Kotlin out must be present in the implementation class to make the type covariant

SampleTest.kt

interface Source<out T> {

    fun nextT(): T
}

class SourceImpl<out T>(private val param: T) : Source<T> {

    override fun nextT(): T {
        return param
    }
}


class SampleTest {

    @Test
    fun foo() {
        val s: SourceImpl<Int> = SourceImpl(1)
        
        
        // without "out" in SourceImpl, this is not allowed.
        val s2: SourceImpl<Number> = s
        println(s2.nextT()) 
    }
}