You want to add some Norm for that with_gen/2 goodness, but Dialyzer got there first. The first argument of with_gen/2 isn't opaque, t:StreamData.t/0 is, and your console is all red:

The call Norm.with_gen(#{'__struct__':='Elixir.Norm.Core.Spec', 'f':=fun((_) -> boolean()), 'generator':='is_atom', 'predicate':=<<_:72>>},'Elixir.StreamData':t(_)) contains an opaque term in 2nd argument when terms of different types are expected in these positions}.

If it wouldn't make @keathley break out in a rash, maybe an @opaque of our own and a sprinkling of @spec would make it easier?