After blaming the failed test code, it appears the src file blame is messed up on GH and theactual PR which introduced thisdoes have tests + proper motivation for it. Now I'm not sure what we should do with this functionality. Deprecating it wouldn't get rid of the ambiguities, but it would set us up for removing it along with possibly more of the implicit grads machinery in the future (if we ever get around to that). Adding the suggested methods in#1484 would work, but feels rather unsatisfying and a little hacky. Maybe we could avoid ambiguities by restricting the list of broadcastable functions?

I wishhttps://docs.julialang.org/en/v1/manual/interfaces/#man-interfaces-broadcasting would be a little clearer on how to implement the interface, especially since this is Base's own internal overloads conflicting with us here. WDYT@CarloLucibello@darsnack?