BUG: Fix padding with large integers

The old way of creating the padded array padded with wrong values for
large integers because the new prepended / appended array was implicitly
created with dtype float64:

>>> (np.zeros(1) + (2 ** 64 - 1)).astype(np.uint64)
array([0], np.uint64)
>>> (np.zeros(1) + (2 ** 63 - 1)).astype(np.int64)
array([-9223372036854775808])

1 files changed, 13 insertions, 0 deletions

diff --git a/numpy/lib/tests/test_arraypad.py b/numpy/lib/tests/test_arraypad.py
index 8be49ce67..8ba0370b0 100644
--- a/numpy/lib/tests/test_arraypad.py
+++ b/numpy/lib/tests/test_arraypad.py
@@ -489,6 +489,19 @@ class TestConstant(object):
         )
         assert_allclose(test, expected)
 
+    def test_check_large_integers(self):
+        uint64_max = 2 ** 64 - 1
+        arr = np.full(5, uint64_max, dtype=np.uint64)
+        test = np.pad(arr, 1, mode="constant", constant_values=arr.min())
+        expected = np.full(7, uint64_max, dtype=np.uint64)
+        assert_array_equal(test, expected)
+
+        int64_max = 2 ** 63 - 1
+        arr = np.full(5, int64_max, dtype=np.int64)
+        test = np.pad(arr, 1, mode="constant", constant_values=arr.min())
+        expected = np.full(7, int64_max, dtype=np.int64)
+        assert_array_equal(test, expected)
+
 
 class TestLinearRamp(object):
     def test_check_simple(self):