Switch dummy_pos depending on compiler version

In OCaml 4.08 the dummy position was changed to have a pos_lnum of 0
instead of 1. We need to match whatever the compiler is using.

Signed-off-by: Patrick Ferris <patrick@sirref.org>

CHANGES.md

@@ -28,6 +28,9 @@ details.
   to what the compiler's `-dparsetree` is.
   (#530, @NathanReb)
 
+- Change the default, dummy position in `Location.ml` to match the compiler's
+  (#540, @ncik-roberts, @patricoferris)
+
 0.33.0 (2024-07-22)
 -------------------
 

astlib/location.ml

@@ -3,6 +3,10 @@ include Ocaml_common.Location
 let set_input_name name = input_name := name
 let set_input_lexbuf lexbuf_opt = input_lexbuf := lexbuf_opt
 
+let dummy_pos =
+  (*IF_NOT_AT_LEAST 407 { Lexing.pos_fname = ""; pos_lnum = 1; pos_bol = 0; pos_cnum = -1; } *)
+  (*IF_AT_LEAST 408 Lexing.dummy_pos *)
+
 module Error = struct
   [@@@warning "-37"]
 

astlib/location.mli

@@ -52,6 +52,10 @@ val set_input_lexbuf : Lexing.lexbuf option -> unit
 val none : t
 (** An arbitrary value of type [t]; describes an empty ghost range. *)
 
+val dummy_pos : Lexing.position
+(** A location that is guaranteed to match the compiler's dummy position for
+    whichever compiler is being used. *)
+
 (** {1 Automatically reporting errors for raised exceptions} *)
 
 val register_error_of_exn : (exn -> Error.t option) -> unit

src/location.ml

@@ -8,7 +8,7 @@ type t = location = {
 }
 
 let in_file name =
-  let loc = { pos_fname = name; pos_lnum = 1; pos_bol = 0; pos_cnum = -1 } in
+  let loc = { L.dummy_pos with pos_fname = name } in
   { loc_start = loc; loc_end = loc; loc_ghost = true }
 
 let set_filename loc fn =