Patches 1, 2 and 3 are logically nothing to do with this feature. I'll wait for them to be reviewed so that we only have a single-patch series, before doing final review of the main patch.
Since the feature was accepted as _Lengthof, that's the form that should be added to GCC; no __lengthof__ variant needed. In general in GCC, although not strictly required by the standard in this case, we use pedwarn_c23 (pass OPT_Wpedantic as the option) to diagnose the use of a new C2Y feature that's not in C23 (if -pedantic with a pre-C2Y standard, or -Wc23-c2y-compat even in C2Y mode), with appropriate testcases to verify this (error with -std=c23 -pedantic-errors, warning with -std=c23 -pedantic, no diagnostic with -std=c23 -pedantic-errors -Wno-c23-c2y-compat, no diagnostic with -std=c2y -pedantic-errors, warning with -std=c2y -pedantic-errors -Wc23-c2y-compat). (pedwarn_c23 handles that logic, you just need the pedwarn_c23 call and the tests for those various cases.) -- Joseph S. Myers josmy...@redhat.com
