What does a player learn when their spell fails for a hidden reason?

Question

(this is effectively this Q&A, but for 5th edition)

Sometimes, a player may try to cast a spell on something, or someone, that is not a valid target or cannot be affected by the spell. Whether it be scrying on a dead creature, targeting an object with a creature-bound spell or being unaware of an immunity (although this last case might be a bit of an outlier), the common consensus seems to be that the spell can be cast, but does not take effect. Whether or not this takes a spell slot still seems to be a case-by-case issue.

However, this brings an important question: what does the character learn about this? Obviously they will most likely notice that the spell didn't work, but how much they learn about this spell failing could drastically change how they continue playing.

If the player thinks they only had a bad roll of the dice, they might keep on trying to use similar spells, when in reality it will never work.

If the player is told that the spell failed because of an invalid target, for example, they might obtain information they weren't supposed to know. Taking the first example of scrying, they could learn that their target is dead or not on the same plane anymore, which could change the direction of the campaign or spoil a key plot element earlier than expected.

The key idea being: information is power, and a lack of information is information itself. So, what does a player learn when their spell fails for a reason they aren't aware of?

Answers that use the Rules As Written are preferred, although any relevant experience on this kind of situation and how it was handled at the table would be greatly appreciated.

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The closest I could find to an existing answer was this answer where the following statements are made:

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.

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 I understand it, this seems to imply that there is no rules regarding this situation and that DMs will have to rule each situation individually.

Answer 1

You only perceive the spell did nothing; if there's a save, you think the target succeeded on the save

By the core rules, this is not specified, so the DM must decide how to handle each case.

Xanathar's Guide to Everything however offers the following optional rule (p. 85f) under the heading Invalid Spell Targets:

A spell specifies what a caster can target with it: any type of creature, a creature of a certain type (humanoid or beast, for instance), an object, an area, the caster, or something else. But what happens if a spell targets something that isn't a valid target? For example, someone might cast charm person on a creature believed to be a humanoid, not knowing that the target is in fact a vampire. If this issue comes up, handle it using the following rule.

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.

As Xanathar's is published later and is often seen as updated rules that close gaps in the core rules, and contains official rules, the closest Rules-As-Written interpretation is that if the spell has a saving throw, the caster thinks the save succeeded, and if it doesn't you perceive that the spell did nothing — so if if there is no perceptible effect, you will not even know that. In either case, the spell slot is lost.