Error message: add hint for unboxed types (#1960)

* generalize hint to all unboxed types

* add regression test

* revert previous change

* extra logic in Ptyp_class

* better error wording

Author: alanechang

ocaml/testsuite/tests/typing-layouts-float64/parsing.ml

@@ -255,9 +255,21 @@ Error: The type constructor float# expects 0 argument(s),
 (* Hint for #float *)
 type t = #float;;
 [%%expect {|
-Line 1, characters 10-15:
+Line 1, characters 9-15:
 1 | type t = #float;;
-              ^^^^^
+             ^^^^^^
+Error: float is neither a polymorphic variant nor a class type.
+       Did you mean the unboxed type float#?
+|}]
+
+(* Hint should not show up in this case *)
+class type floot = object end
+class type c = float
+[%%expect {|
+class type floot = object  end
+Line 2, characters 15-20:
+2 | class type c = float
+                   ^^^^^
 Error: Unbound class type float
-Hint: Did you mean float#?
+Hint: Did you mean floot?
 |}]

ocaml/typing/env.ml

@@ -3888,12 +3888,7 @@ let report_lookup_error _loc env ppf = function
     end
   | Unbound_cltype lid ->
       fprintf ppf "Unbound class type %a" !print_longident lid;
-      begin match lid with
-      | Lident "float" ->
-        Misc.did_you_mean ppf (fun () -> ["float#"])
-      | Lident _ | Ldot _ | Lapply _ ->
-        spellcheck ppf extract_cltypes env lid
-      end;
+      spellcheck ppf extract_cltypes env lid
   | Unbound_instance_variable s ->
       fprintf ppf "Unbound instance variable %s" s;
       spellcheck_name ppf extract_instance_variables env s;

ocaml/typing/typetexp.ml

@@ -77,6 +77,7 @@ type error =
   | Non_sort of
       {vloc : sort_loc; typ : type_expr; err : Jkind.Violation.t}
   | Bad_jkind_annot of type_expr * Jkind.Violation.t
+  | Did_you_mean_unboxed of Longident.t
 
 exception Error of Location.t * Env.t * error
 exception Error_forward of Location.error
@@ -715,6 +716,16 @@ and transl_type_aux env policy mode styp =
           let path, decl = Env.find_type_by_name lid2 env in
           ignore(Env.lookup_cltype ~loc:lid.loc lid.txt env);
           (path, decl, false)
+        with Not_found -> try
+          (* Raise a different error if it matches the name of an unboxed type *)
+          let lid3 =
+            match lid.txt with
+              Longident.Lident s     -> Longident.Lident (s ^ "#")
+            | Longident.Ldot(r, s)   -> Longident.Ldot (r, s ^ "#")
+            | Longident.Lapply(_, _) -> fatal_error "Typetexp.transl_type"
+          in
+          ignore (Env.find_type_by_name lid3 env : Path.t * Types.type_declaration);
+          raise (Error (styp.ptyp_loc, env, Did_you_mean_unboxed lid.txt))
         with Not_found ->
           ignore (Env.lookup_cltype ~loc:lid.loc lid.txt env); assert false
       in
@@ -1388,6 +1399,9 @@ let report_error env ppf = function
     fprintf ppf "@[<b 2>Bad layout annotation:@ %a@]"
       (Jkind.Violation.report_with_offender
          ~offender:(fun ppf -> Printtyp.type_expr ppf ty)) violation
+  | Did_you_mean_unboxed lid ->
+    fprintf ppf "@[%a is neither a polymorphic variant nor a class type.@ \
+                 Did you mean the unboxed type %a#?@]" longident lid longident lid
 
 let () =
   Location.register_error_of_exn

ocaml/typing/typetexp.mli

@@ -118,6 +118,7 @@ type error =
   | Non_sort of
       {vloc : sort_loc; typ : type_expr; err : Jkind.Violation.t}
   | Bad_jkind_annot of type_expr * Jkind.Violation.t
+  | Did_you_mean_unboxed of Longident.t
 
 exception Error of Location.t * Env.t * error