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I'm sorry to report that this analysis is completely wrong, and demonstrates a lack of understanding of the R object model. The class that is provided by tibble does not implement all of data.frame, and the OP is correct.

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(S3 -- see footnote) Classes don't "implement" anything in R the way they would in other languages. They are labels that tell dispatch functions how to deal with an object. A tibble is internally a data frame. The last example in my post makes this exactly clear.

The other OO systems in R do act closer to traditional classes, but all the tidyverse stuff is S3.

(But the OP was correct in another sense related to the example narrowly!)

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