I just received a used copy of Canon's 16mm f/2.8 STM lens and here are some of its first images.
This lens has a weak lower left corner at apertures from wide open to f/8 (significantly better at f/8), but surprisingly sharp large central area, even wide open. Color and contrast are very good from f/2.8. LRc used for PP.
Certainly a great value for the used purchase price! 
Canon EOS R RF16mm F2.8 STM lens 16mm f/2.8 1/1250s 100 ISO 0.0 EV
Canon EOS R RF16mm F2.8 STM lens 16mm f/2.8 1/1600s 100 ISO +0.7 EV
Canon EOS R RF16mm F2.8 STM lens 16mm f/5.6 1/160s 100 ISO +1.0 EV
Canon EOS R RF16mm F2.8 STM lens 16mm f/8.0 1/125s 100 ISO +0.3 EV
Canon EOS R RF16mm F2.8 STM lens 16mm f/8.0 1/160s 100 ISO +0.3 EV