Improved error message for invalid private row type declaration

Changes

@@ -184,6 +184,11 @@ Working version
 
 ### Compiler user-interface and warnings:
 
+- #8732, improved error messages for invalid private row type definitions.
+  For instance, [ type t = private [< `A > `A ] ] .
+  (Florian Angeletti, review by Jacques Garrigue, Thomas Refis,
+   and Gabriel Scherer)
+
 - #9407: added warning for missing mli interface file
   (Anukriti Kumar, review by Florian Angeletti)
 

testsuite/tests/typing-private/invalid_private_row.ml

@@ -0,0 +1,52 @@
+(* TEST
+   * expect
+*)
+
+(** Error message for trying to make private a row type variable
+    that only exists syntactically *)
+
+type a = [`A | `C | `D]
+type b = [`B | `D | `E]
+type c = private [< a | b > `A `B `C `D `E]
+[%%expect {|
+type a = [ `A | `C | `D ]
+type b = [ `B | `D | `E ]
+Line 6, characters 0-43:
+6 | type c = private [< a | b > `A `B `C `D `E]
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: This private row type declaration is invalid.
+       The type expression on the right-hand side reduces to
+         [ `A | `B | `C | `D | `E ]
+       which does not have a free row type variable.
+       Hint: If you intended to define a private type abbreviation,
+       write explicitly
+         private [ `A | `B | `C | `D | `E ]
+|}]
+
+type u = private < x:int; .. > as 'a constraint 'a = < x: int > ;;
+[%%expect {|
+Line 1, characters 0-63:
+1 | type u = private < x:int; .. > as 'a constraint 'a = < x: int > ;;
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: This private row type declaration is invalid.
+       The type expression on the right-hand side reduces to
+         < x : int >
+       which does not have a free row type variable.
+       Hint: If you intended to define a private type abbreviation,
+       write explicitly
+         private < x : int >
+|}]
+
+type u = private [> `A ] as 'a constraint 'a = [< `A ] ;;
+[%%expect {|
+Line 1, characters 0-54:
+1 | type u = private [> `A ] as 'a constraint 'a = [< `A ] ;;
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: This private row type declaration is invalid.
+       The type expression on the right-hand side reduces to
+         [ `A ]
+       which does not have a free row type variable.
+       Hint: If you intended to define a private type abbreviation,
+       write explicitly
+         private [ `A ]
+|}]

typing/typedecl.ml

@@ -54,7 +54,6 @@ type error =
   | Rebind_private of Longident.t
   | Variance of Typedecl_variance.error
   | Unavailable_type_constructor of Path.t
-  | Bad_fixed_type of string
   | Unbound_type_var_ext of type_expr * extension_constructor
   | Val_in_structure
   | Multiple_native_repr_attributes
@@ -65,6 +64,7 @@ type error =
   | Bad_unboxed_attribute of string
   | Boxed_and_unboxed
   | Nonrec_gadt
+  | Invalid_private_row_declaration of type_expr
 
 open Typedtree
 
@@ -166,8 +166,11 @@ let is_fixed_type sd =
       sd.ptype_private = Private &&
       has_row_var sty
 
-(* Set the row variable in a fixed type *)
-let set_fixed_row env loc p decl =
+(* Set the row variable to a fixed type in a private row type declaration.
+   (e.g. [ type t = private [< `A | `B ] ] or [type u = private < .. > ])
+   Require [is_fixed_type decl] as a precondition
+*)
+let set_private_row env loc p decl =
   let tm =
     match decl.type_manifest with
       None -> assert false
@@ -179,15 +182,20 @@ let set_fixed_row env loc p decl =
         let row = Btype.row_repr row in
         Btype.set_type_desc tm
           (Tvariant {row with row_fixed = Some Fixed_private});
-        if Btype.static_row row then Btype.newgenty Tnil
+        if Btype.static_row row then
+          (* the syntax hinted at the existence of a row variable,
+             but there is in fact no row variable to make private, e.g.
+             [ type t = private [< `A > `A] ] *)
+          raise (Error(loc, Invalid_private_row_declaration tm))
         else row.row_more
     | Tobject (ty, _) ->
-        snd (Ctype.flatten_fields ty)
-    | _ ->
-        raise (Error (loc, Bad_fixed_type "is not an object or variant"))
+        let r = snd (Ctype.flatten_fields ty) in
+        if not (Btype.is_Tvar r) then
+          (* a syntactically open object was closed by a constraint *)
+          raise (Error(loc, Invalid_private_row_declaration tm));
+        r
+    | _ -> assert false
   in
-  if not (Btype.is_Tvar rv) then
-    raise (Error (loc, Bad_fixed_type "has no row variable"));
   Btype.set_type_desc rv (Tconstr (p, decl.type_params, ref Mnil))
 
 (* Translate one type declaration *)
@@ -431,7 +439,7 @@ let transl_declaration env sdecl (id, uid) =
               (Longident.Lident(Ident.name id ^ "#row")) env
         with Not_found -> assert false
       in
-      set_fixed_row env sdecl.ptype_loc p decl
+      set_private_row env sdecl.ptype_loc p decl
     end;
   (* Check for cyclic abbreviations *)
     begin match decl.type_manifest with None -> ()
@@ -1403,7 +1411,8 @@ let transl_value_decl env loc valdecl =
    In particular, note that [sig_env] is an extension of
    [outer_env].
 *)
-let transl_with_constraint id row_path ~sig_env ~sig_decl ~outer_env sdecl =
+let transl_with_constraint id ?fixed_row_path ~sig_env ~sig_decl ~outer_env
+    sdecl =
   Env.mark_type_used sig_decl.type_uid;
   reset_type_variables();
   Ctype.begin_def();
@@ -1485,9 +1494,8 @@ let transl_with_constraint id row_path ~sig_env ~sig_decl ~outer_env sdecl =
       type_uid = Uid.mk ~current_unit:(Env.get_unit_name ());
     }
   in
-  begin match row_path with None -> ()
-  | Some p -> set_fixed_row env loc p new_sig_decl
-  end;
+  Option.iter (fun p -> set_private_row env sdecl.ptype_loc p new_sig_decl)
+    fixed_row_path;
   begin match Ctype.closed_type_decl new_sig_decl with None -> ()
   | Some ty -> raise(Error(loc, Unbound_type_var(ty, new_sig_decl)))
   end;
@@ -1825,8 +1833,6 @@ let report_error ppf = function
              (variance v2) (variance v1))
   | Unavailable_type_constructor p ->
       fprintf ppf "The definition of type %a@ is unavailable" Printtyp.path p
-  | Bad_fixed_type r ->
-      fprintf ppf "This fixed type %s" r
   | Variance Typedecl_variance.Varying_anonymous ->
       fprintf ppf "@[%s@ %s@ %s@]"
         "In this GADT definition," "the variance of some parameter"
@@ -1875,6 +1881,14 @@ let report_error ppf = function
   | Nonrec_gadt ->
       fprintf ppf
         "@[GADT case syntax cannot be used in a 'nonrec' block.@]"
+  | Invalid_private_row_declaration ty ->
+      Format.fprintf ppf
+        "@[<hv>This private row type declaration is invalid.@ \
+         The type expression on the right-hand side reduces to@;<1 2>%a@ \
+         which does not have a free row type variable.@]@,\
+         @[<hv>@[Hint: If you intended to define a private type abbreviation,@ \
+         write explicitly@]@;<1 2>private %a@]"
+        Printtyp.type_expr ty Printtyp.type_expr ty
 
 let () =
   Location.register_error_of_exn

typing/typedecl.mli

@@ -38,8 +38,10 @@ val transl_value_decl:
     Env.t -> Location.t ->
     Parsetree.value_description -> Typedtree.value_description * Env.t
 
+(* If the [fixed_row_path] optional argument is provided,
+   the [Parsetree.type_declaration] argument should satisfy [is_fixed_type] *)
 val transl_with_constraint:
-    Ident.t -> Path.t option ->
+    Ident.t -> ?fixed_row_path:Path.t ->
     sig_env:Env.t -> sig_decl:Types.type_declaration ->
     outer_env:Env.t -> Parsetree.type_declaration ->
     Typedtree.type_declaration
@@ -89,7 +91,6 @@ type error =
   | Rebind_private of Longident.t
   | Variance of Typedecl_variance.error
   | Unavailable_type_constructor of Path.t
-  | Bad_fixed_type of string
   | Unbound_type_var_ext of type_expr * extension_constructor
   | Val_in_structure
   | Multiple_native_repr_attributes
@@ -100,6 +101,7 @@ type error =
   | Bad_unboxed_attribute of string
   | Boxed_and_unboxed
   | Nonrec_gadt
+  | Invalid_private_row_declaration of type_expr
 
 exception Error of Location.t * error
 

typing/typemod.ml

@@ -554,7 +554,7 @@ let merge_constraint initial_env loc sg lid constr =
           Env.add_type ~check:false id_row decl_row initial_env
         in
         let tdecl =
-          Typedecl.transl_with_constraint id (Some(Pident id_row))
+          Typedecl.transl_with_constraint id ~fixed_row_path:(Pident id_row)
             ~sig_env ~sig_decl:decl ~outer_env:initial_env sdecl in
         let newdecl = tdecl.typ_type in
         check_type_decl sig_env sdecl.ptype_loc id row_id newdecl decl rs rem;
@@ -568,7 +568,7 @@ let merge_constraint initial_env loc sg lid constr =
        (With_type sdecl | With_typesubst sdecl as constr))
       when Ident.name id = s ->
         let tdecl =
-          Typedecl.transl_with_constraint id None
+          Typedecl.transl_with_constraint id
             ~sig_env ~sig_decl ~outer_env:initial_env sdecl in
         let newdecl = tdecl.typ_type and loc = sdecl.ptype_loc in
         check_type_decl sig_env loc id row_id newdecl sig_decl rs rem;