There is no subtyping relationship between the types (or their non-freshened
variants), so they can not be merged.
Fixes#22645Fixes#24352Fixes#23825
Should fix#25235 (no test in issue).
Should fix#19976 (test is outdated).
trait matching more tailored. We now detect recursion where the
obligations "match" -- meaning basically that they are the same for some
substitution of any unbound type variables.
