Fix not returning negative double from box cache (#2777)

Src/Newtonsoft.Json.Tests/Issues/Issue2768.cs

@@ -74,6 +74,22 @@ namespace Newtonsoft.Json.Tests.Issues
             Assert.AreEqual("0.0", d.ToString());
         }
 
+        [Test]
+        public void Test_Deserialize_Negative()
+        {
+            decimal d = JsonConvert.DeserializeObject<decimal>("-0.0");
+
+            Assert.AreEqual("0.0", d.ToString());
+        }
+
+        [Test]
+        public void Test_Deserialize_NegativeNoTrailingZero()
+        {
+            decimal d = JsonConvert.DeserializeObject<decimal>("-0");
+
+            Assert.AreEqual("0", d.ToString());
+        }
+
         [Test]
         public void Test_Deserialize_MultipleTrailingZeroes()
         {
@@ -100,18 +116,18 @@ namespace Newtonsoft.Json.Tests.Issues
                 FloatParseHandling = FloatParseHandling.Decimal
             };
 
-            decimal? parsedDecimal = null;
+            decimal? parsedValue = null;
             
             while (reader.Read())
             {
                 if (reader.TokenType == JsonToken.Float)
                 {
-                    parsedDecimal = (decimal)reader.Value;
+                    parsedValue = (decimal)reader.Value;
                     break;
                 }
             }
 
-            Assert.AreEqual("0.0", parsedDecimal.ToString());
+            Assert.AreEqual("0.0", parsedValue.ToString());
         }
 
         [Test]
@@ -144,7 +160,42 @@ namespace Newtonsoft.Json.Tests.Issues
 #else
             Assert.IsFalse(object.ReferenceEquals(boxedDecimals[0], boxedDecimals[1]));
 #endif
+        }
 
+        [Test]
+        public void Test_Deserialize_Double_Negative()
+        {
+            double d = JsonConvert.DeserializeObject<double>("-0.0");
+
+#if NETCOREAPP3_1_OR_GREATER
+            Assert.AreEqual("-0", d.ToString());
+#else
+            Assert.AreEqual("0", d.ToString());
+#endif
+        }
+
+        [Test]
+        public void Test_Deserialize_Double_NegativeNoTrailingZero()
+        {
+            double d = JsonConvert.DeserializeObject<double>("-0");
+
+#if NETCOREAPP3_1_OR_GREATER
+            Assert.AreEqual("-0", d.ToString());
+#else
+            Assert.AreEqual("0", d.ToString());
+#endif
+        }
+
+        [Test]
+        public void JValueDouble_ToString()
+        {
+            var d = new JValue(-0.0d);
+
+#if NETCOREAPP3_1_OR_GREATER
+            Assert.AreEqual("-0", d.ToString());
+#else
+            Assert.AreEqual("0", d.ToString());
+#endif
         }
     }
 }

Src/Newtonsoft.Json/Utilities/BoxedPrimitives.cs

@@ -131,7 +131,8 @@ namespace Newtonsoft.Json.Utilities
         {
             if (value == 0.0d)
             {
-                return DoubleZero;
+                // Double supports -0.0. Detection logic from https://stackoverflow.com/a/4739883/11829.
+                return double.IsNegativeInfinity(1.0 / value) ? DoubleNegativeZero : DoubleZero;
             }
             if (double.IsInfinity(value))
             {
@@ -147,6 +148,7 @@ namespace Newtonsoft.Json.Utilities
         internal static readonly object DoubleNaN = double.NaN;
         internal static readonly object DoublePositiveInfinity = double.PositiveInfinity;
         internal static readonly object DoubleNegativeInfinity = double.NegativeInfinity;
-        internal static readonly object DoubleZero = (double)0;
+        internal static readonly object DoubleZero = 0.0d;
+        internal static readonly object DoubleNegativeZero = -0.0d;
     }
 }