I think we really need a decision here. For me it would be an inconsistent and and odd behavior.
For example, if for<'a> Tr<'a, Assoc = impl Copy + 'a> allows Assoc to be paramterized by 'a, I would expect the same behavior from for<'a> Tr<'a, Assoc = impl Copy> as well! Adding a lifetime bound shouldn't change semantics in this regard.