Tags: buster sid
synapse fails to build from source with vala 0.36 which is currently
in experimental and will be uploaded to unstable soon.
You can find a build log from Ubuntu 17.10 Alpha at
There were a few commits on the upstream bzr branch to fix some of the
vala 0.36 issues but I don't think all were fixed.
On behalf of the Debian Vala maintainers,