Created attachment 370082 [details]
reproducer

With glibc 2.26, the attached test program that exercises Posix.major() and minor() builds with a warning, but still works:

$ valac --pkg posix -o /tmp/out major.vala 
/home/martin/src/test/vala/major.vala.c: In Funktion »_vala_main«:
/home/martin/src/test/vala/major.vala.c:43:13: Warnung: In the GNU C Library, "major" is defined
 by <sys/sysmacros.h>. For historical compatibility, it is
 currently defined by <sys/types.h> as well, but we plan to
 remove this soon. To use "major", include <sys/sysmacros.h>
 directly. If you did not intend to use a system-defined macro
 "major", you should undefine it after including <sys/types.h>.
  _tmp6_ = major (_tmp5_);
             ^~~~~~~~~~~~~                                                                                                                                                                                                                                                                                                                                                           
/home/martin/src/test/vala/major.vala.c:46:13: Warnung: In the GNU C Library, "minor" is defined
 by <sys/sysmacros.h>. For historical compatibility, it is
 currently defined by <sys/types.h> as well, but we plan to
 remove this soon. To use "minor", include <sys/sysmacros.h>
 directly. If you did not intend to use a system-defined macro
 "minor", you should undefine it after including <sys/types.h>.
  _tmp9_ = minor (_tmp8_);
             ^~~~~~~~~~~~~                                            

But with glibc 2.27 (as in Fedora Rawhide) it does not work any more, as sys/types.h is now not sufficient any more:

$ valac --pkg posix -o /tmp/out major.vala
/home/martin/src/test/vala/major.vala.c: In function ‘_vala_main’:
/home/martin/src/test/vala/major.vala.c:48:45: warning: implicit declaration of function ‘major’ [-Wimplicit-function-declaration]
  fprintf (_tmp3_, "major: %u, minor: %u\n", major (_tmp5_), minor (_tmp7_));
                                             ^~~~~
/home/martin/src/test/vala/major.vala.c:48:61: warning: implicit declaration of function ‘minor’; did you mean ‘mknod’? [-Wimplicit-function-declaration]
  fprintf (_tmp3_, "major: %u, minor: %u\n", major (_tmp5_), minor (_tmp7_));
                                                             ^~~~~
                                                             mknod
/tmp/ccZtOWCJ.o: In function `_vala_main':
major.vala.c:(.text+0x37a): undefined reference to `minor'
major.vala.c:(.text+0x38d): undefined reference to `major'
collect2: error: ld returned 1 exit status