想得美

xiǎngdeměi
to have an unrealistically optimistic expectation; (as an interjection) what a dreamer!

CEDICT

Used to express that someone is thinking of something that is favorable but impossible

Wiktionary

想法过于乐观美好，常用以表示事情绝不可能。

重編國語辭典

Entry Composition

1 想 xiǎng
  1. to think (about)
  2. to think of; to devise
  3. to think (that); to believe (that)
  4. to desire; to want (to)
  5. to miss (feel wistful about the absence of)
2 得 de structural particle: used after a verb (or adjective as main verb), linking it to following phrase indicating effect, degree, possibility etc
3 美 měi
  1. beautiful
  2. very satisfactory; good
  3. to beautify
  4. to be pleased with oneself