[System.Numerics] Contract the values array when decoding an array with a negative number and a trailer that evals to zero. Fixes #20456.

2 files changed, 15 insertions, 8 deletions

diff --git a/mcs/class/System.Numerics/System.Numerics/BigInteger.cs b/mcs/class/System.Numerics/System.Numerics/BigInteger.cs
index b067997cb7c..329f91eed1f 100644
--- a/mcs/class/System.Numerics/System.Numerics/BigInteger.cs
+++ b/mcs/class/System.Numerics/System.Numerics/BigInteger.cs
@@ -315,12 +315,14 @@ namespace System.Numerics {
 					word = (uint)sub;
 					borrow = (uint)(sub >> 32) & 0x1u;
 
-					data [data.Length - 1] = ~word & store_mask;
+					if ((~word & store_mask) == 0)
+						data = Resize (data, data.Length - 1);
+					else
+						data [data.Length - 1] = ~word & store_mask;
 				}
 				if (borrow != 0) //FIXME I believe this can't happen, can someone write a test for it?
 					throw new Exception ("non zero final carry");
 			}
-
 		}
 
 		public bool IsEven {
diff --git a/mcs/class/System.Numerics/Test/System.Numerics/BigIntegerTest.cs b/mcs/class/System.Numerics/Test/System.Numerics/BigIntegerTest.cs
index 4aa0ffbd029..817d13e40c1 100644
--- a/mcs/class/System.Numerics/Test/System.Numerics/BigIntegerTest.cs
+++ b/mcs/class/System.Numerics/Test/System.Numerics/BigIntegerTest.cs
@@ -550,13 +550,18 @@ namespace MonoTests.System.Numerics
 		{
 			long[] values = new long [] {
 				0, long.MinValue, long.MaxValue, -1, 1L + int.MaxValue, -1L + int.MinValue, 0x1234, 0xFFFFFFFFL, 0x1FFFFFFFFL, -0xFFFFFFFFL, -0x1FFFFFFFFL,
-				0x100000000L, -0x100000000L, 0x100000001L, -0x100000001L };
+				0x100000000L, -0x100000000L, 0x100000001L, -0x100000001L, 4294967295L, -4294967295L, 4294967296L, -4294967296L };
 			foreach (var val in values) {
-				var a = new BigInteger (val);
-				var b = new BigInteger (a.ToByteArray ());
-
-				Assert.AreEqual (val, (long)a, "#a_" + val);
-				Assert.AreEqual (val, (long)b, "#b_" + val);
+				try {
+					var a = new BigInteger (val);
+					var b = new BigInteger (a.ToByteArray ());
+
+					Assert.AreEqual (val, (long)a, "#a_" + val);
+					Assert.AreEqual (val, (long)b, "#b_" + val);
+					Assert.AreEqual (a, b, "#a  == #b (" + val + ")");
+				} catch (Exception e) {
+					Assert.Fail ("could not roundtrip {0}", val);
+				}
 			}
 		}