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An example might be a car that is fast, luxurious, reliable, gets great gas mileage, and is very cheap. Clearly we'd all love to own such a car, but it doesn't exist, and probably never will. There's also some naiveté in assuming it would.

Is there a metaphor, idiom, or other phrase that connotes this?

"Gee, that sounds like a wonderful car, but unfortunately, it's a _____."


UPDATE:

With regard to the proposed dupe - that question has the same accepted answer, but it's not the same question. Two different questions can have the same answer. Please let me know if you disagree.

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4  
    
it's a mirage or it's too good to be true. – Graffito Feb 2 at 23:45
    
Santa Claus... – Drew Feb 3 at 2:30
2  
Another possible dupe: english.stackexchange.com/questions/62299/… – ermanen Feb 3 at 3:07
4  
"Two different questions can have the same answer. Please let me know if you disagree." The standard on most Stack Exchange sites isn't whether the questions are the same, but rather whether an answer to the other question precisely answers this question. If so, the question is closed as a duplicate. I don't know the standards here at ELU, but that's the usual one on SE sites. – T.J. Crowder Feb 4 at 12:34
up vote 74 down vote accepted

It's just a...

pipe dream - an unattainable or fanciful hope or scheme
Example usage from oxforddictionaries:
free trade in international aviation will remain a pipe dream

Origin: Late 19th century: referring to a dream experienced when smoking an opium pipe.

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You may say it is just wishful thinking:

  • Thinking in which what one wishes were the case is believed to be real or likely to become real.

(American Heritage® Dictionary of the English Language)

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My favourite:

Pie in the sky

"...an idea, thought or dream that is extremely unrealistic, even to the point where it begins to seem ludicrous."

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This was my first thought. – delliottg Feb 3 at 18:59

Fantasy

  1. The faculty or activity of imagining things, especially things that are impossible or improbable.

    • the product of imagining impossible or improbable things.

    • a fanciful mental image, typically one on which a person dwells at length or repeatedly and which reflects their conscious or unconscious wishes.

While "fantasy" doesn't always describe something a person wants to be true, it often does. Context can make it clear.

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An apt answer to the OP, but it needs a link. – ab2 Feb 2 at 23:34
    
@ab2 link added – Jessa Feb 2 at 23:45
    
+1 for an answer with a link. – ab2 Feb 2 at 23:48

Consider chimera

A thing that is hoped or wished for but in fact is illusory or impossible to achieve: the economic sovereignty you claim to defend is a chimera

It is derived from the Chimera of Greek mythology, a fire-breathing female monster with a lion’s head, a goat’s body, and a serpent’s tail.

Oxford Dictionaries Online

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Unfortunately that dictionary has mangled the description of the beast, and the connotation I derive has more to two with having three heads and being dangerous than being impossible, s – Ben Voigt Feb 2 at 23:24
    
@BenVoigt Wikipedia claims that Homer's is the earliest description: a thing of immortal make, not human, lion-fronted and snake behind, a goat in the middle, and snorting out the breath of the terrible flame of bright fire. It goes on to say The term chimera has come to describe any mythical or fictional animal with parts taken from various animals, or to describe anything composed of very disparate parts, or perceived as wildly imaginative, implausible, or dazzling. – bib Feb 3 at 0:41
    
@BenVoigt If I heard someone say that a economic idea was a "chimera", I would assume it was a mashup of two or more widely disparate concepts - I wouldn't necessarily assume it was "dangerous", but neither would I assume it was "fantastical", just that it took things that totally didn't belong together, and mashed them together anyway. – neminem Feb 4 at 21:41
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@neminem: That's my main point -- it means significantly different things to different readers, and thus writers should avoid using it where they wish to impute meaning. – Ben Voigt Feb 4 at 21:57

Unobtanium. But that's mostly about materials or components that you wish existed or that you could afford.

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No. Unobtanium is a movie-literature-television trope- a generic term for whatever it is that motivates the protagonist and/or antogonist into conflict. Similar but not always identical to macguffin. – cobaltduck Feb 3 at 15:00
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@cobaltduck - The idea of unobtanium significantly pre-dates its use in films. The terms was originally used in engineering to describe a substance that the designer wishes existed, but that doesn't currently exist, such as high-tensile carbon nanotubes – Richard Feb 3 at 18:39
    
Yes, unobtanium is from engineering, not film. – Simon White Feb 4 at 15:27
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@cobaltduck you're actually thinking of phlebotinum, unobtanium is something else. – Cat'r'pillar Feb 4 at 22:45

Gee, that sounds like a wonderful car, but unfortunately, it's just a castle in the air.

A fanciful or impractical notion or hope; daydream. [1570–80] Random House Kernerman Webster's College Dictionary

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I like the term "will-o'-the-wisp". It is perhaps a little archaic.

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Please fully explain your answer. Remember: you're answering the question for someone who doesn't know the answer. – Matt E. Эллен Feb 7 at 19:26

A little more risqué definition: Wet Dream

  1. Errotic Dream
  2. nocturnal emission
  3. (idiomatic, by extension) An exciting fantasy; a very appealing, ideal thing, person, or state-of-affairs.
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protected by Matt E. Эллен Feb 7 at 19:26

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