Canon EF 40mm f/2.8 STM
Below are some comparison MTF charts of the 50 f/1.8 II, 50 f/1.4 & 40 f/2.8 STM.
The lens looks pretty decent in the center, and tails off a bit in the corners. However, it looks better than both of the current non L 50′s in the Canon lineup.
I also dig the moniker “Shorty Forty” for the new pancake. Taemobig on our forum may or may not have come up with the name.