Comment 2 for bug 1819187

I'm afraid you're right: http://c0x.coding-guidelines.com/6.5.7.html#1185

None the less I think this behavior is pretty dangerous without a warning, as I have seen this kind of stuff pretty often.
From the STM32-Hal where I got this code from:

  /* Bits of channels in single or differential mode are set only for */
  /* differential mode (for single mode, mask of bits allowed to be set is */
  /* shifted out of range of bits of channels in single or differential mode. */

As this is a gcc issue and not arm-gcc (the behavior can be seen with x86, too), I discuss that there.