I think we should have this, see this issue for a description of what those are and my argument for why it should be in stdlib.
These are currently used in ReverseDiff.jl. See my simulator of a MOS 6502 CPU for another fun application. You will find that you need them every time you need to efficiently unbox a function (e.g. if you hold functions in a container), which seems like a fairly generic use case to me.
Opening this thread is nothing other than redundant, shameless promotion. Hopefully you’ll all forgive me for abusing discourse slightly .