Fix custom handling of assert_that(inherits(.)) testing multiple inheritance

NEWS.md

@@ -1,5 +1,9 @@
 # assertthat (development version)
 
+* `assert_that(inherits(x, c("a", "b")))`, where inheritance is tested
+  against multiple target classes, correctly handles failed
+  assertions (@MichaelChirico, [#73](https://github.com/hadley/assertthat/issues/73))
+
 # assertthat 0.2.1
 
 * `assert_that()` no longer throws a fatal error on very long custom 

R/base-is.r

@@ -42,8 +42,12 @@ base_fs$is.recursive <- is_not("a recursive object")
 base_fs$is.symbol <- is_not("a name")
 
 
-# Catch all
 base_fs$inherits <- function(call, env) {
   class <- eval(call$what, env)
-  paste0(deparse(call$x), " does not inherit from class ", class)
+  x_str <- paste(deparse(call$x), collapse = " ")
+  if (length(class) == 1L) {
+    paste(x_str, "does not inherit from class", class)
+  } else {
+    paste(x_str, "does not inherit from any of these classes:", toString(class))
+  }
 }

tests/testthat/test-base.R

@@ -17,3 +17,11 @@ test_that("all message is useful", {
 test_that("custom message is printed", {
   expect_equal(validate_that(FALSE, msg = "Custom message"), "Custom message")
 })
+
+test_that("inherits on multiple classes fails gracefully", {
+  expect_error(
+    assert_that(inherits(NULL, c("a", "b"))),
+    "does not inherit from any of these classes",
+    fixed = TRUE
+  )
+})