This question already has an answer here:
Assume a player with two weapons: a whip and a dagger.
A hostile creature stands right next to the player. On the creature's turn, it moves away 5ft. So the creature left the reach of the dagger, but is still in the reach of the whip.
Does this movement provoke an opportunity attack?
I am interested in an RAW answer, not in speculation about what a (any) sensible DM would rule.
Note: Assume Mearls' interpretation of reach provoking OAs in general, as discussed here.