test.cpp

/* check that exp(i * pi) == -1 */
#include <math.h>        /* for atan */
#include <stdio.h>
#include <complex.h>
#undef complex // complex.h has a macro define for "_Complex" -> "complex" which will break c++ compilation

int
main(void)
{
   double pi = 4 * atan(1.0);
   double _Complex z = cexp(I * pi); // this will error, see this: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59087 (suggestion to avoid the use of "I" in code)
   printf("%f + %f * i\n", creal(z), cimag(z));
}

/*
$ clang++ --version
Apple LLVM version 7.0.0 (clang-700.0.72)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

$ clang++ test.cpp -o test_example
test.cpp:11:29: error: use of undeclared identifier 'I'
   double _Complex z = cexp(I * pi);
                            ^
1 error generated.

*/