Clarify types in class expectations (#2168)

Fixes #1989

Author: hadley

R/expect-inheritance.R

@@ -20,16 +20,17 @@
 #' See [expect_vector()] for testing properties of objects created by vctrs.
 #'
 #' @param type String giving base type (as returned by [typeof()]).
-#' @param class
-#'  * `expect_type()`: a single string giving an R base type.
-#'  * `expect_s3_class()`: a character vector of class names or `NA` to assert
-#'    that `object` isn't an S3 object. If you provide multiple class names,
-#'    the test will pass if `object` inherits from any of them, unless
-#'    `exact = TRUE`.
-#'  * `expect_s4_class()`: a character vector of class names or `NA` to assert
-#'    that `object` isn't an S4 object.
-#'  * `expect_r6_class()`: a string.
-#'  * `expect_s7_class()`: an [S7::S7_class()] object.
+#' @param class The required type varies depending on the function:
+#'   * `expect_type()`: a string.
+#'   * `expect_s3_class()`: a string or character vector. The behaviour of
+#'     multiple values (i.e. a character vector) is controlled by the
+#'     `exact` argument.
+#'   * `expect_s4_class()`: a string.
+#'   * `expect_r6_class()`: a string.
+#'   * `expect_s7_class()`: an [S7::S7_class()] object.
+#'
+#'   For historical reasons, `expect_s3_class()` and `expect_s4_class()` also
+#'   take `NA` to assert that the `object` is not an S3 or S4 object.
 #' @inheritParams expect_that
 #' @family expectations
 #' @examples
@@ -85,7 +86,7 @@ expect_type <- function(object, type) {
 #' @rdname inheritance-expectations
 #' @param exact If `FALSE`, the default, checks that `object` inherits
 #'   from any element of `class`. If `TRUE`, checks that object has a class
-#'   that's identical to `class`.
+#'   that exactly matches `class`.
 expect_s3_class <- function(object, class, exact = FALSE) {
   check_bool(exact)