[wasm-gc] Make table type restrictions explicit

Bug: v8:7748
Change-Id: Ib5beeb55c3a9eba8fb23680e3845f80d55f3f154
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2274633
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68646}

src/wasm/module-decoder.cc

@@ -749,7 +749,13 @@ class ModuleDecoderImpl : public Decoder {
       if (!AddTable(module_.get())) break;
       module_->tables.emplace_back();
       WasmTable* table = &module_->tables.back();
+      const byte* type_position = pc();
       table->type = consume_reference_type();
+      if (table->type.kind() != ValueType::kOptRef) {
+        // TODO(7748): Implement other table types.
+        error(type_position,
+              "Currently, only nullable references are allowed as table types");
+      }
       uint8_t flags = validate_table_flags("table elements");
       consume_resizable_limits(
           "table elements", "elements", FLAG_wasm_max_table_size,