Implementation by delegation serializes only the child proto if the interface also implements KtMessage

[andrewmellen-toast]

Description

Because of how the Kotlin compiler ends up building the output code for implementation by delegation, calling .serialize() on a proto class that implements an interface by delegation that also implements KtMessage ends up calling the wrong .serialize() method and produces an unexpected result.

Reproduction steps

syntax = "proto3"

package mypkg.v1;

import "protokt/protokt.proto";

option java_package = "com.mypkg.v1";

message ImplementerProto {
      option (protokt.class).implements = "com.mypkg.MyInterface";

      String value = 1;
}

message DelegationProto {
     option (protokt.class).implements = "com.mypkg.MyInterface by metadata";

     ImplementerProto metadata = 1;
     String more_data = 2;
}

interface MyInterface : KtMessage {
    val value: String
}

fun test() {
    val delegationProto = DelegationProto {
        metadata = ImplementerProto {
            value = "some value"
        }
        moreData = "some other value"
    }

    // Bug here: This is actually calling ImplementerProto's serialize() method, not DelegationProto's because
    // Kotlin's compiler overrides serialize() when doing the implementation by delegation
    val serializedProto = delegationProto.serialize()

    // this throws an exception
    DelegationProto.deserialize(serializedProto)

    // this succeeds
    ImplementerProto.deserialize(serializedProto)

    // doing this gets the correct value, because we're calling into serialize(KtSerializer),
    // which each generated proto class overrides explicitly
    val s = ByteArray(serializedProto.messageSize).apply {
            serializedProto.serialize(serializer(CodedOutputStream.newInstance(this)))
     }
}