It‘s a superset of ANSI-C. One needs to be carful to mention this. I‘m not aware that this had changed and also added some small grieve back in the days for me. But I sadly can‘t remember anymore what didn‘t work for me.
I think it was specifically in .m files. Obj-c is using ANSI-C (c89) which lacks certain features and keywords. Mixing with c99 files is of course possible. Maybe that was also an issue in older versions of clang.
I haven't run into such issues intermingling C11 features in a .m file. Possibly you just needed to set the right compiler flags, or it was a bug in older versions of clang.
