Fix test

csharp/src/Google.Protobuf.Test/JsonParserTest.cs

@@ -551,13 +551,9 @@ namespace Google.Protobuf
         }
 
         [Test]
-        // Skip these test cases in .NET 5 because floating point parsing supports bigger values.
-        // These big values won't throw an error in the test.
-#if !NETCOREAPP3_1_OR_GREATER
         [TestCase("1.7977e308")]
         [TestCase("-1.7977e308")]
         [TestCase("1e309")]
-#endif
         [TestCase("1,0")]
         [TestCase("1.0.0")]
         [TestCase("+1")]

csharp/src/Google.Protobuf.Test/JsonTokenizerTest.cs

@@ -199,12 +199,8 @@ namespace Google.Protobuf
         [TestCase("1e-")]
         [TestCase("--")]
         [TestCase("--1")]
-        // Skip these test cases in .NET 5 because floating point parsing supports bigger values.
-        // These big values won't throw an error in the test.
-#if !NETCOREAPP3_1_OR_GREATER
         [TestCase("-1.7977e308")]
         [TestCase("1.7977e308")]
-#endif
         public void InvalidNumberValue(string json)
         {
             AssertThrowsAfter(json);

csharp/src/Google.Protobuf/JsonTokenizer.cs

@@ -471,9 +471,18 @@ namespace Google.Protobuf
                 // TODO: What exception should we throw if the value can't be represented as a double?
                 try
                 {
-                    return double.Parse(builder.ToString(),
+                    double result = double.Parse(builder.ToString(),
                         NumberStyles.AllowLeadingSign | NumberStyles.AllowDecimalPoint | NumberStyles.AllowExponent,
                         CultureInfo.InvariantCulture);
+
+                    // .NET Core 3.0 and later returns infinity if the number is too large or small to be represented.
+                    // For compatibility with other Protobuf implementations the tokenizer should still throw.
+                    if (double.IsInfinity(result))
+                    {
+                        throw reader.CreateException("Numeric value out of range: " + builder);
+                    }
+
+                    return result;
                 }
                 catch (OverflowException)
                 {