GH-45362: [C++] Fix identity cast for time and list scalar

cpp/src/arrow/scalar.cc

@@ -1177,14 +1177,16 @@ enable_if_duration<To, Result<std::shared_ptr<Scalar>>> CastImpl(
 }
 
 // time to time
-template <typename To, typename From, typename T = typename To::TypeClass>
+template <typename To, typename From,
+          typename T = typename TypeTraits<To>::ScalarType::TypeClass>
 enable_if_time<To, Result<std::shared_ptr<Scalar>>> CastImpl(
     const TimeScalar<From>& from, std::shared_ptr<DataType> to_type) {
   using ToScalar = typename TypeTraits<To>::ScalarType;
   ARROW_ASSIGN_OR_RAISE(
       auto value, util::ConvertTimestampValue(AsTimestampType<From>(from.type),
                                               AsTimestampType<To>(to_type), from.value));
-  return std::make_shared<ToScalar>(value, std::move(to_type));
+  return std::make_shared<ToScalar>(static_cast<typename To::c_type>(value),
+                                    std::move(to_type));
 }
 
 constexpr int64_t kMillisecondsInDay = 86400000;
@@ -1288,10 +1290,11 @@ CastImpl(const StructScalar& from, std::shared_ptr<DataType> to_type) {
 }
 
 // casts between variable-length and fixed-length list types
-template <typename To, typename From>
+template <typename To, typename FromScalar,
+          typename From = typename FromScalar::TypeClass>
 std::enable_if_t<is_list_type<To>::value && is_list_type<From>::value,
                  Result<std::shared_ptr<Scalar>>>
-CastImpl(const From& from, std::shared_ptr<DataType> to_type) {
+CastImpl(const FromScalar& from, std::shared_ptr<DataType> to_type) {
   if constexpr (sizeof(typename To::offset_type) < sizeof(int64_t)) {
     if (from.value->length() > std::numeric_limits<typename To::offset_type>::max()) {
       return Status::Invalid(from.type->ToString(), " too large to cast to ",

cpp/src/arrow/scalar_test.cc

@@ -36,6 +36,7 @@
 #include "arrow/status.h"
 #include "arrow/testing/extension_type.h"
 #include "arrow/testing/gtest_util.h"
+#include "arrow/testing/random.h"
 #include "arrow/testing/util.h"
 #include "arrow/type_traits.h"
 
@@ -156,6 +157,50 @@ TEST(TestBooleanScalar, Cast) {
   }
 }
 
+TEST(TestScalar, IdentityCast) {
+  random::RandomArrayGenerator gen(/*seed=*/42);
+  auto test_identity_cast_for_type =
+      [&gen](const std::shared_ptr<arrow::DataType>& data_type) {
+        auto tmp_array = gen.ArrayOf(data_type, /*size=*/1, /*null_probability=*/0.0);
+        ARROW_SCOPED_TRACE("data type = ", data_type->ToString());
+        ASSERT_OK_AND_ASSIGN(auto scalar, tmp_array->GetScalar(0));
+        ASSERT_OK_AND_ASSIGN(auto casted_scalar, scalar->CastTo(data_type));
+        ASSERT_TRUE(casted_scalar->Equals(*scalar));
+        ASSERT_TRUE(scalar->Equals(*casted_scalar));
+      };
+  for (auto& type : PrimitiveTypes()) {
+    test_identity_cast_for_type(type);
+  }
+  for (auto& type : DurationTypes()) {
+    test_identity_cast_for_type(type);
+  }
+  for (auto& type : IntervalTypes()) {
+    test_identity_cast_for_type(type);
+  }
+  for (auto& type : {
+           arrow::fixed_size_list(arrow::int32(), 20), arrow::list(arrow::int32()),
+           arrow::large_list(arrow::int32()),
+           // TODO(GH-45430): CastTo for ListView is not implemented yet.
+           // arrow::list_view(arrow::int32()), arrow::large_list_view(arrow::int32())
+           // TODO(GH-45431): CastTo for ComplexType is not implemented yet.
+           // arrow::map(arrow::binary(), arrow::int32()),
+           // struct_({field("float", arrow::float32())}),
+       }) {
+    test_identity_cast_for_type(type);
+  }
+  // TODO(GH-45429): CastTo for Decimal is not implemented yet.
+  /*
+  for (auto& type: {
+    arrow::decimal32(2, 2),
+    arrow::decimal64(4, 4),
+    arrow::decimal128(10, 10),
+    arrow::decimal128(20, 20),
+  }) {
+    test_identity_cast_for_type(type);
+  }
+  */
+}
+
 template <typename T>
 class TestNumericScalar : public ::testing::Test {
  public:
@@ -866,6 +911,9 @@ TEST(TestTimeScalars, Basics) {
     ASSERT_TRUE(first->Equals(*MakeScalar(ty, 5).ValueOrDie()));
     ASSERT_TRUE(last->Equals(*MakeScalar(ty, 42).ValueOrDie()));
     ASSERT_FALSE(last->Equals(*MakeScalar("string")));
+
+    ASSERT_OK_AND_ASSIGN(auto casted, first->CastTo(ty));
+    ASSERT_TRUE(casted->Equals(*first));
   }
 }