As far as I know these are tenses that you do not often use. Am I right?
Will have been + verb+ing
Would have been + verb+ing
You are right that the two constructions are not often used. This is because they express uncommon ideas. Both are grammatical, however, and could be used as follows:
By the time I retire I will have been working here for 45 years.
If you had called me 10 minutes ago, I would have been sleeping.
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