Don't you say "not fail to" in this way?
"The frame won't fail to make my puzzle beautiful." I mean the frame will surely make my puzzle beautiful. Don't you say "not fail to" in this way? When do you say it?
You can indeed. It's a more indirect and understated way of saying it in a way that is quite natural to English. The two negatives in meaning cancel each other out and in so doing actually reinforce the point.
February 12, 2016
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