R Under development (unstable) (2023-06-30 r84625 ucrt) -- "Unsuffered Consequences"
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Platform: x86_64-w64-mingw32/x64

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> library(testthat)
> if (Sys.info()[["sysname"]] == "Darwin") {
+   test_that("Warning if multibyte support is not detected", {
+     withr::with_locale(new = c("LC_CTYPE" = "en_US.US-ASCII"), code = {
+       expect_message(library(datefixR), paste0(
+         "The current locale does not support multibyte characters. ",
+         "You may run into difficulties if any months are given as ",
+         "non-English language names. \n"
+       ))
+     })
+   })
+ }
> 
> proc.time()
   user  system elapsed 
   0.48    0.07    0.54