Fails to build with Vala 0.43.6
Using Vala 0.43.6, I see a lot of compile errors of that type:
src/engine/api/geary-folder.vala:476.41-476.71: error: Synchronous out-parameters are not supported in async methods
BUILDSTDERR: out Geary.EmailIdentifier? low, out Geary.EmailIdentifier? high,
BUILDSTDERR: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
See https://kojipkgs.fedoraproject.org//work/tasks/6418/32386418/build.log.