Fix import of signatures with mutually recursive types

src/ppx_import.ml

@@ -378,20 +378,27 @@ let type_declaration ~tool_name mapper type_decl =
     end
   | _ -> default_mapper.type_declaration mapper type_decl
 
-let rec psig_of_tsig ~subst ?(trec=[]) tsig =
+let rec cut_tsig_block_of_rec_types accu tsig =
+  match tsig with
+  | Sig_type (id, ttype_decl, Trec_next) :: rest ->
+      cut_tsig_block_of_rec_types ((id, ttype_decl) :: accu) rest
+  | _ ->
+      (List.rev accu, tsig)
+
+let rec psig_of_tsig ~subst tsig =
   match tsig with
-  | (Sig_type (_, _, Trec_first) | _) :: _ when trec <> [] ->
-    let psig_desc = Psig_type(Recursive, trec) in
-    { psig_desc; psig_loc = Location.none } :: psig_of_tsig ~subst tsig
   | Sig_type (id, ttype_decl, rec_flag) :: rest ->
-    let ptype_decl = ptype_decl_of_ttype_decl ~manifest:None ~subst (Location.mknoloc (Ident.name id)) ttype_decl in
-    begin match rec_flag with
-    | Trec_not ->
-      let psig_desc = Psig_type(Nonrecursive, [ptype_decl]) in
+      let accu = [(id, ttype_decl)] in
+      let (rec_flag, (block, rest)) =
+        match rec_flag with
+        | Trec_not -> (Nonrecursive, (accu, rest))
+        | Trec_first -> (Recursive, cut_tsig_block_of_rec_types accu rest)
+        | Trec_next -> assert false in
+      let block = block |> List.map (fun (id, ttype_decl) ->
+        ptype_decl_of_ttype_decl ~manifest:None ~subst
+          (Location.mknoloc (Ident.name id)) ttype_decl) in
+      let psig_desc = Psig_type(rec_flag, block) in
       { psig_desc; psig_loc = Location.none } :: psig_of_tsig ~subst rest
-    | Trec_first | Trec_next ->
-      psig_of_tsig ~subst ~trec:(ptype_decl :: trec) rest
-    end
   | Sig_value (id, { val_type; val_kind; val_loc; val_attributes }) :: rest ->
     let pval_prim =
       match val_kind with

src_test/stuff.ml

@@ -20,3 +20,8 @@ module MI = struct
 end
 open MI
 type nonrec i = I of i
+
+module type S_rec = sig
+  type t = A of u
+  and u = B of t
+end

src_test/test_ppx_import.ml

@@ -11,6 +11,7 @@ type 'b g' = [%import: 'b Stuff.g]
 type h = [%import: Stuff.h]
 module MI = Stuff.MI
 type i = [%import: Stuff.i]
+module type S_rec = [%import: (module Stuff.S_rec)]
 
 let test_constr _ctxt =
   ignore ([A1; A2 "a"]);