Use the same logic when guessing as we use when parsing

Not sure why the logic ended up so much different here, I think it was
legacy behavior of some-kind

Fixes #240

NEWS.md

@@ -1,5 +1,7 @@
 # vroom (development version)
 
+* Datetime formats used when guessing now match those used when parsing (#240)
+
 * `vroom_lines()` now works with empty files (#285)
 
 * `vroom_write()` now works when the delimiter is empty, e.g. `delim = ""` (#287).

src/guess_type.cc

@@ -98,11 +98,8 @@ static bool isDateTime(const std::string& x, LocaleInfo* pLocale) {
   if (!ok)
     return false;
 
-  if (!parser.compactDate())
-    return true;
-
-  // Values like 00014567 are unlikely to be dates, so don't guess
-  return parser.year() > 999;
+  DateTime dt = parser.makeDateTime();
+  return dt.validDateTime();
 }
 
 std::string guess_type__(

tests/testthat/test-datetime.R

@@ -218,7 +218,7 @@ test_that("DDDD-DD not parsed as date (i.e. doesn't trigger partial date match)"
 })
 
 test_that("leading zeros don't get parsed as date without explicit separator", {
-  expect_type(vroom("00010203\n", col_names = FALSE, delim = "\n", col_types = list())[[1]], "character")
+  expect_type(vroom("00010203\n", col_names = FALSE, delim = "\n", col_types = list())[[1]], "double")
   expect_s3_class(vroom("0001-02-03\n", col_names = FALSE, delim = "\n", col_types = list())[[1]], "Date")
 })
 
@@ -244,3 +244,7 @@ test_that("subsetting works with both double and integer indexes", {
   expect_equal(x$X1[NA_integer_], na_dt)
   expect_equal(x$X1[NA_real_], na_dt)
 })
+
+test_that("guessing datetime uses the same logic as parsing", {
+  expect_s3_class(vroom("date\n2015-06-14T09Z\n2015-06-14T09Z", delim=",")[[1]], "character")
+})