Accept only existing editions

An error is thrown when trying to set an edition that does not exist.
Fixes r-lib#1547.

R/edition.R

@@ -51,6 +51,14 @@ edition_name <- function(x) {
   }
 }
 
+is_valid_edition <- function(x) {
+  if (is_zap(x)) {
+    TRUE
+  } else {
+    is.numeric(x) && length(x) == 1 && x %in% c(2, 3)
+  }
+}
+
 #' Temporarily change the active testthat edition
 #'
 #' `local_edition()` allows you to temporarily (within a single test or
@@ -62,7 +70,9 @@ edition_name <- function(x) {
 #' @param .env Environment that controls scope of changes. For expert use only.
 #' @keywords internal
 local_edition <- function(x, .env = parent.frame()) {
-  stopifnot(is_zap(x) || (is.numeric(x) && length(x) == 1))
+  if (!is_valid_edition(x)) {
+    stop("Available editions are 2 and 3", call. = FALSE)
+  }
   old <- edition_set(x)
   withr::defer(edition_set(old), envir = .env)
 }

tests/testthat/test-edition.R

@@ -6,6 +6,17 @@ test_that("can locally override edition", {
   expect_equal(edition_get(), 2)
 })
 
+test_that("only existing editions can be set", {
+  expect_error(
+    local_edition(5),
+    regexp = "Available editions are 2 and 3"
+  )
+  expect_error(
+    local_edition(-1),
+    regexp = "Available editions are 2 and 3"
+  )
+})
+
 test_that("deprecation only fired for newer edition", {
   local_edition(2)
   expect_warning(edition_deprecate(3, "old stuff"), NA)