gcc/testsuite/gcc.dg/asm-1.c

/* { dg-do compile { target i?86-*-* } } */

struct x {
  int selector;
};

void f()
{
  struct x* addr;
  __asm__("mov %%gs,%0":"=r" ((unsigned short)addr->selector)); /* { dg-error "lvalue" "casts make rvalues" } */
}