I'd be curious if you can remember what differences there were. It's compatible with all C99 and C11 syntax I've tried
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.
Could be. My obj-c days are long over though. The time I worked with was between 2009 and 2015.