resurrect half of the Bad_fixed_type error

but make the error message much more explicit
about what is happening.

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

@@ -64,6 +64,7 @@ type error =
   | Wrong_unboxed_type_float
   | Boxed_and_unboxed
   | Nonrec_gadt
+  | Invalid_private_row_declaration of type_expr
 
 open Typedtree
 
@@ -164,7 +165,7 @@ let is_fixed_type sd =
 
 (* Set the row variable in a fixed type. Require
    [is_fixed_type decl] as a precondition *)
-let set_fixed_row env p decl =
+let set_fixed_row env p loc decl =
   let tm =
     match decl.type_manifest with
       None -> assert false
@@ -174,10 +175,19 @@ let set_fixed_row env p decl =
     match tm.desc with
       Tvariant row ->
         let row = Btype.row_repr row in
+        if Btype.static_row row then
+          (* the syntax hinted to the existence of a row variable,
+             but there are in fact no row variable to make private, e.g.
+             [ type t = private [< `A > `A] ] *)
+          raise (Error(loc, Invalid_private_row_declaration tm));
         tm.desc <- Tvariant {row with row_fixed = Some Fixed_private};
         row.row_more
     | Tobject (ty, _) ->
-        snd (Ctype.flatten_fields ty)
+        let r = snd (Ctype.flatten_fields ty) in
+        if not (Btype.is_Tvar r) then
+          (* a syntactically open object was closed by a later constraint *)
+          raise (Error(loc, Invalid_private_row_declaration tm));
+        r
     | _ -> assert false
   in
   rv.desc <- Tconstr (p, decl.type_params, ref Mnil)
@@ -516,7 +526,7 @@ let transl_declaration env sdecl id =
               (Longident.Lident(Ident.name id ^ "#row")) env
         with Not_found -> assert false
       in
-      set_fixed_row env p decl
+      set_fixed_row env p sdecl.ptype_loc decl
     end;
   (* Check for cyclic abbreviations *)
     begin match decl.type_manifest with None -> ()
@@ -1507,7 +1517,8 @@ let transl_with_constraint env id ?fixed_row_path orig_decl sdecl =
       type_unboxed;
     }
   in
-  Option.iter (fun p -> set_fixed_row env p decl) fixed_row_path;
+  Option.iter (fun p -> set_fixed_row env p sdecl.ptype_loc decl)
+    fixed_row_path;
   begin match Ctype.closed_type_decl decl with None -> ()
   | Some ty -> raise(Error(sdecl.ptype_loc, Unbound_type_var(ty,decl)))
   end;
@@ -1856,6 +1867,14 @@ 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

@@ -99,6 +99,7 @@ type error =
   | Wrong_unboxed_type_float
   | Boxed_and_unboxed
   | Nonrec_gadt
+  | Invalid_private_row_declaration of type_expr
 
 exception Error of Location.t * error