fix type coercion in bmerge

R/bmerge.R

@@ -71,7 +71,13 @@ 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, xclass, iname, iclass)
     }
     if (xclass == iclass) {
-      if (verbose) catf("%s has same type (%s) as %s. No coercion needed.\n", iname, xclass, xname)
+      if (length(icols)>1 && is(x[[xc]], "Date") && is(i[[ic]], "Date")) {
+        set(x, j=xc, value=as.double(x[[xc]]))
+        set(i, j=ic, value=as.double(i[[ic]]))
+        if (verbose) catf("%s and %s are both Dates. R does not guarentee a type for Date internally, hence, coercing to double.\n",  iname, xname)
+      } else {
+        if (verbose) catf("%s has same type (%s) as %s. No coercion needed.\n", iname, xclass, xname)
+      }
       next
     }
     if (xclass=="character" || iclass=="character" ||

inst/tests/tests.Rraw

@@ -15727,7 +15727,8 @@ DT = data.table(z = 1i)
 test(2069.33, DT[DT, on = 'z'], error = "Type 'complex' is not supported for joining/merging")
 
 # forder verbose message when !isReallyReal Date, #1738
-DT = data.table(d=sample(seq(as.Date("2015-01-01"), as.Date("2015-01-05"), by="days"), 20, replace=TRUE))
+date_dbl = as.Date(as.double(seq(as.Date("2015-01-01"), as.Date("2015-01-05"), by="days")), origin="1970-01-01")
+DT = data.table(d=sample(date_dbl, 20, replace=TRUE))
 test(2070.01, typeof(DT$d), "double")
 test(2070.02, DT[, .N, keyby=d, verbose=TRUE], output="Column 1.*date.*8 byte double.*no fractions are present.*4 byte integer.*to save space and time")
 
@@ -20596,3 +20597,12 @@ test(2295.3, is.data.table(d2))
 
 # #6588: .checkTypos used to give arbitrary strings to stopf as the first argument
 test(2296, d2[x %no such operator% 1], error = '%no such operator%')
+
+# coerce Dates to double if join on multiple columns, #6602
+date_int = seq(as.Date('2015-01-01'), as.Date('2015-01-01'), by="day")
+date_dbl = as.Date('2015-01-01')
+x = data.table(a=date_int, b=1L)
+y = data.table(c=date_int, d=date_dbl)
+
+test(2297.1, options=c(datatable.verbose=TRUE), y[x, on=.(c == a)], data.table(c=date_int, d=date_dbl, b=1L), output="No coercion needed.")
+test(2297.2, options=c(datatable.verbose=TRUE), y[x, on=.(c == a, d == a)], data.table(c=date_int, d=date_int, b=1L), output="coercing to double.")