I'm going to quote an interesting passage from the book "Undoing Depression" by Richard O'Connor (page 158, 1st edition).

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What's wrong with these conversations?

SHE: What time is the concert?
HE: You have to be ready by seven-thirty.

SHE: How many people are coming to dinner?
HE: Don't worry, there's plenty of food.

HE: Are you just about finished?
SHE: Do you want dinner now?

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This kind of thing used to drive me crazy, but I have never seen it pointed out in literature and still don't know what to call it. Does it have a name?

- vital