gcc/testsuite/gcc.dg/cpp/c2x-scope-1.c

/* Test :: token in C2x.  */
/* { dg-do preprocess } */
/* { dg-options "-std=c2x -pedantic-errors" } */

#define CONCAT(x, y) x ## y

CONCAT (:, :)
CONCAT (::, >) /* { dg-error "does not give a valid preprocessing token" } */