Parse error of "as" keyword

[tunguski]

Simple form:

balanceHelp tree =
  case tree of
    RBNode_elm_builtin Black as d -> x

elm-lang/core Dict:

balanceHelp : Dict k v -> Dict k v
balanceHelp tree =
  case tree of
    -- handle double blacks
    RBNode_elm_builtin BBlack xk xv a (RBNode_elm_builtin NBlack zk zv (RBNode_elm_builtin Black yk yv b c) (RBNode_elm_builtin Black _ _ _ _ as d)) ->
      RBNode_elm_builtin Black yk yv (RBNode_elm_builtin Black xk xv a b) (balance Black zk zv c (redden d))

Does not parse.