gcc/testsuite/gcc.dg/cpp/warn-undef-2.c

   1 // { dg-do preprocess }
   2 // { dg-options "-std=gnu99 -fdiagnostics-show-option -Werror=undef" }
   3
   4 #if x  // { dg-error "\"x\" is not defined .-Wundef." }
   5 #endif