> But why put in unreachable? Doesn't make any sense to me.
Because sometimes you don't have a choice e.g. say you have a switch/case, if you don't do anything and none of the cases match, then it's equivalent to having an empty `default`. But you may want a `default: unreachable()` instead, to tell the compiler that it needs no fallback.
> If a branch is truly not supposed to ever happen, why have a branch at all? Just remove that code from the source entirely- that helps the optimizer even more, because the most optimal code is of course no code at all.
Except the compiler may compile code with the assumption that it needs to handle edge cases you "know" are not valid. By providing these branches-which-are-not, you're giving the compiler more data to work with. That extra data might turn out to be useless, but it might not.
Because sometimes you don't have a choice e.g. say you have a switch/case, if you don't do anything and none of the cases match, then it's equivalent to having an empty `default`. But you may want a `default: unreachable()` instead, to tell the compiler that it needs no fallback.
> If a branch is truly not supposed to ever happen, why have a branch at all? Just remove that code from the source entirely- that helps the optimizer even more, because the most optimal code is of course no code at all.
Except the compiler may compile code with the assumption that it needs to handle edge cases you "know" are not valid. By providing these branches-which-are-not, you're giving the compiler more data to work with. That extra data might turn out to be useless, but it might not.