Add shift() error for Date/POSIXct mismatch (#7174)

* add NEWS

* add fix

* add tests

* add tests

* tweak

---------

Co-authored-by: Michael Chirico <chiricom@google.com>

Author: ben-schwen

NEWS.md

@@ -84,6 +84,8 @@
 
 13. Reference to `.SD` in `...` arguments to `lapply()`, e.g. ``lapply(list_of_tables, `[`, j=.SD[1L])`` is evaluated correctly, [#2982](https://github.com/Rdatatable/data.table/issues/2982). Thanks @franknarf1 for the report and @MichaelChirico for the fix.
 
+14. Filling columns of class Date with POSIXct (and vice versa) using `shift()` now yields a clear, informative error message specifying the class mismatch, [#5218](https://github.com/Rdatatable/data.table/issues/5218). Thanks @ashbaldry for the report and @ben-schwen for the fix.
+
 ### NOTES
 
 1. The following in-progress deprecations have proceeded:

inst/tests/tests.Rraw

@@ -6851,6 +6851,13 @@ test(1463.78, shift(x,1:2,   type="cyclic"), list(as.raw(c(5, 1:4)), as.raw(c(4:
 test(1463.79, shift(x,-1L,   type="cyclic"), as.raw(c(2:5, 1)))
 test(1463.80, shift(x,-(1:2),type="cyclic"), list(as.raw(c(2:5, 1)), as.raw(c(3:5,1:2))))
 
+# shift incompatible types (e.g. Date and POSIXct)
+d = .Date(0:4)
+p = .POSIXct(1:5)
+test(1463.81, shift(d, fill=p[1L]), error="Filling Date with POSIXct .* unsupported.*")
+test(1463.82, shift(p, fill=d[1L]), error="Filling POSIXct with Date .* unsupported.*")
+test(1463.83, shift(d, fill=as.IDate(2000L)), .Date(c(2000L, 0:3)))
+
 # FR #686
 DT = data.table(a=rep(c("A", "B", "C", "A", "B"), c(2,2,3,1,2)), foo=1:10)
 # Seemingly superfluous 'foo' is needed to test fix for #1942

src/shift.c

@@ -40,6 +40,12 @@ SEXP shift(SEXP obj, SEXP k, SEXP fill, SEXP type)
     SEXP elem  = VECTOR_ELT(x, i);
     size_t size  = RTYPE_SIZEOF(elem);
     R_xlen_t xrows = xlength(elem);
+    if (INHERITS(elem, char_Date) && INHERITS(fill, char_POSIXct) || INHERITS(elem, char_POSIXct) && INHERITS(fill, char_Date)) {
+      const char* elem_type = INHERITS(elem, char_Date) ? "Date" : "POSIXct";
+      const char* fill_type = INHERITS(fill, char_Date) ? "Date" : "POSIXct";
+      error(_("Filling %s with %s using shift() is unsupported. Please convert fill to %s first."),
+            elem_type, fill_type, elem_type);
+    }
     SEXP thisfill = PROTECT(coerceAs(fill, elem, ScalarLogical(0)));  // #4865 use coerceAs for type coercion
     switch (TYPEOF(elem)) {
     case INTSXP: case LGLSXP: {