Address review: use std::get_if and capture Validate status once

Author: huan233usc

src/iceberg/schema_field.cc

@@ -174,12 +174,14 @@ Status ValidateDefault(const SchemaField& field, const Literal& value,
   // so a value that does not fit the field precision would pass the type check above yet
   // be rejected when the metadata is parsed back. Reject it here to keep the default
   // consistent with what the reader accepts.
-  if (field.type()->type_id() == TypeId::kDecimal &&
-      std::holds_alternative<Decimal>(value.value())) {
-    const auto& decimal_type = internal::checked_cast<const DecimalType&>(*field.type());
-    if (!std::get<Decimal>(value.value()).FitsInPrecision(decimal_type.precision())) {
-      return InvalidSchema("{} of field {} does not fit precision {}", kind, field.name(),
-                           decimal_type.precision());
+  if (field.type()->type_id() == TypeId::kDecimal) {
+    if (const auto* dec = std::get_if<Decimal>(&value.value())) {
+      const auto& decimal_type =
+          internal::checked_cast<const DecimalType&>(*field.type());
+      if (!dec->FitsInPrecision(decimal_type.precision())) {
+        return InvalidSchema("{} of field {} does not fit precision {}", kind,
+                             field.name(), decimal_type.precision());
+      }
     }
   }
   return {};

src/iceberg/test/schema_field_test.cc

@@ -179,8 +179,9 @@ TEST(SchemaFieldTest, ValidateRejectsDecimalDefaultExceedingPrecision) {
   SchemaField field(/*field_id=*/1, /*name=*/"d", decimal(2, 1),
                     /*optional=*/true, /*doc=*/"",
                     std::make_shared<const Literal>(Literal::Decimal(999, 2, 1)));
-  EXPECT_THAT(field.Validate(), IsError(ErrorKind::kInvalidSchema));
-  EXPECT_THAT(field.Validate(), HasErrorMessage("does not fit precision"));
+  auto status = field.Validate();
+  EXPECT_THAT(status, IsError(ErrorKind::kInvalidSchema));
+  EXPECT_THAT(status, HasErrorMessage("does not fit precision"));
 }
 
 TEST(SchemaFieldTest, ValidateAcceptsDecimalDefaultWithinPrecision) {