test: cover forbidden nested type promotions

Assert that promotion helpers reject nested type targets for unknown and regular primitive source types.

Co-authored-by: Codex <codex@openai.com>

Author: manuzhang

src/iceberg/test/update_schema_test.cc

@@ -20,6 +20,7 @@
 #include "iceberg/update/update_schema.h"
 
 #include <memory>
+#include <vector>
 
 #include <gtest/gtest.h>
 
@@ -29,6 +30,7 @@
 #include "iceberg/test/update_test_base.h"
 #include "iceberg/type.h"
 #include "iceberg/util/checked_cast.h"
+#include "iceberg/util/type_util.h"
 
 namespace iceberg {
 
@@ -1068,6 +1070,22 @@ TEST_F(UpdateSchemaTest, UpdateColumnUnknownToPrimitive) {
   EXPECT_EQ(field_opt->get().doc(), "A null-only placeholder");
 }
 
+TEST_F(UpdateSchemaTest, UpdateColumnPromotionToNestedTypesForbidden) {
+  std::vector<std::shared_ptr<Type>> nested_types = {
+      struct_({SchemaField::MakeOptional(3, "name", string())}),
+      list(SchemaField::MakeOptional(4, "element", string())),
+      map(SchemaField::MakeRequired(5, "key", string()),
+          SchemaField::MakeOptional(6, "value", string())),
+  };
+
+  for (const auto& nested_type : nested_types) {
+    SCOPED_TRACE(nested_type->ToString());
+
+    EXPECT_FALSE(IsPromotionAllowed(unknown(), nested_type));
+    EXPECT_FALSE(IsPromotionAllowed(int32(), nested_type));
+  }
+}
+
 TEST_F(UpdateSchemaTest, AddRequiredUnknownColumnFails) {
   ICEBERG_UNWRAP_OR_FAIL(auto update, table_->NewUpdateSchema());
   update->AllowIncompatibleChanges().AddRequiredColumn("mystery", unknown());