<+ operator in Kotlin gives "Unresolved reference....

# stackoverflow

[user]

+ operator in Kotlin gives "Unresolved reference. None of the following candidates is applicable because of receiver type mismatch:"
I am learning Kotlin by doing exercises on exercism.com. I am currently working on triangles. The code has the following test that I'm trying to make pass:
class TriangleTest {
@Test(expected = IllegalArgumentException::class)
fun `triangle inequality violation - last is greater then sum of others `() {
Triangle(1, 1, 3)
}
}

My solution:
class Triangle(private val a: T, private...