How to determine the target(s) of Catapult?
In the D&D 5th Edition Player's Handbook Spellcasting section, under Casting a Spell, we find a loose definition of "target" for spells.
A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect [...]
Knowing what a spell targets, or at least having a way to consistently adjudicate edge cases is important. Not only for adjudicating the effects of the spell, but also for interaction with other game mechanics, such as the sorcerer's Twin Spell Metamagic feature, which requires a spell to be incapable of targeting more than one creature, or the charmed condition, which prevents a creature from targeting the charmer with harmful abilities or magical effects.
The relevant sections from the description of catapult seem to be as follows:
Choose one object weighing 1 to 5 pounds within range that isn't being worn or carried. The object flies in a straight line up to 90 feet in a direction you choose[...]
[...] If the object would strike a creature, that creature must make a Dexterity saving throw. On a failed save, the object strikes the target and stops moving. [...]
Given that a creature that fails the Dexterity saving throw is referred to as a target specifically, that seems pretty clear. But what about a creature who succeeds? Should that creature be considered a target? What about a creature who never makes a saving throw because the object stopped before it reached them?
If a Mage catapults an object toward Alice, Bob and Charlie, who are standing in a straight line, as shown:
Then Alice succeeds on her saving throw, Bob fails his and the object stops, so Charlie never makes one. Are Alice and Bob both targets of the spell? Just Bob? Alice, Bob and Charlie?
Would it make a difference if the Mage is charmed by Charlie?