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

// { dg-do preprocess }
// { dg-options "-std=gnu99 -fdiagnostics-show-option -Werror=undef" }
/* { dg-message "some warnings being treated as errors" "" {target "*-*-*"} 0 } */
#if x  // { dg-error "\"x\" is not defined .-Werror=undef." }
#endif