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 _____."
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.