expect_is() is an older form that uses
inherits() without checking
x is S3, S4, or neither. Instead, I'd recommend using
expect_s4_class() to more clearly
convey your intent.
expect_is(object, class, info = NULL, label = NULL)
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.
expect_is() is formally deprecated in the 3rd edition.