test(longobject): add INT64_MIN boundary tests for _PyLong_TryAsInt64Exact

Verify that _PyLong_TryAsInt64Exact correctly handles INT64_MIN (abs_val == INT64_MAX + 1 with negative sign), INT64_MAX, and that values outside the int64 range gracefully fall back to the slow path.

Author: KRRT7

Lib/test/test_long.py

@@ -1140,6 +1140,30 @@ def test_small_ints(self):
         self.assertIs(i - i, 0)
         self.assertIs(0 * i, 0)
 
+    @support.cpython_only
+    def test_int64_boundary_add(self):
+        # Verify _PyLong_TryAsInt64Exact handles INT64 extremes correctly.
+        INT64_MAX = (1 << 63) - 1
+        INT64_MIN = -(1 << 63)
+
+        # INT64_MAX: addition that stays within range
+        self.assertEqual(INT64_MAX + 0, INT64_MAX)
+        self.assertEqual(INT64_MAX + (-1), INT64_MAX - 1)
+
+        # INT64_MIN: extraction must succeed and arithmetic must be correct
+        self.assertEqual(INT64_MIN + 0, INT64_MIN)
+        self.assertEqual(INT64_MIN + 1, INT64_MIN + 1)
+
+        # Adding two values that overflow int64 falls back to slow path
+        self.assertEqual(INT64_MAX + 1, 1 << 63)
+        self.assertEqual(INT64_MIN + (-1), INT64_MIN - 1)
+
+        # Values just outside int64 range are not handled by the fast path
+        beyond_max = INT64_MAX + 2
+        beyond_min = INT64_MIN - 2
+        self.assertEqual(beyond_max + 1, INT64_MAX + 3)
+        self.assertEqual(beyond_min + (-1), INT64_MIN - 3)
+
     def test_bit_length(self):
         tiny = 1e-10
         for x in range(-65000, 65000):