When should I use type bound in generic class in scala?

Question

I have a generic class like this and I only want function "one" to be called when T is Int.

class A[T] {
    def one[T <: Int] = 1
}
val a = new A[String]
a.one

But this compiles.

I find that I can do this:

class A[T] {
    def one(implicit ev: T <:< Int) = 1
}
val a = new A[String]
a.one
<console>:14: error: Cannot prove that String <:< Int.

Why?

Answer 1

In the first case you have two different and unrelated T parameters: one on the class, another on the method. So when you call a.one , one 's T is Int .

In the second case one doesn't have its own T parameter, so in T <:< Int you have A 's T . When you call a.one , A 's T is String which doesn't satisfy the bound.