expect_is() is an older form that uses inherits() without checking whether x is S3, S4, or neither. Instead, I'd recommend using expect_type(), expect_s3_class() or expect_s4_class() to more clearly convey your intent.

expect_is(object, class, info = NULL, label = NULL)

## Arguments

object Object to test. Supports limited unquoting to make it easier to generate readable failures within a function or for loop. See quasi_label for more details. Either a character vector of class names, or for expect_s3_class() and expect_s4_class(), an NA to assert that object isn't an S3 or S4 object.

## 3rd edition

expect_is() is formally deprecated in the 3rd edition.