flambda-backend: Unboxed records may be null (#2648)

* Tests

* Fix incorrect assumption about records

---------

Co-authored-by: Diana Kalinichenko <dkalinichenko@janestreet.com>

Author: dkalinichenko-js

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

@@ -127,3 +127,47 @@ let _ = my_id4 (Fake_or_null.some 4)
 - : int Fake_or_null.t = <abstr>
 - : int Fake_or_null.t = <abstr>
 |}]
+
+(* Check behavior of type arguments and unboxed annotations. *)
+
+module M2 : sig
+  type 'a t = { v : 'a } [@@unboxed]
+
+  val box : 'a -> 'a t
+  val unbox : 'a t -> 'a
+end = struct
+  type 'a t = { v : 'a } [@@unboxed]
+
+  let box v = { v }
+  let unbox { v } = v
+end
+
+[%%expect{|
+module M2 :
+  sig
+    type 'a t = { v : 'a; } [@@unboxed]
+    val box : 'a -> 'a t
+    val unbox : 'a t -> 'a
+  end
+|}]
+
+module M3 : sig
+  type 'a t = V of 'a [@@unboxed]
+
+  val box : 'a -> 'a t
+  val unbox : 'a t -> 'a
+end = struct
+  type 'a t = V of 'a [@@unboxed]
+
+  let box v = V v
+  let unbox (V v) = v
+end
+
+[%%expect{|
+module M3 :
+  sig
+    type 'a t = V of 'a [@@unboxed]
+    val box : 'a -> 'a t
+    val unbox : 'a t -> 'a
+  end
+|}]

testsuite/tests/typing-layouts/basics_alpha.ml

@@ -693,15 +693,15 @@ module M9_4 = struct
     | ({vur_void = _},i) -> i
 end;;
 [%%expect {|
-Line 4, characters 8-16:
+Line 4, characters 7-21:
 4 |     | ({vur_void = _},i) -> i
-            ^^^^^^^^
-Error: The record field vur_void belongs to the type void_unboxed_record
-       but is mixed here with fields of type ('a : non_null_value)
+           ^^^^^^^^^^^^^^
+Error: This pattern matches values of type void_unboxed_record
+       but a pattern was expected which matches values of type ('a : value)
        The layout of void_unboxed_record is void, because
          of the definition of t_void at line 6, characters 0-19.
-       But the layout of void_unboxed_record must be a sublayout of non_null_value, because
-         it's a boxed record type.
+       But the layout of void_unboxed_record must be a sublayout of value, because
+         it's the type of a tuple element.
 |}];;
 
 module M9_5 = struct

typing/typecore.ml

@@ -2669,7 +2669,9 @@ and type_pat_aux
             let ty = generic_instance expected_ty in
             Some (p0, p, is_principal expected_ty), ty
         | Maybe_a_record_type ->
-          None, newvar (Jkind.non_null_value ~why:Boxed_record)
+          (* We can't assume that the jkind of a record type is [non_null_value]
+             because of unboxed records. *)
+          None, newvar (Jkind.any ~why:Dummy_jkind)
         | Not_a_record_type ->
           let error = Wrong_expected_kind(Record, Pattern, expected_ty) in
           raise (Error (loc, !env, error))