There is a pervasive equivocation on "constructive" (as in criticism) that is typically overlooked.
When people ask for "constructive criticism", they mean both 1) "helpful", and 2) "assisting in the further construction of the work". Unfortunately, often the most helpful criticism that can be given is that the work is misconceived or misexecuted, and should be abandoned or restarted. Actually saying this leads to such howls for "constructiveness" that many people won't even try.
Here are some fine examples of constructive criticism in that sense:
When carving a statue of an elephant, one must start with a block of material and cut away anything that doesn't look like an elephant. You have already cut away too much.
A reply to a review, by the musician Max Reger (also attributed to many others):
I am sitting in the smallest room in my house. Your review is before me. Soon it will be behind me.
In the 18th century, an anonymous playwright sent a English theatre manager his verse tragedy on Mariamne, the wife of King Herod. The play, to say the least, did not meet contemporary standards of commercial value. The manager therefore published the following epigram:
Poet, whoe'er thou art, God damn thee;
Go hang thyself, and burn thy Mariamne.