fread drop when NULL in colClasses (#3277)

inst/tests/tests.Rraw

@@ -11143,6 +11143,11 @@ test(1825.15, fread(str, colClasses=list(numeric=2, NULL=3:5), drop=1),
               data.table(x2=c(2,4), x3=c(1.5,2.5), x4=c("T","F"), x5=c("cc","ff")),
               warning="Ignoring the NULL item in colClasses= because select= or drop= has been used")   # warning because drop != colClasses$`NULL`
 test(1825.16, fread(str, colClasses=(cl<-list(numeric=2, NULL=3:5)), drop=cl$`NULL`), data.table(x1=INT(1,3), x2=c(2,4)))  # cover commit f0bd6e3
+# NULL didn't work in 1.11.0-1.11.8 so some usage exists where drop= is used to respecify the NULLs. The warning could be reintroduced in future.
+# https://github.com/Rdatatable/data.table/issues/3233#issuecomment-453674647
+test(1825.17, fread(str, colClasses=c("integer","integer","NULL","character","NULL"), drop=3), data.table(x1=INT(1,3), x2=INT(2,4), x4=c("T","F")))
+test(1825.18, fread(str, colClasses=c("integer","numeric","NULL","character","NULL"), drop=3:4), data.table(x1=INT(1,3), x2=c(2,4)))
+test(1825.19, fread(str, drop=6), data.table(x1=INT(1,3), x2=INT(2,4), x3=c(1.5,2.5), x4=c("T","F"), x5=c("cc","ff")), warning="Column number 6 (drop[1]) is out of range [1,ncol=5]")
 
 # issue 2351
 set.seed(1)

src/freadR.c

@@ -281,7 +281,10 @@ _Bool userOverride(int8_t *type, lenOff *colNames, const char *anchor, int ncol)
         if (k<1 || k>ncol) {
           DTWARN("Column number %d (drop[%d]) is out of range [1,ncol=%d]",k,j+1,ncol);
         } else {
-          if (type[k-1] == CT_DROP) STOP("Duplicates detected in drop");
+          // if (type[k-1] == CT_DROP) DTWARN("drop= contains duplicates");
+          // NULL in colClasses didn't work between 1.11.0 and 1.11.8 so people have been using drop= to re-specify the NULL columns in colClasses. Now that NULL in colClasses works
+          // from v1.12.0 there is no easy way to distinguish dups in drop= from drop overlapping with NULLs in colClasses. But it's unambiguous that it was intended to remove these
+          // columns, so no need for warning.
           type[k-1] = CT_DROP;
         }
       }