"Method return type must not include a type variable or wildcard " it seems to be clear

You cannot. Type information needs to be fully known at runtime in order for deserialization to work.

would be great

So for example, a request such as:
Call<T[]> call = apiService.getResources(...); where T extends Resource would not be possible? (Resource being a class from the moshi-jsonapi library, which allows you to extend a class as a Resource.)
Would I then need to create a different retrofit API interface for each of these Resources?

Sorry to bring up old issues...

So... If I have 6 different types of resources, I'm going to create 6 different interfaces, and this is how people have been doing it the whole time? I'm sorry I'm just not sure what the alternative is here when trying to keep my code DRY.

@lordplagus02
How about this, define the Call so as to return a JsonElement as the class type, something like this,
Call<JsonElement> getObjectByID(); and then parse the jsonElement yourself

CustomObject1 customObject1 = new Gson().fromJson(response.body().getAsJsonObject().toString(), CustomObject1.class);

CustomObject2 customObject2 = new Gson().fromJson(response.body().getAsJsonObject().toString(), CustomObject2.class);

So here, the parsing dependency is upon you and not retrofit. Hope it helps.

this way you can create just one interface for all object by ID queries, alternatively you can also add query map to add additional options and set null if not required.

oh This will be very troublesome

You can create these methods once and then you're done. A utility sort of thing

yes It will work but May I have some more in explanation if possible with example with 2 or more service call
Thank you shahsurajk

I get this error because of proguard. In my interface I had a function that was not used anymore. Also the class of the return type was not used anymore in the project. I think proguard removed the return type class and made the return type of the function Call or Call<?>. The result was the same IllegalArgumentException as soon as the first http call started.

It needs to fix proguard rules

It has worked for me using Kotlin. But is this the best way to do this? Doesn't this generic form bring problems with memory leaks?