Test_2.scala:7: error: type mismatch;
 found   : Iso[Test.Foo,(Int, String, Boolean)]
 required: Iso[Test.Foo,Nothing]
Note: (Int, String, Boolean) >: Nothing, but trait Iso is invariant in type U.
You may wish to define U as -U instead. (SLS 4.5)
    val equiv = foo(Foo(23, "foo", true))
                   ^
one error found