This sentence tells nothing about the future. "Will" here is used to describe what happened in the past / present.
No chance of finding him sober now; he will have been drinking all day.
Can you explain me this kind of usage of will?
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"Will" has several functions. One of them is to express likelihood. As the Cobuild English Grammar states:
You use 'will' when you are assuming that something is the case and you do not think there is any reason to doubt it.
You can use will + continuous in this way to express your assumption about the future, present and past.
No doubt he'll be drinking when we arrive.
Knowing him he will be drinking already.
Small wonder he had a hangover. He will have been drinking all day the day before.
The grammatical tense is one thing, the time it refers to is another thing. Not always tense and time correspond. Present tense can be used for future, and assumptions about the past can be expressed in future perfect:
It actually does say something about the future.
The sentence means that when (in the future) you find him, you will see that he has been drinking all day.
The reference time for the clause is the time at which you find him, i.e., sometime in the future, and the speaker's prediction for that time, is that you will observe that he has been drinking all day.