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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:

  • 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.

  • 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.

  • A wizard casts Acid Splash on a mannequin, trying to damage it with acid.

Possible outcomes I can think of are:

  1. The caster cannot even try to cast the spell, regardless of their information about what the target actually is.
  2. The caster tries to cast the spell, the casting fails, not expending the spell slot.
  3. The caster casts the spell, it expends the spell slot, then fails.
  4. The caster casts the spell, but it hits the original and not the illusion.
  5. 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:

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.
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    \$\begingroup\$ I think 'Magic Missile' is a special case because the missiles seek out the chosen target rather than the caster having to aim them (or, in functional terms, because there is no attack roll) \$\endgroup\$ – DaveMongoose Oct 3 '17 at 15:59
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There are 2 ways to resolve this that have been discussed by WOTC designers in an official capacity: One older option on a Sage Advice segment of one of their Dragon Talk podcasts here (specifically at about 13:12 for ~7.5 minutes), and one newer one in an optional rule provided by Xanathar's Guide to Everything's Dungeon Master's Tools chapter.

The Sage Advice Way

To summarize Jeremy Crawford's statements in the older 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, there is an intent for how it should be handled, which is the cast should still take up the casting time but the spell will not occur and not consume a spell slot (I.E., option 2 listed in the question).

There are enough corner cases with this at the moment that Crawford still recommends that a DM adjudicates each individual occurrence 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 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, and 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's Way

As of Xanathar's Guide to Everything's release, the (optional) rule for resolving invalid spell targeting is as follows:

If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn’t attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

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.

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    \$\begingroup\$ I find this ruling in conflict with the rules about "readying a spell". Which implies that a spell slot is consumed even when the trigger does not occur. Which first implies that the effect of the spell is not relevant for the spell slot and second with the mentioned ruling it would mean that when an illegal target triggers your readied spell you would get your spell slot back. \$\endgroup\$ – Thyzer Mar 13 '17 at 18:23
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    \$\begingroup\$ Player: "I cast 'detect mimic' on the tavern table." DM: "What?" Player: "It is a ray of frost, but it will only work if the table is a mimic." \$\endgroup\$ – Mindwin Nov 12 '17 at 5:20
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This came up in a recent conversation on a forum about using spells like Sacred Flame as a way of determining if statues were really gargoyles or something like that.

As I recall, the rules do not have a terribly satisfying answer to this. The spells are written the way they are to avoid awkward situations like a continuous stream of acid splash cantrips burning their way through the wall of a castle or something. In practice though, most people seem to find the idea of characters using metagame knowledge like "it only targets creatures" in order to deduce other information as distasteful at best.

The few times I've seen it come up in play DMs have basically handwaved it away, letting the approximate effect take place even if you targeted an object instead of a creature, but not allowing characters to take it to absurd lengths like burning their way through a castle wall with a cantrip.

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  • \$\begingroup\$ In the case of cantrips, I could see an argument that the caster has had so much practice and experience with the cantrip (that's the fluff for why they're at-will, after all) that they'd know what it works on and what it doesn't. \$\endgroup\$ – Pilchard123 Oct 26 '18 at 14:37
  • \$\begingroup\$ Sure, but that's assuming that the character has any way at all to know if it would work or not. If you can't tell the difference between a statue and a gargoyle other than the fact that Eldritch Blast can't target a statue, it's hard to justify it by saying "the caster knows it won't work". \$\endgroup\$ – Garrett Rooney Oct 27 '18 at 18:19
  • \$\begingroup\$ I meant more in the sense that they would try to cast it and it may fail to cast correctly. If the target gets the magical equivalent of high-velocity lead poisoning, great. If it fails, then the target was not a valid one. (Or there was some other reason it failed, like a counterspell or anti-magic field. Plot fuel!) \$\endgroup\$ – Pilchard123 Oct 27 '18 at 20:06
  • \$\begingroup\$ Yes, that's the point. It's pretty clear that by introducing limits on what you can target the designers didn't intend to turn 3/4 of the cantrips in the game into a "tell if this thing is a creature" spell. \$\endgroup\$ – Garrett Rooney Oct 28 '18 at 21:04
  • \$\begingroup\$ Sure, and I agree with you. I'm just not certain that in games where an invalid target does stop a spell being cast "this cantrip only works on creatures" is necessarily metagaming because the caster knows the spell so well and that's the rules of the world the caster lives in. Certain spells only work on non-undead creatures - after realising that they don't work, the character wouldn't try any more. In an invalid-target-stops-cast world, perhaps the caster would know that some cantrip "requires a connection to the life-force of it's target" or some such fluff. 1/2 \$\endgroup\$ – Pilchard123 Oct 29 '18 at 11:43
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If a spell has restrictions on what kinds of targets it can have, this tells you which kinds of targets it can affect, and it has no effect on targets who don't meet this restriction. For instance, Charm Person specifies a Humanoid target. You can cast Charm Person on a Beast creature, or on a door, and it will have zero effect on either of these targets, but will still expend the spell slot and will be cast normally. It only has the effect described if you cast it on a Humanoid target.

Similarly, Eldritch Blast specifies it can only target creatures. This suggests that if a beam from Eldritch Blast hits an object, it will not affect it or deal any damage, and the eldritch energy will simply dissipate harmlessly. But you can still aim the spell at said object, intentionally; there's nothing preventing you from doing that.

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