Just a note that, by and large, a literal translation mostly works here as well, although the construction sounds a little English (vs. American) to me. To wit: "They will have gone to bed upon arriving at the hotel because the trip was very long" is perhaps an unusual phrasing in modern conversational (American) English, but certainly not an unintelligible one, and I think it carries the same meaning.
I agree, and it doesn't seem that unusual to me (a Brit). I do get the impression it's less common in American English. However, I don't think it can be used in the first person, as in this example:
Lo habré conocido antes porque su cara me suena.
I must have met him before because his face is familiar.
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To be clear, I am American which was why I was qualifying that I thought it sounded linguistically unusual, but not unintelligible, to me. I suspect a colloquialism like "are you down?" would be well understood in most of the 50 states (and even Puerto Rico where I currently am). Whether it would be equally understood in England or Ghana or India, I don't know. (At the least, it seems likely if the person in question is a consumer of American media.) "They" which had long been informally used as a gender-neutral pronoun is now having that status codified in various style guides, in no small part for sociopolitical reasons. But since there's no English equivalent to the RAE, that I'm aware of, it's definitely a fragmentary process.
R and Alan, I agree; As a native English speaker [though Irish] “will have” works very well for me too, and makes sense in the context. That said, for me, it conveys a degree of confidence about the supposition which seems fine.
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