Miscompilation of method delegation

[alainfrisch]

module W = struct
  let r = ref (object method m x = Printf.printf "BAD %i\n%!" x end)
end

let proxy = object method m = (!W.r) # m end

let () =
  W.r := object method m x = Printf.printf "OK %i\n%!" x end;
  proxy # m 3

On 4.12.1 and on trunk, both with ocamlc and ocamlopt, this program prints "BAD 3" instead of "OK 3". A first look at the output of -dlambda suggests that the method m on proxy is compiled using CamlinternalOO's SendConst, reading from the W.r reference when the proxy object is created. This is not correct, because this read is impure.

[alainfrisch]

Translclass.module_path decides that reading (Pfield) from a value accessible through a global path gives a constant, which is wrong.

[alainfrisch]

AFAICT, this has been around since 2003 (f209562).