The iPhone X is said to feature a fingerprint sensor hidden under the OLED screen. "Apple may switch to Synaptec's optical-based fingerprint reader for the new Touch ID sensor, citing it as "currently the only workable solution" for detecting a fingerprint through a smartphone screen."
Just released Yole Developpement report "Fingerprint Sensor Applications and Technologies – Consumer Market Focus" too points to the optical sensing as one of contenders for under the screen fingreprint devices:
Thanks to AM for the link!