I've been using a code that pass -U__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ during compilation. The code was compiling well for a while. I haven't been using that project for a while, but recently found out that my project start failing to build with errors: In file included from my_project/main_app.cpp:52: In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/mach/mach_time.h:32: In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/mach/mach_types.h:109: In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/mach/vm_region.h:47: In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/mach/machine/vm_param.h:33: In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/mach/i386/vm_param.h:97: /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/mach/vm_page_size.h:59:44: error: expected ';' after top level declarator extern vm_size_t vm_kernel_page_size __OSX_AVAILABLE_STARTING(__MAC_10_9, __IPHONE_7_0); My /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/Availability.h file contains #ifdef __IPHONE_OS_VERSION_MIN_REQUIRED ... #elif defined(__MAC_OS_X_VERSION_MIN_REQUIRED) ... #else #define __OSX_AVAILABLE_STARTING(_osx, _ios) #define __OSX_AVAILABLE_BUT_DEPRECATED(_osxIntro, _osxDep, _iosIntro, _iosDep) #define __OSX_AVAILABLE_BUT_DEPRECATED_MSG(_osxIntro, _osxDep, _iosIntro, _iosDep, _msg) #endif but the last #else case is never hit because /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/AvailabilityInternal.h contains: #ifndef __MAC_OS_X_VERSION_MIN_REQUIRED #if defined(__has_builtin) && __has_builtin(__is_target_os) #if __is_target_os(macos) #define __MAC_OS_X_VERSION_MIN_REQUIRED __ENVIRONMENT_OS_VERSION_MIN_REQUIRED__ #define __MAC_OS_X_VERSION_MAX_ALLOWED __MAC_14_2 #endif #elif __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ #define __MAC_OS_X_VERSION_MIN_REQUIRED __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ #define __MAC_OS_X_VERSION_MAX_ALLOWED __MAC_14_2 #endif /* __has_builtin(__is_target_os) && __is_target_os(macos) */ #endif /* __MAC_OS_X_VERSION_MIN_REQUIRED */ So in combination with my initial -U__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ option the #define __MAC_OS_X_VERSION_MIN_REQUIRED __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ produce an empty define. So the question to apple developers: could you please update #elif __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ to #elif defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)? Or even change #ifndef __MAC_OS_X_VERSION_MIN_REQUIRED to #if !defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)? Otherwise the #else case in Availability.h described above does not make any sense. Best regards, Petro