It looks like the enabling of debugging code in clang/llvm was flipped around during the stable/12 branch. See the definition of NDEBUG in lib/clang/llvm.build.mk line 20: current before r339436: #CFLAGS+= -DNDEBUG after r339436: CFLAGS+= -DNDEBUG stable/12 at r339435: #CFLAGS+= -DNDEBUG i.e. debugging code is now disabled in current and enabled in stable/12.