Update comments and tests

Author: d-kalinichenko

ocaml/testsuite/tests/typing-layouts-non-null-value/arguments.ml

@@ -72,13 +72,28 @@ end = struct
   let mk x = x
 end
 
-(* By default, type parameters to abstract types are nullable: *)
+(* CR layouts v3.0: abstract types and type parameters to
+   abstract types should default to non-null: *)
+
+let _ = Fake_or_null.some (M1.mk 2)
+;;
+[%%expect{|
+module M1 : sig type 'a t val mk : 'a -> 'a t end
+Line 14, characters 26-35:
+14 | let _ = Fake_or_null.some (M1.mk 2)
+                               ^^^^^^^^^
+Error: This expression has type int M1.t
+       but an expression was expected of type ('a : non_null_value)
+       The layout of int M1.t is value, because
+         of the definition of t at line 2, characters 2-11.
+       But the layout of int M1.t must be a sublayout of non_null_value, because
+         of the definition of some at line 5, characters 2-23.
+|}]
 
 let _ = M1.mk (Fake_or_null.some 5)
 ;;
 
 [%%expect{|
-module M1 : sig type 'a t val mk : 'a -> 'a t end
 - : int Fake_or_null.t M1.t = <abstr>
 |}]
 

ocaml/testsuite/tests/typing-layouts-non-null-value/basics.ml

@@ -268,7 +268,7 @@ Error: This expression has type <  > but an expression was expected of type
          of the definition of id_non_null_value at line 3, characters 4-21.
 |}]
 
-(* CR layouts v3.0: [lazy_t] possibly should be non-null. *)
+(* CR layouts v3.0: [lazy_t] should be non-null. *)
 
 let _ = id_non_null_value (lazy 3)
 ;;

ocaml/typing/jkind.mli

@@ -148,9 +148,10 @@ val any : why:any_creation_reason -> t
 (** Value of types of this jkind are not retained at all at runtime *)
 val void : why:void_creation_reason -> t
 
-(** This is the jkind of normal ocaml values *)
+(** This is the jkind of normal ocaml values and null pointers. *)
 val value : why:value_creation_reason -> t
 
+(** This is the jkind of normal ocaml values. They have sort Value. *)
 val non_null_value : why:non_null_value_creation_reason -> t
 
 (** Values of types of this jkind are immediate on 64-bit platforms; on other