CNet reports that TCL was supposed to demonstrate a novel smartphone design next week (at the cancelled MWC event) that uses an "expandable" sliding OLED display. The idea is basically the same as in the first crop of foldable OLEDs - provide a phone that can open up to a tablet-size device:
Apparently TCL was supposed to show a working demonstration. Such a design will likely require a screen that is folded in two places or some sort of rollable display - which is possible but more difficult to actually commercialize compared to a screen that folds inwardly or outwardly in a single position.
TCL has been planning foldable OLED smartphone prototypes for 2020, but these hasn't been officially launched yet. The company has demonstrated several foldable OLED prototypes - in addition to rollable displays as well (this was shown by the display arm of TCL, rather than the device business unit).