Recursive module test + location fix

Author: Roman Leshchinskiy

ocaml/testsuite/tests/typing-modules/strengthening.ml

@@ -340,3 +340,14 @@ module Remove_aliases :
       end
   end
 |}]
+
+module Recursive = struct
+  module rec M : sig end with N = N
+  and N : sig end = struct end
+end
+[%%expect{|
+Line 2, characters 30-31:
+2 |   module rec M : sig end with N = N
+                                  ^
+Error: Illegal recursive module reference
+|}]

ocaml/typing/typemod.ml

@@ -911,8 +911,10 @@ and approx_modtype_jane_syntax env = function
   | Jane_syntax.Module_type.Jmty_strengthen { mty = smty; mod_id } ->
     let mty = approx_modtype env smty in
     let path =
+      (* CR-someday: potentially improve error message for strengthening with
+         a mutually recursive module. *)
       Env.lookup_module_path ~use:false ~load:false
-        ~loc:smty.pmty_loc mod_id.txt env
+        ~loc:mod_id.loc mod_id.txt env
     in
     let aliasable = (not (Env.is_functor_arg path env)) in
     Mty_strengthen (mty, path, Aliasability.aliasable aliasable)