A fool's paradise
A state of happiness based on false hope.
This is an early phrase, first recorded in the Paston Letters, 1462:
"I wold not be in a folis paradyce."
Shakespeare later used it in Romeo and Juliet, 1592.
Now, afore God, I am so vexed, that every part about me quivers. Scurvy knave! Pray you, sir, a word: and as I told you, my young lady bade me inquire you out; what she bade me say, I will keep to myself: but first let me tell ye, if ye should lead her into a fool's paradise, as they say, it were a very gross kind of behavior, as they say: for the gentlewoman is young; and, therefore, if you should deal double with her, truly it were an ill thing to be offered to any gentlewoman, and very weak dealing.
See other phrases and sayings from Shakespeare.