Ensure that all [val]s are [value]s.

ocaml/testsuite/tests/typing-layouts/basics_alpha.ml

@@ -170,13 +170,13 @@ module F2 (X : sig val x : t_void end) = struct
   let f () = X.x
 end;;
 [%%expect{|
-Line 2, characters 8-16:
-2 |   let f () = X.x
-            ^^^^^^^^
-Error: Non-value detected in [value_kind].
-       Please report this error to the Jane Street compilers team.
-       t_void has layout void, which is not a sublayout of value.
+Line 1, characters 27-33:
+1 | module F2 (X : sig val x : t_void end) = struct
+                               ^^^^^^
+Error: This type signature for x is not a value type.
+       x has layout void, which is not a sublayout of value.
 |}];;
+(* CR layouts v5: the test above should be made to work *)
 
 module F2 (X : sig val f : void_record -> unit end) = struct
   let g z = X.f { vr_void = z; vr_int = 42 }

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

@@ -261,3 +261,8 @@ end;;
 [%%expect {|
 module F : sig end -> sig end
 |}];;
+
+(****************************************)
+(* Test 8: [val]s must be representable *)
+
+(* CR layouts: Bring this test back from modules_alpha *)

ocaml/testsuite/tests/typing-layouts/modules_alpha.ml

@@ -470,3 +470,20 @@ end;;
 [%%expect {|
 module F : sig end -> sig end
 |}];;
+
+(****************************************)
+(* Test 8: [val]s must be representable *)
+
+module type S = sig val x : t_any end
+
+module M = struct
+  let x : t_void = assert false
+end
+
+[%%expect{|
+Line 1, characters 28-33:
+1 | module type S = sig val x : t_any end
+                                ^^^^^
+Error: This type signature for x is not a value type.
+       x has layout any, which is not a sublayout of value.
+|}]

ocaml/testsuite/tests/typing-layouts/modules_beta.ml

@@ -405,3 +405,8 @@ end;;
 [%%expect {|
 module F : sig end -> sig end
 |}];;
+
+(****************************************)
+(* Test 8: [val]s must be representable *)
+
+(* CR layouts: Bring this test back from modules_alpha *)

ocaml/typing/typedecl.ml

@@ -71,6 +71,7 @@ type error =
       ; err : Layout.Violation.t
       }
   | Layout_empty_record
+  | Non_value_in_sig of Layout.Violation.t * string
   | Separability of Typedecl_separability.error
   | Bad_unboxed_attribute of string
   | Boxed_and_unboxed
@@ -1957,6 +1958,13 @@ let check_unboxable env loc ty =
 (* Translate a value declaration *)
 let transl_value_decl env loc valdecl =
   let cty = Typetexp.transl_type_scheme env valdecl.pval_type in
+  (* CR layouts v5: relax this to check for representability. *)
+  begin match Ctype.constrain_type_layout env cty.ctyp_type
+                (Layout.value ~why:Structure_element) with
+  | Ok () -> ()
+  | Error err ->
+    raise(Error(cty.ctyp_loc, Non_value_in_sig(err, valdecl.pval_name.txt)))
+  end;
   let ty = cty.ctyp_type in
   let v =
   match valdecl.pval_prim with
@@ -2506,6 +2514,9 @@ let report_error ppf = function
          ~offender:(fun ppf -> Printtyp.type_expr ppf typ)) err
   | Layout_empty_record ->
     fprintf ppf "@[Records must contain at least one runtime value.@]"
+  | Non_value_in_sig (err, val_name) ->
+    fprintf ppf "@[This type signature for %s is not a value type.@ %a@]"
+      val_name (Layout.Violation.report_with_name ~name:val_name) err
   | Bad_unboxed_attribute msg ->
       fprintf ppf "@[This type cannot be unboxed because@ %s.@]" msg
   | Separability (Typedecl_separability.Non_separable_evar evar) ->

ocaml/typing/typedecl.mli

@@ -110,6 +110,7 @@ type error =
       ; err : Layout.Violation.t
       }
   | Layout_empty_record
+  | Non_value_in_sig of Layout.Violation.t * string
   | Separability of Typedecl_separability.error
   | Bad_unboxed_attribute of string
   | Boxed_and_unboxed