Deriving fails over poly variants whose type name clashes with those in Ppx_deriving_runtime

[cemerick]

This works:

type arr = [`A of int]
[@@deriving eq]

type foo = [ | arr ]
[@@deriving eq]

but this doesn't:

type array = [`A of int]
[@@deriving eq]

type foo = [ | array ]
[@@deriving eq]

8 | type foo = [ | array ]
                   ^^^^^
Error: The type array is not a variant type

Similar errors result if arr is renamed to anything defined in Ppx_deriving_runtime, e.g. list, int64, etc. Looking at the expanded output, the open! of that module is what dooms the match clause that names the poly variant (#arr here):

let rec equal_foo : foo -> foo -> Ppx_deriving_runtime.bool =
  let __0 () = equal_arr in
  ((let open! ((Ppx_deriving_runtime)[@ocaml.warning "-A"]) in
      fun lhs ->
        fun rhs ->
          match (lhs, rhs) with
          | ((#arr as lhs), (#arr as rhs)) -> ((fun x -> (__0 ()) x)) lhs rhs
          | _ -> false)

(This might be related to #254, and thus #260?)