From 776ee2ea1ea147311ef9a3923c00e9a8c045c5b6 Mon Sep 17 00:00:00 2001
From: Conor Hoekstra <codereport@outlook.com>
Date: Thu, 23 Sep 2021 12:36:35 -0400
Subject: [PATCH 1/3] Fix and test

---
 cpp/include/cudf/fixed_point/fixed_point.hpp |  8 +++---
 cpp/tests/unary/cast_tests.cpp               | 26 ++++++++++++++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/cpp/include/cudf/fixed_point/fixed_point.hpp b/cpp/include/cudf/fixed_point/fixed_point.hpp
index e3694dc289a..af2715d1290 100644
--- a/cpp/include/cudf/fixed_point/fixed_point.hpp
+++ b/cpp/include/cudf/fixed_point/fixed_point.hpp
@@ -279,9 +279,11 @@ class fixed_point {
             typename cuda::std::enable_if_t<cuda::std::is_integral<U>::value>* = nullptr>
   explicit constexpr operator U() const
   {
-    // Don't cast to U until converting to Rep because in certain cases casting to U before shifting
-    // will result in integer overflow (i.e. if U = int32_t, Rep = int64_t and _value > 2 billion)
-    return static_cast<U>(detail::shift<Rep, Rad>(_value, scale_type{-_scale}));
+    // Cast to the larger of the two types (of U and Rep) before converting to Rep because in
+    // certain cases casting to U before shifting will result in integer overflow (i.e. if U =
+    // int32_t, Rep = int64_t and _value > 2 billion)
+    auto const value = std::common_type_t<U, Rep>(_value);
+    return static_cast<U>(detail::shift<Rep, Rad>(value, scale_type{-_scale}));
   }
 
   CUDA_HOST_DEVICE_CALLABLE operator scaled_integer<Rep>() const
diff --git a/cpp/tests/unary/cast_tests.cpp b/cpp/tests/unary/cast_tests.cpp
index c71f6aa2019..f3b0a231b34 100644
--- a/cpp/tests/unary/cast_tests.cpp
+++ b/cpp/tests/unary/cast_tests.cpp
@@ -848,3 +848,29 @@ TYPED_TEST(FixedPointTests, FixedPointToFixedPointDifferentTypeidUpNullMask)
 
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected, result->view());
 }
+
+TEST_F(FixedPointTestSingleType, AvoidOverflowDecimal32ToDecimal64)
+{
+  using namespace numeric;
+  using fp_wrapper32 = cudf::test::fixed_point_column_wrapper<int32_t>;
+  using fp_wrapper64 = cudf::test::fixed_point_column_wrapper<int64_t>;
+
+  auto const input    = fp_wrapper32{{9999999}, scale_type{3}};
+  auto const expected = fp_wrapper64{{9999999}, scale_type{3}};
+  auto const result   = cudf::cast(input, make_fixed_point_data_type<decimal64>(3));
+
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected, result->view());
+}
+
+TEST_F(FixedPointTestSingleType, AvoidOverflowDecimal32ToInt64)
+{
+  using namespace numeric;
+  using fp_wrapper32 = cudf::test::fixed_point_column_wrapper<int32_t>;
+  using fw_wrapper64 = cudf::test::fixed_width_column_wrapper<int64_t>;
+
+  auto const input    = fp_wrapper32{{9999999}, scale_type{3}};
+  auto const expected = fw_wrapper64{{9999999000}};
+  auto const result   = cudf::cast(input, cudf::data_type{cudf::type_id::INT64});
+
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected, result->view());
+}

From 96d3dd4e4dc60d9856bec7f91952fd533929cfff Mon Sep 17 00:00:00 2001
From: Conor Hoekstra <codereport@outlook.com>
Date: Fri, 24 Sep 2021 11:05:47 -0400
Subject: [PATCH 2/3] Python fix

---
 python/cudf/cudf/tests/test_decimal.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/python/cudf/cudf/tests/test_decimal.py b/python/cudf/cudf/tests/test_decimal.py
index 51f05e1b876..488217e9fde 100644
--- a/python/cudf/cudf/tests/test_decimal.py
+++ b/python/cudf/cudf/tests/test_decimal.py
@@ -14,6 +14,7 @@
     FLOAT_TYPES,
     INTEGER_TYPES,
     NUMERIC_TYPES,
+    SIGNED_TYPES,
     _decimal_series,
     assert_eq,
 )
@@ -228,7 +229,7 @@ def test_typecast_to_from_decimal(data, from_dtype, to_dtype):
     "from_dtype",
     [Decimal64Dtype(7, 2), Decimal64Dtype(11, 4), Decimal64Dtype(17, 10)],
 )
-@pytest.mark.parametrize("to_dtype", NUMERIC_TYPES)
+@pytest.mark.parametrize("to_dtype", SIGNED_TYPES)
 def test_typecast_from_decimal(data, from_dtype, to_dtype):
     got = data.astype(from_dtype)
     pa_arr = got.to_arrow().cast(to_dtype, safe=False)

From c2fdd23b6ac34fd36f1343e28298b194b3b8bdae Mon Sep 17 00:00:00 2001
From: Conor Hoekstra <codereport@outlook.com>
Date: Fri, 24 Sep 2021 11:15:23 -0400
Subject: [PATCH 3/3] unused import

---
 python/cudf/cudf/tests/test_decimal.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/python/cudf/cudf/tests/test_decimal.py b/python/cudf/cudf/tests/test_decimal.py
index 488217e9fde..c37381a3af9 100644
--- a/python/cudf/cudf/tests/test_decimal.py
+++ b/python/cudf/cudf/tests/test_decimal.py
@@ -13,7 +13,6 @@
 from cudf.testing._utils import (
     FLOAT_TYPES,
     INTEGER_TYPES,
-    NUMERIC_TYPES,
     SIGNED_TYPES,
     _decimal_series,
     assert_eq,