DerivedSources.make in WebCore and WebKit2 still specify -std=gnu++11 instead of -std=gnu++14. These commands are only run for "ifeq ($(OS),MACOS)", so it won't affect Windows or other ports.
Created attachment 307753 [details] Patch
Comment on attachment 307753 [details] Patch LGTM.
Comment on attachment 307753 [details] Patch Tests pass, so cq+
Comment on attachment 307753 [details] Patch Clearing flags on attachment: 307753 Committed r215629: <http://trac.webkit.org/changeset/215629>
All reviewed patches have been landed. Closing bug.