Fix ProtoTypeAdapter being unable to deserialize certain repeated f…

…ields

For example a `List<Long>` Protobuf field might internally have the
Protobuf-internal `LongList` interface type.
Previously Gson's ConstructorConstructor was nonetheless creating an
ArrayList for this, which is wrong but worked. Now with the changes in
ConstructorConstructor Gson will fail to create an instance, so this commit
tries to solve this properly in ProtoTypeAdapter.

proto/src/main/java/com/google/gson/protobuf/ProtoTypeAdapter.java

@@ -20,6 +20,7 @@
 
 import com.google.common.base.CaseFormat;
 import com.google.common.collect.MapMaker;
+import com.google.common.reflect.TypeToken;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.JsonArray;
 import com.google.gson.JsonDeserializationContext;
@@ -44,6 +45,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentMap;
@@ -331,8 +333,15 @@ public Message deserialize(JsonElement json, Type typeOfT, JsonDeserializationCo
             String protoArrayFieldName =
                 protoFormat.to(CaseFormat.LOWER_CAMEL, fieldDescriptor.getName()) + "_";
             Field protoArrayField = protoClass.getDeclaredField(protoArrayFieldName);
-            Type protoArrayFieldType = protoArrayField.getGenericType();
-            fieldValue = context.deserialize(jsonElement, protoArrayFieldType);
+
+            @SuppressWarnings("unchecked")
+            TypeToken<? extends List<?>> protoArrayFieldType =
+                (TypeToken<? extends List<?>>) TypeToken.of(protoArrayField.getGenericType());
+            // Get the type as `List<E>`, otherwise type might be Protobuf internal interface for
+            // which no instance can be created
+            Type protoArrayResolvedFieldType =
+                protoArrayFieldType.getSupertype(List.class).getType();
+            fieldValue = context.deserialize(jsonElement, protoArrayResolvedFieldType);
             protoBuilder.setField(fieldDescriptor, fieldValue);
           } else {
             Object field = defaultInstance.getField(fieldDescriptor);