There are many spells which explicitly target creatures:
Choose one creature within range
The target creature is normally supposed to make a saving throw in this case.
What happens when the caster chooses a creature, that is actually an object? For instance, a scarecrow, a mannequin, an illusion, or a corpse.
Examples:
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An evil mage is hiding in the bushes. He casts an illusion of himself on the road, using the Silent Image spell. A PC sorcerer walks down the road, sees the illusion and casts Magic Missile on it.
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A cleric is standing near a pile of dead bodies. He has suspicions that a few corpses are actually undead creatures. He casts Sacred Flame on every corpse, see which if them succeed, hence, reveals the undead.
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A wizard casts Acid Splash on a mannequin, trying to damage it with acid.
Possible outcomes I can think of are:
- The caster cannot even try to cast the spell, regardless of their information about what the target actually is.
- The caster tries to cast the spell, the casting fails, not expending the spell slot.
- The caster casts the spell, it expends the spell slot, then fails.
- The caster casts the spell, but it hits the original and not the illusion.
- The caster casts the spell, it flies to the target illusion but deals no damage to it (passes through it).
References in the rules and Sage advice
I was trying to investigate, but it didn't make things clearer:
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Does magic missile hit silent image's illusory creature? has an answer, that Magic Missile won't hit. It says nothing about if it can be targeted at all, it leaves it up to the DM.
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Magic Missile automatically finds the original, even when the caster targets an illusion. At least when the illusion was created by the Mirror Image spell. Now confirmed by Jeremy Crawford. This supports the counter-intuitive option 4.
on the other hand
- Casters don't automatically know when a spell fails, if there were no perceivable effects (let's say you can try to cast the Command spell on an illusion). That assumes that casters can waste their spells on illusion, which means they at least can target it. This supports options 3 and 5.
Best Answer
There are 2 ways to resolve this that have been discussed by WOTC designers in an official capacity: An older option on a Sage Advice segment of one of their Dragon Talk podcasts from January 2017 (specifically at about 13:12 for ~7.5 minutes), and a newer optional rule provided by Xanathar's Guide to Everything's Dungeon Master's Tools chapter.
Sage Advice: Intent is that spell effect doesn't happen, action cost applies, and DM judges whether spell slot is used
To summarize Jeremy Crawford's statements in the January 2017 podcast, "illegal targeting" is a gap in the written rules (as of the date of the podcast) and it's mostly open to DMs to choose how to handle it. That said, Crawford says the intent for how it should be handled is that the spell should still take up the casting time but the spell effect will not occur and not consume a spell slot (I.E., option 2 listed in the question).
There are enough corner cases with this solution at the time of the podcast that Crawford still recommends that a DM adjudicate each individual occurrence on a case-by-case basis until there is eventually an official printed rule. As an example of why, he says spells such as those which require a spell attack probably should still consume their spell slot since there'd be some dissonance with the fact that those spells can miss, unlike saving throw spells which always "hit" but the target can resist their effects.
(He does not clarify what should happen if something like Eldritch Blast, which targets only creatures, actually hits a non-creature in this case.)
The flavor reason for this is that he views spells as essentially trying to make a magical connection between the caster and target. When that connection is established, the energy of the spell is consumed in producing the spell's effect - but if the target isn't one the spell can make a connection to, nothing happens and that energy isn't expended.
He views spell attack spells as a different category; if I had to guess why, it's likely because they mainly produce some effect that then follows standard attack rules in trying to reach the target.
Xanathar's Guide to Everything (optional): No spell effect, action cost applies, and spell slot is expended
As of Xanathar's Guide to Everything's release in November 2017, the (optional) rule for resolving invalid spell targets states (p. 85-86):
This is in slight contrast to Crawford's earlier statements on the topic above, in that the spell still occurs and consumes a spell slot with no apparent effect.