Student or Learner
It is like a dream come true.
Why is 'come' used instead of 'comes'?
But the problem is that it is an active thought. So why it should not be, "It is like a dream coming true."?
Because of its being a song? or Is it a complex sentence(imagined complex sentence)?
For example if a certain man asked a woman to marry him, she might say, "It's like a dream come true". She's obviously assuming that the wedding will go ahead etc. Enough of the dream has come true to assume that the dream will play out to the end.
Anyway, regardless of the explanation, that's the idiom.
It's one of those set expressions.