Reapply custom option accessors from previous PR along with CustomOptions accessor deprecations

csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs

@@ -41,6 +41,8 @@ using static UnitTest.Issues.TestProtos.ComplexOptionType2.Types;
 using static UnitTest.Issues.TestProtos.DummyMessageContainingEnum.Types;
 using static Google.Protobuf.Test.Reflection.CustomOptionNumber;
 
+#pragma warning disable CS0618
+
 namespace Google.Protobuf.Test.Reflection
 {
     // Internal enum to allow us to use "using static" for convenience.

csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs

@@ -128,17 +128,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this enum.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<EnumOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -148,6 +147,5 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
     }
 }
\ No newline at end of file

csharp/src/Google.Protobuf/Reflection/EnumValueDescriptor.cs

@@ -73,17 +73,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this enum value.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<EnumValueOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -93,7 +92,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
     }
 }
  
\ No newline at end of file

csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs

@@ -277,17 +277,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this field.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<FieldOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -297,7 +296,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         /// <summary>
         /// Look up and cross-link all field types etc.

csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs

@@ -504,17 +504,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this file.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<FileOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -524,7 +523,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         /// <summary>
         /// Performs initialization for the given generic type argument.

csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs

@@ -243,17 +243,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this message.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<MessageOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -263,7 +262,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         /// <summary>
         /// Looks up and cross-links all fields and nested types.

csharp/src/Google.Protobuf/Reflection/MethodDescriptor.cs

@@ -73,17 +73,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this method.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<MethodOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -93,7 +92,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         internal MethodDescriptor(MethodDescriptorProto proto, FileDescriptor file,
                                   ServiceDescriptor parent, int index)

csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs

@@ -94,17 +94,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this service.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<ServiceOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -114,7 +113,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         internal void CrossLink()
         {