The Unearthed Arcana Paladin class ability "Channel Divinity: Guided Strike" says:
Guided Strike. You can use your Channel Divinity to strike with supernatural accuracy. When you make an attack roll, you can use your Channel Divinity to gain a +10 bonus to the roll. You make this choice after you see the roll, but before the DM says whether the attack hits or misses.
When a paladin strikes an opponent who is able to cast Shield, who has to announce first whether Guided Strike/Shield is used? I am assuming I cannot wait until the DM announces whether he uses Shield or not to announce my own Guided Strike?