = sym.field.orElse(newField).asTerm
     if (sym.is(Accessor, butNot = NoFieldNeeded))
       if (sym.isGetter) {
-        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)
+        def skipBlocks(t: Tree): Tree = t match {
+          case Block(a, b) if a.forall(isIdempotentExpr) => skipBlocks(b)
+          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)
+        }
       }
       else if (sym.isSetter) {
         if (!sym.is(ParamAccessor)) { val Literal(Constant(())) = tree.rhs } // this is intended as an assertion