Yeah, some things not very clear...

Untested.

Sigh...

Next commit needs this.  I never fully get this, sigh.

Not needed anymore.  We just use operator= now.

Now that we require C++11, no need to macro.

Not planning on using it.  So remove.  Can add later if needed.

Every time I have to study these to understand why a change is right..

Fuzzer caught it:

==14==ERROR: AddressSanitizer: stack-use-after-return on address 0x7fca2ed7a3e0 at pc 0x0000006057aa bp 0x7ffc3290f1d0 sp 0x7ffc3290f1c8
READ of size 4 at 0x7fca2ed7a3e0 thread T0
SCARINESS: 55 (4-byte-read-stack-use-after-return)
    #0 0x6057a9 in OT::SingleSubstFormat2::subset(hb_subset_context_t*) const /src/harfbuzz/src/./hb-ot-layout-gsub-table.hh:194:40
    #1 0x5ff921 in hb_subset_context_t::return_t OT::SingleSubst::dispatch<hb_subset_context_t>(hb_subset_context_t*) const /src/harfbuzz/src/./hb-ot-layout-gsub-table.hh:256:13

I can't reproduce locally, but many of the bots are failing because of this
as well.

It's a pity that operator-> must return pointer.  Ugh.  Why?!

Not sure why I don't get it, but this warning:

warning: base class ‘struct hb_iter_fallback_mixin_t<hb_array_t<const OT::UVSMapping>, const OT::UVSMapping&>’ should be explicitly initialized in the copy constructor [-Wextra]

Using decltype() instead.