This works as intended but: the exact index raised was not the one inferred since the top-level partition itself is not a default partition hence always empty hence it's index is always empty; an empty index cannot raise uniqueness violation exceptions then the one raised must be the leaf partition's (whichever it is).
There seems to be a mix-up regarding the uniqueness restriction (a constraint) and the way it is implemented (unique indexes, wherever they may exist). That might be partly the documentation's fault for using index in improper places but I haven't fully checked that out. In any case, the unique constraint on the partitioned table can very well be enforced even if that named relation doesn't actually have data nor a backing unique index. The point of the partitioned table is to abstract that away from the user.
It would be nice if the ON CONFLICT behavior could be abstracted away as well, and you are right in your expectation that it would. But it is not a bug that our implementation is lacking in that area, though it could maybe be better documented, as evidenced by this thread.
