Using 45W Power Adapter with MacBook Pro Retina 15
It will charge. However, it will do so very slowly.
I am using the original apple 45W charger from my old MB Air with the magsafe 2 adapter on my MBP 2016 and I haven't had any problems in the last 2.5 years.
I've been using the MacBook Air 45W power adapter with my MacBook Pro Retina 15" and it works perfectly. Charging takes a little longer than with the original power adapter. I recommend the 45W adapter for travel.