Fix externref trait-object cast path for wasm-gc

Fixes moonbitlang/moonbit-docs#1123.

The wasm-gc backend was emitting invalid cast sequences when trait-object conversion involved #external values across supertrait chains.

This change adds a Pidentity lowering path in clam_of_core and tightens Pcast lowering to avoid redundant casts and preserve source/target type intent.

In wasm_of_clam_gc, Lcast now handles Ref_extern<->Ref_any with the dedicated conversion ops (Extern_convert_any / Any_convert_extern) instead of generic ref_cast where needed.

Author: bikallem

src/clam_of_core.ml

@@ -250,14 +250,28 @@ let rec transl_expr ~name_hint ~mtype_defs ~addr_tbl ~type_defs ~object_methods
                | Ref _ | Ref_lazy_init _ | Ref_nullable _ | Ref_extern
                | Ref_string | Ref_bytes | Ref_func | Ref_any ->
                    Prefeq)
+         | Pidentity -> (
+             match[@warning "-fragile-match"] args with
+             | arg :: [] ->
+                 let source_type = transl_type (Mcore.type_of_expr arg) in
+                 let target_type = transl_type ty in
+                 if Ltype.equal source_type target_type then go arg
+                 else
+                   bind arg (fun source ->
+                       Lcast { expr = Lvar { var = source }; target_type })
+             | _ -> assert false)
          | Pcast { kind } -> (
              match[@warning "-fragile-match"] args with
              | arg :: [] -> (
                  match kind with
                  | Constr_to_enum | Make_newtype -> go arg
                  | Unfold_rec_newtype | Enum_to_constr ->
+                     let source_type = transl_type (Mcore.type_of_expr arg) in
                      let target_type = transl_type ty in
-                     Lcast { expr = go arg; target_type })
+                     if Ltype.equal source_type target_type then go arg
+                     else
+                       bind arg (fun source ->
+                           Lcast { expr = Lvar { var = source }; target_type }))
              | _ -> assert false)
          | Penum_field { index; tag = _ } -> (
              let tid =

src/wasm_of_clam_gc.ml

@@ -95,6 +95,12 @@ let addr_to_string = Basic_fn_address.to_wasm_name
 let add_cst = Wasmir_util.add_cst
 let add_dummy_i32 rest = add_cst 0 rest
 
+let cast_operand_type (e : Clam.lambda) =
+  match Clam_util.no_located e with
+  | Lvar { var } -> Some (Ident.get_type var)
+  | Lcast { target_type; _ } -> Some target_type
+  | _ -> None
+
 type loop_info = { params : Ident.t list; break_used : bool ref }
 
 type ctx = {
@@ -574,9 +580,16 @@ and compileExpr0 ~(tail : bool) ~ctx ~global_ctx ~type_defs (body : Clam.lambda)
        let tid_base = tid in
        generic_get var @> struct_get tid_base 0 @: generic_get var
        @> call closure_ffi_name @: rest
-   | Lcast { expr; target_type = Ref_any } -> got expr rest
-   | Lcast { expr; target_type } ->
-       gon expr (ref_cast (result target_type) @: rest)
+  | Lcast { expr; target_type = Ref_any } -> (
+      match cast_operand_type expr with
+      | Some Ref_extern -> gon expr (Ast.Extern_convert_any @: rest)
+      | _ -> got expr rest)
+  | Lcast { expr; target_type = Ref_extern } -> (
+      match cast_operand_type expr with
+      | Some Ref_any -> gon expr (Ast.Any_convert_extern @: rest)
+      | _ -> gon expr (ref_cast (result Ref_extern) @: rest))
+  | Lcast { expr; target_type } ->
+      gon expr (ref_cast (result target_type) @: rest)
    | Lcatch { body; on_exception; type_ } ->
        assert !Basic_config.test_mode;
        let body = gon body [] in