Scala lower bound type parameter does not work when type argument is not specified explicitly

Question

I have a below class which uses covariance annotation and another type parameter for its method to which class type parameter is lower bound

class MyQueue[+T]{
  def add[U >: T](arg:U):Unit= {
    println("Arg value :"+ arg)
  }
}

Given above code, I don't understand why below lines execute successfully. As per my understanding of lower bounds, method "add" should only accept value of type Int or its super-type.

val q1:MyQueue[Int] = new MyQueue[Int]
q1.add("string")

However, it gives expected compilation error (String do not conform to method add's type parameter bounds) if we explicitly specify the type argument as below

q1.add[String]("string")

Answer 1

String is not a supertype of Int but there is a common supertype shared between Int and String and that is Any

val q1:MyQueue[Int] = new MyQueue[Int]
q1.add("string")

is equivalent to

val q1:MyQueue[Int] = new MyQueue[Int]
q1.add[Any]("string")

on the other hand, if you explicitly pass String as the type parameter a compilation error will happen because String is not a supertype of Int

Answer 2

I'll just add to @MikelSanVicente's answer that if you want q1.add("string") not to compile you should replace type bound with implicit type constraint

class MyQueue[+T]{
  def add[U](arg: U)(implicit ev: T <:< U): Unit= {
    println("Arg value :"+ arg)
  }
}

val q1: MyQueue[Int] = new MyQueue[Int]
q1.add("string") // doesn't compile

q1.add[String]("string") // doesn't compile

q1.add[Any]("string") // compiles

https://blog.bruchez.name/2015/11/generalized-type-constraints-in-scala.html