Resolve intermittent failure in test_long (#984)

Author: slozier

Src/IronPython/Modules/Builtin.cs

@@ -643,7 +643,7 @@ public static string hex(object? o) {
 
             static string ToHexString(BigInteger b) {
                 var val = b.ToString("x");
-                if (val[0] == '0') val = val.TrimStart('0'); // BigInteger.ToString("x") sometimes pads with 0 so we need to trim it (https://github.com/dotnet/runtime/issues/43269)
+                if (val[0] == '0') val = val.TrimStart('0'); // positive values with the most significant bit set are padded with a 0
                 return val;
             }
         }

Src/IronPython/Runtime/Operations/LongOps.cs

@@ -369,11 +369,19 @@ public static BigInteger op_LeftShift(BigInteger x, int y) {
             return x << y;
         }
 
+        private static readonly bool hasShiftBug = BigInteger.Parse("-18446744073709543424") >> 32 == 0; // https://github.com/dotnet/runtime/issues/43396
+
         [SpecialName]
         public static BigInteger op_RightShift(BigInteger x, int y) {
             if (y < 0) {
                 throw PythonOps.ValueError("negative shift count");
             }
+            if (hasShiftBug && x.IsNegative()) {
+                if (y == 0) return x;
+                if (y % 32 == 0) {
+                    return (x >> (y - 1)) >> 1;
+                }
+            }
             return x >> y;
         }
 

Tests/test_ironmath.py

@@ -234,4 +234,10 @@ def test_misc(self):
         self.assertEqual(BigInteger(1).CompareTo(None), 1)
         self.assertEqual(BigInteger(1).CompareTo(True), 0)
 
+    def test_rightshiftby32_negative_bug(self):
+        # test workaround for https://github.com/dotnet/runtime/issues/43396
+        from System.Numerics import BigInteger
+        self.assertEqual(BigInteger.Parse("-18446744073709543424") >> 32, -4294967296)
+        self.assertEqual(BigInteger.Parse("-79228162514264337593543917568") >> 32, -18446744073709551616)
+
 run_test(__name__)