fread: Improve validation of the dec argument

NEWS.md

@@ -78,6 +78,8 @@
 
 18. `example(local=TRUE)` where the example uses `[.data.table` works again (e.g. `example(':=', package='data.table', local=TRUE, echo=FALSE)`), [#7855](https://github.com/Rdatatable/data.table/issues/7855) re-fixing [#2972](https://github.com/Rdatatable/data.table/issues/2972). Thanks @michaelChirico for the fix.
 
+19. `fread()` returns a clearer error message when `dec = NA` is used, [#7737](https://github.com/Rdatatable/data.table/issues/7737). Thanks @mcol for the report and the fix.
+
 ### Notes
 
 1. {data.table} now depends on R 3.5.0 (2018).

R/fread.R

@@ -55,7 +55,11 @@ yaml=FALSE, tmpdir=tempdir(), tz="UTC")
     else stopifnot( nchar(sep)==1L )  # otherwise an actual character to use as sep
   }
   stopifnot( is.character(dec), length(dec)==1L)
-  if (dec == "auto") dec = "" else stopifnot(nchar(dec) == 1L)
+  if (identical(dec, "auto")) {
+    dec = ""
+  } else if (!is.character(dec) || length(dec) != 1L || is.na(dec) || nchar(dec) != 1L) {
+    stopf('dec= must be a single non-NA character or "auto".')
+  }
   # handle encoding, #563
   if (length(encoding) != 1L || !encoding %chin% c("unknown", "UTF-8", "Latin-1")) {
     stopf("Argument 'encoding' must be 'unknown', 'UTF-8' or 'Latin-1'.")

inst/tests/tests.Rraw

@@ -6449,8 +6449,9 @@ for (t in seq_len(nrow(all))) {
 
 }
 
-# fread dec=',' e.g. France
-test(1439, fread("A;B\n1;2,34\n", dec="12"), error=base_messages$stopifnot("nchar(dec) == 1L"))
+# validation of dec argument, issue #7738
+test(1439.1, fread("A;B\n1;2,34\n", dec="12"), error='dec= must be a single non-NA character or "auto".')
+test(1439.2, fread(input = "whatever.csv", dec = NA_character_), error='dec= must be a single non-NA character or "auto".')
 test(1440, fread("A;B\n8;2,34\n", dec="1"), data.table(A=8L, B="2,34"))
 test(1441, fread("A;B\n8;2,34\n", dec=","), data.table(A=8L, B=2.34))
 test(1442, fread("A;B\n1;2,34\n", sep=".", dec="."), error="sep == dec ('.') is not allowed")