TypeConstraints

class A
class B extends A
class C
class D
implicit def d2a(d: D) = new A

val (a, b, c, d) = (new A, new B, new C, new D)

//def u[T <: A](x: T): A = x
def u[T](x: T)(implicit f: T <:< A): A = x
u(b)  // ok
u(d)  // error
// a, b => ok, c, d => error

def e[T](x: T)(implicit f: T =:= A): A = x
// a => ok, b, c, d => error

// def v[T <% A](x: T): A = x
// def v[T](x: T)(implicit f: T => A): A = x
def v[T](x: T)(implicit f: T <%< A): A = x
// a, b, d => ok, c => error