Revert ocaml-ppx#128 and reapply the original fix from ocaml-ppx#126

src/ppx_deriving_yojson.ml

@@ -321,7 +321,7 @@ let ser_str_of_record ~loc varname labels =
       | None ->
           [%expr [%e result] :: fields]
       | Some default ->
-          [%expr if Pervasives.(=) [%e field] [%e default] then fields else [%e result] :: fields])
+          [%expr if [%e field] = [%e default] then fields else [%e result] :: fields])
   in
   let assoc =
     List.fold_left

src/ppx_deriving_yojson_runtime.mli

@@ -1,5 +1,3 @@
-include module type of Ppx_deriving_runtime
-
 type 'a error_or = ('a, string) Result.result
 
 val ( >>= ) : 'a error_or -> ('a -> 'b error_or) -> 'b error_or
@@ -10,3 +8,17 @@ val map_bind : ('a -> 'b error_or) -> 'b list -> 'a list -> 'b list error_or
     computes it tail-recursively so that large list lengths don't
     cause a stack overflow *)
 val safe_map : ('a -> 'b) -> 'a list -> 'b list
+
+val ( = ) : 'a -> 'a -> bool (* NOTE: Used for [@default ...] *)
+module List : (module type of List)
+module String : (module type of String)
+module Bytes : (module type of Bytes)
+module Int32 : (module type of Int32)
+module Int64 : (module type of Int64)
+module Nativeint : (module type of Nativeint)
+module Array : (module type of Array)
+module Result : sig
+  type ('a, 'b) result = ('a, 'b) Result.result =
+    | Ok of 'a
+    | Error of 'b
+end

src_test/test_ppx_yojson.ml

@@ -510,11 +510,13 @@ let test_equality_redefined ctxt =
     module Pervasives = struct
       let (=) : int -> int -> bool = fun a b -> a = b
       let _ = 1 = 1 (* just dummy usage of `=` to suppress compiler warning *)
+
+      let never_gonna_be_in_pervasives = None
     end
     let (=) : int -> int -> bool = fun a b -> a = b
     let _ = 1 = 1 (* just dummy usage of `=` to suppress compiler warning *)
 
-    type t = {field : int option [@default None]} [@@deriving to_yojson]
+    type t = {field : int option [@default Pervasives.never_gonna_be_in_pervasives]} [@@deriving to_yojson]
     let x = {field = Some 42}
   end
   in