Remove warning 30 from the standard set.

Warning 30 (same constructor/label name used in two mutually-recursive
declarations) is an annoying warning that makes no sense since we
introduced type-based disambiguation of fields and constructors. It
was disabled by default in OCaml 4.10

  ocaml/ocaml#9046

and there is no reason to keep it in Dune.

Signed-off-by: Gabriel Scherer <gabriel.scherer@gmail.com>
Co-authored-by: Rudi Grinberg <me@rgrinberg.com>

doc/changes/9568.md

@@ -0,0 +1,2 @@
+- Remove warning 30 from default set for projects where dune lang is at least
+  3.13 (#9568, @gasche)

src/dune_rules/ocaml_flags.ml

@@ -13,6 +13,8 @@ let dev_mode_warnings =
       (Int.Set.of_list
          [ 4; 29; 40; 41; 42; 44; 45; 48; 58; 59; 60; 63; 64; 65; 66; 67; 68; 69; 70 ])
   in
+  let add n range = Int.Set.add range n in
+  let remove n range = Int.Set.remove range n in
   let warnings_range ws =
     let wrange_to_flag (x, y) =
       if x = y then sprintf "@%d" x else sprintf "@%d..%d" x y
@@ -25,10 +27,22 @@ let dev_mode_warnings =
     |> List.rev_map ~f:wrange_to_flag
     |> String.concat ~sep:""
   in
-  let pre_3_3 = lazy (warnings_range all) in
-  let post_3_3 = lazy (warnings_range (Int.Set.union all (Int.Set.of_list [ 67; 69 ]))) in
+  let and_warnings lazy_range =
+    lazy_range, lazy (warnings_range (Lazy.force lazy_range))
+  in
+  let range_pre_3_3, pre_3_3 = and_warnings @@ lazy all in
+  let range_pre_3_13, pre_3_13 =
+    and_warnings @@ lazy (Lazy.force range_pre_3_3 |> add 67 |> add 69)
+  in
+  let _range_later, later =
+    and_warnings @@ lazy (Lazy.force range_pre_3_13 |> remove 30)
+  in
   fun ~dune_version ->
-    if dune_version >= (3, 3) then Lazy.force post_3_3 else Lazy.force pre_3_3
+    if dune_version < (3, 3)
+    then Lazy.force pre_3_3
+    else if dune_version < (3, 13)
+    then Lazy.force pre_3_13
+    else Lazy.force later
 ;;
 
 let vendored_warnings = [ "-w"; "-a" ]

test/blackbox-tests/test-cases/default-ocaml-flags-3-13.t

@@ -0,0 +1,25 @@
+  $ cat >dune <<EOF
+  > (library
+  >  (modes byte)
+  >  (name foo))
+  > EOF
+
+  $ cat >foo.ml <<EOF
+  > type a = { a : int }
+  > and b = { a : int ; b : bool }
+  > EOF
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.12)
+  > EOF
+  $ dune build foo.cma
+  File "foo.ml", line 2, characters 10-19:
+  2 | and b = { a : int ; b : bool }
+                ^^^^^^^^^
+  Error (warning 30 [duplicate-definitions]): the label a is defined in both types a and b.
+  [1]
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.13)
+  > EOF
+  $ dune build foo.cma