#ifdef __clang__
#  define FALLTHROUGH [[fallthrough]]
#  define __builtin_fallthrough() (void)0
#else
#  define FALLTHROUGH __builtin_fallthrough ()
#endif

void f0 (void)
{
  // This would be an invalid [[fallthrough]] and GCC rejects it
  // as expected with:
  //   error: invalid use of '__builtin_fallthrough ()'
  FALLTHROUGH;

  {
    // This would be invalid [[fallthrough]] but is silently accepted 
    // by GCC.
    0 ? __builtin_fallthrough () : (void)0;
  }
  {
    switch (0)
      // This is invalid with [[fallthrough]] because there's no
      // preceding statement within the switch statement, and no
      // subsequent one.  Clang rejects it but GCC silently accepts
      // it (it issues -Wswitch-unreachable).
      FALLTHROUGH;
  }

  {
    // This is invalid but GCC gives:
    //   warning: not preceding a label
    // It seems that it should give an error like in the first case
    // above.  I suspect it's confused by the preceding switch.
    // Clang gives an error.
    FALLTHROUGH;
  }
}

void f1 (void)
{
  {
    switch (0)
      // This also seems invalid because there is no preceding
      // statement and no subsequent case-labeled statement but
      // GCC silently accepts it (as does Clang).  It seems that
      // GCC gets tricked by the A label.
      default: FALLTHROUGH; A: ;
  }

  {
    // Unlike the same block at the end of f0 this is an error
    // as it should be`.
    FALLTHROUGH;
  }
}

void f2 (int i)
{
  {
    switch (i)
    {
      case 0:
        // Both of the following are invalid with [[fallthrough]]
        // because neither is followed by a case-labeled statement.
        // GCC only only rejects the second of the two.  Clang
        // accepts both.
        FALLTHROUGH;
        FALLTHROUGH;
    }
  }
}

// The following function triggers an ICE.
void f3 (void)
{
  {
    // Invalid but without the ICE silently accepted by GCC.
    int i = 0;
    if (i) __builtin_fallthrough ();
  }
  {
    switch (0)
      __builtin_fallthrough ();
  }

  __builtin_fallthrough ();
}