extension GenericClass where T: OptionalType {
  func valueOrThrow() throws -> T.Wrapped {
    if let unwrapped = self.value.value {
      return unwrapped
    }

    throw ErrorImpl("No value found")
  }
}

func testGenericClass() {
  let gc1 = GenericClass<Int>(1)
  // gc1.valueOrThrow() - does not compile.

  let gc2 = GenericClass<Int?>(1)

  do {
    let value = try gc2.valueOrThrow()        // This is of type Int
  } catch let e {
    ...
  }
}