Hi, How to figure out this compiler error? ```Type parameter output is declared as 'out' but occurs ...

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[Muhammad Usman]

Hi, How to figure out this compiler error?

Type parameter output is declared as 'out' but occurs in 'invariant' position in type Flow<PagingData<output>>

abstract class PaginatedFlowUseCase<in Input, out output>(
    private val dispatcher: CoroutineDispatcher
) {
    operator fun invoke(input: Input): Flow<PagingData<output>> {
        return execute(input).flowOn(dispatcher)
    }

    protected abstract fun execute(input: Input): Flow<PagingData<output>>
}

But it works when changing PagingData to kolin.Result class

[Youssef Shoaib [MOD]]

It's likely that PagingData is declared with an invariant type parameter. You can either make output invariant by removing out, or you can replace Flow<PagingData<output>> with Flow<PagingData<out output>> . Also, please use PascalCase for your type parameters, it's the usual convention, so Output instead of output .

[Muhammad Usman]

abstract class PaginatedFlowUseCase<in Input, Output>(
    private val dispatcher: CoroutineDispatcher
) {
    operator fun invoke(input: Input): Flow<PagingData<Output>> {
        return execute(input).flowOn(dispatcher)
    }

    protected abstract fun execute(input: Input): Flow<PagingData<Output>>
}

Type argument is not within its bound
Expected: Any
Found: Output

[Stephan Schroeder]

i'm 80% sure that the issue is, that you didn't limit Output to extend from nonnullable Any (right now it's Any? by default). Try abstract class PaginatedFlowUseCase<in Input, Output: Any>

[Muhammad Usman]

Thanks, Stephan that works.