gistfile1.txt

scala> trait X { type A }
defined trait X

scala> implicit def convert(a:X)(implicit ev: a.A =:= String): Int = 42
convert: (a: X)(implicit ev: =:=[a.A,String])Int

scala> (new X {}) : Int
<console>:10: error: Cannot prove that a.A =:= String.
              (new X {}) : Int
               ^

scala> (new X { type A = String }) : Int
res11: Int = 42