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

// { dg-do preprocess }
// { dg-options "-std=gnu99 -fdiagnostics-show-option -Wtrigraphs" }

??=  // { dg-warning "trigraph \\?\\?= ignored, use -trigraphs to enable .-Wtrigraphs." }