flambda-backend: Use the right kinds when building shapes for unboxed variants (#2735)

Author: lthls

testsuite/tests/mixed-blocks/variant_unboxing.ml

@@ -0,0 +1,28 @@
+(* TEST
+   flags = "-extension layouts_beta";
+   native;
+*)
+
+(* Unboxing of variants with mixed constructors *)
+
+type t =
+  | Const_t
+  | Boxed of Float.t
+
+type u =
+  | Const_u
+  | Unboxed of float#
+
+external unbox : float -> float# = "%unbox_float"
+
+let t_to_u (t : t) : u =
+  match t with
+  | Const_t -> Const_u
+  | Boxed field -> Unboxed (unbox field)
+;;
+
+let f t =
+  (* The result of [t_to_u] is speculatively unboxed *)
+  let _ = t_to_u t in
+  ()
+;;