https://github.com/googlefonts/harfbuzz/commit/9f610ae239a11e86f94621e26bc15849b65ce41b#commitcomment-33944686

To check for assignment overflows.

Let's see if bots happy.

Not sure where else we can use these.  Mm.  Maybe in hb_invoke().

If they're not, it's a bug in our subsetting logic somewhere.  So check.

Calls obj.copy() or obj.operator=() in that order.

Tested, but feels fragile :(.

Ugh.  Need to think of something else.

Rarely used, but used, in name table or similar constructs.