unchecked-impossible.scala:5: warning: fruitless type test: a value of type T2[Int,Int] cannot also be a Seq[A]
    case Seq(x) =>
            ^
unchecked-impossible.scala:5: error: pattern type is incompatible with expected type;
 found   : Seq[A]
 required: T2[Int,Int]
    case Seq(x) =>
            ^
one warning found
one error found