The different impls are all guarded by cfg-flags, and the revisions could be
used to cover the full power-set of combinations.
(I only included 20 of the possible 32 cases here; the null-set is not
interesting, and the remaining 11 all yielded ambiguous method resolution errors
which did not mix well with this testing strategy; I'm not trying to check UI
for the resolution diagnostics; I'm trying to create checkpoint of current
resolution semantics when compilation succeeds.)
