He must have seen it, mustn't he?

[joham]

English Lessons - Grammar

In the above link we can see such a sentence:

He must have seen it, mustn't he?

This is the first time I have seen such a question tag as with 'must have done'. In China's English tests or examinations, such a tag is thought to be absolutely wrong. They say we must use 'hasn't he?'. They also say we must say 'He must have seen it yesterday, didn't he?'

I thought 'mustn't he?' was fine, but I had never found any proof. Now that I've got this sentence, I'd like to know how authoritative it is and whether we can use this sentence at all.

Might I ask native English teachers to help me clarify this puzzle of mine?

Thank you very much.

 

[BobK]

...
He must have seen it, mustn't he?

There's nothing wrong with this. Any other tag question implies an ellipsis such as 'He must have done it. [Tell me I'm not wrong. (Beginning to be unsure) He did...] Didn't he?'. In that case, it's not really a tag question at all, as it's not asking for routine confirmation of a certainty.

b