For specialization to be applicable here, there needsto be a "clear hierarchy" between the trait bounds:
either all T : Dummy are also T : Into<u32>, and then Into<u32> can have a more specialized impl than Dummy's (thus the latter is the one needing the default qualifier)
this can be achieved with, for instance, a super trait
trait Dummy : Into<u32> {}
or all T : Into<u32> are also T : Dummy, and then Dummy can have a more specialized impl than Into<u32>'s (thus the latter is the one needing the default qualifier)
this can be achieved with, for instance, a generic impl