error: lifetime may not live long enough
  --> tests/fail/invariance-bad-function.rs:15:5
   |
14 | fn shrink<'a, 'b: 'a>(long: Box<S<'b>>) -> Box<dyn Any<Inv<'a>> + 'b> {
   |           --  -- lifetime `'b` defined here
   |           |
   |           lifetime `'a` defined here
15 |     long
   |     ^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
   |
   = help: consider adding the following bound: `'a: 'b`
   = note: requirement occurs because of the type `transient::Inv<'_>`, which makes the generic argument `'_` invariant
   = note: the struct `transient::Inv<'a>` is invariant over the parameter `'a`
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance