Add warning for joins on mixed time classes

NEWS.md

@@ -131,6 +131,8 @@ rowwiseDT(
 
 18. `as.data.table()` method for `data.frame`s (especially those with extended classes) is more consistent with `as.data.frame()` with respect to rention of attributes, [#5699](https://github.com/Rdatatable/data.table/issues/5699). Thanks @jangorecki for the report and fix.
 
+19. Joining with incompatible column times (e.g., `Date` with `POSIXt`) now provides a clear warning, [#6605](https://github.com/Rdatatable/data.table/issues/6605). Thanks to @al-obrien for the report and @r2evans for the PR.
+
 ## NOTES
 
 1. There is a new vignette on joins! See `vignette("datatable-joins")`. Thanks to Angel Feliz for authoring it! Feedback welcome. This vignette has been highly requested since 2017: [#2181](https://github.com/Rdatatable/data.table/issues/2181).

R/bmerge.R

@@ -82,6 +82,14 @@ bmerge = function(i, x, icols, xcols, roll, rollends, nomatch, mult, ops, verbos
       }
       stopf("Incompatible join types: %s (%s) and %s (%s). Factor columns must join to factor or character columns.", xname, x_merge_type, iname, i_merge_type)
     }
+    # data.table::as.ITime, chron::times, nanotime::nanotime
+    time_classes = c("Date", "POSIXt", "ITime", "times", "nanotime")
+    x_class_time = intersect(class(x[[xc]]), time_classes)
+    i_class_time = intersect(class(i[[ic]]), time_classes)
+    if (length(x_class_time) > 0L && length(i_class_time) > 0L && !identical(sort(x_class_time), sort(i_class_time))) {
+      warningf("Attempting to join column %s (%s) with column %s (%s). They are likely to be incompatible, so we suggest you convert one to the other's class.",
+               xname, toString(x_class_time), iname, toString(i_class_time))
+    }
     # we check factors first to cater for the case when trying to do rolling joins on factors
     if (x_merge_type == i_merge_type) {
       if (verbose) catf("%s has same type (%s) as %s. No coercion needed.\n", iname, x_merge_type, xname)

inst/tests/tests.Rraw

@@ -20703,6 +20703,9 @@ x = data.table(a=1, b=2L)
 y = data.table(c=1.5, d=1L)
 test(2297.31, y[x, on=.(c == a, d == a), nomatch=NULL], output="Empty data.table (0 rows and 3 cols): c,d,b")
 
+# #6605: Joins do not warn user when using POSc and Date comparisons
+test(2297.41, data.table(a = .POSIXct(20000L), v1 = 1)[data.table(a = .Date(20000L), v2 = 2), on = "a"], output = ".+", warning = "incompatible")
+
 # rbindlist(l, use.names=TRUE) should handle different colnames encodings #5452
 x = data.table(a = 1, b = 2, c = 3)
 y = data.table(x = 4, y = 5, z = 6)