It seems that the default value for VALA_MAX_API_VERSION is still on 0.36 , this causes old vala version being pulled in (for example by app-i18n/ibus). Please update to latest API version (0.42), ideally maybe always automatically set that to the latest available version, individual ebuilds could then set VALA_MAX_API_VERSION if it is not compatible with the latest vala. Reproducible: Always
Ah crap, forget it ... I seem to be blind... it is at 0.44 already
ibus needs fixes to succeed build with newer vala. The relevant bug was closed after restricting the max vala per-ebuild; it still needs a patch or upstream fix release to work with newer vala.