This is rather subtle. When we are casting a value to an instantiated trait like a as trait<'r>, regionck already ensures that any references that appear in the type of a are bounded by 'r (ed.: rem FIXME(#5723)). However, it is possible that there are type parameters in the type of a, and those type parameters may have references within them. We have to guarantee that the regions which appear in those type parameters are not obscured.