gistfile1.sbt

scala> def a[T](i:T)(implicit ev: =:=[String, T]) = println(i)
a: [T](i: T)(implicit ev: =:=[String,T])Unit

scala> def test[T <: Any](a : T)(implicit ev : T =:= String) = println(a)
test: [T](a: T)(implicit ev: =:=[T,String])Unit

scala> test("aa")
aa

scala> test(1)
<console>:12: error: Cannot prove that Int =:= String.
              test(1)