Using the latest version of Segger Embedded Studio which supports SAMD21 CMSIS out the box. I can see that samd21.h is importing samd21g18au.h which is correct.
EIC->INTFLAG.reg = 0xFFFF;
'cannot compile this l-value expression yet'
expanded from macro 'EIC'
#define EIC ((Eic *)0x40001800UL) /**< \brief (EIC) APB Base Address */
If I'm typing it finds EIC->INTFLAG but not the .reg part.
Any ideas where to start?