This package fails to build on an unstable system. Attached relevant logs and emerge --info
Created attachment 784484 [details] emerge_info emerge --info
Created attachment 784487 [details] subversion-1.14.2-r1:20220613-010617.log.xz build log (compressed because it exceeds attachment limit, use 'xzless' to read it)
It uses sed to remove -Wunused and it mangles -Wunused-value
That looks like a clear upstream subversion problem... Could you please also file a bug there, and add here a link to it? Thanks!