Current webkit2gtk packages (such as libjavascriptcoregtk-4.0-18)
depends on libc++1-16, which conflicts against libc++1-11 and
libc++-dev is uninstallable.

$ sudo apt install libjavascriptcoregtk-4.0-18 libc++-dev
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
libc++-dev is already the newest version (1:11.0-51+nmu5).
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libc++-dev : Depends: libc++-11-dev (>= 11~) but it is not going to
be installed
E: Unable to correct problems, you have held broken packages.

(Alternative idea would be having llvm-defaults/bullseye-backports
pointing to llvm-toolchain-16)

Hi,

I'm afraid we cannot bump the llvm-defaults version, as that would imply
changing the build for several packages, which could cause build failures or
worse runtime issues.

Why is this an issue for you? Are you trying to build software that requires
libc++-dev ? Perhaps you could do so inside a chroot, or maybe locally build
llvm-defaults pointing to llvm 16 and use that, depending on what you're trying
to do.

Ideally libc++-16-dev wouldn't conflict with libc++-11-dev et al, but I see it
ships the unversioned library in /usr/lib/<triplet>, just like the rest of the
packages, hence the conflict. I'm not sure if that's necessary given that rdeps
can just gain a RPATH, like webkit did, but that's a change that should happen
in sid (if at all), so wouldn't solve your issue.

Cheers,
Emilio