Fix generic base codec generic type parameter resolution

src/Orleans.Serialization/Serializers/CodecProvider.cs

@@ -543,7 +543,7 @@ private IFieldCodec CreateCodecInstance(Type fieldType, Type searchType)
             }
             else if (searchType.BaseType is object
                 && CreateCodecInstance(
-                    fieldType,
+                    fieldType.BaseType,
                     searchType.BaseType switch
                     {
                         { IsConstructedGenericType: true } => searchType.BaseType.GetGenericTypeDefinition(),

test/Orleans.Serialization.UnitTests/GenericBaseClassTest.cs

@@ -0,0 +1,30 @@
+using Microsoft.Extensions.DependencyInjection;
+using Xunit;
+
+namespace Orleans.Serialization.UnitTests
+{
+    [GenerateSerializer]
+    abstract class Base<T>;
+
+    class Derived : Base<int>;
+
+    [Trait("Category", "BVT")]
+    public class GenericBaseClassTest
+    {
+        private readonly ServiceProvider _services;
+        private readonly Serializer _serializer;
+        public GenericBaseClassTest()
+        {
+            _services = new ServiceCollection()
+                .AddSerializer()
+                .BuildServiceProvider();
+            _serializer = _services.GetRequiredService<Serializer>();
+        }
+
+        [Fact]
+        public void DerivedNoGeneric()
+        {
+            Assert.False(_serializer.CanSerialize(typeof(Derived)));
+        }
+    }
+}