Fixed bug #28980: the result of ROUND(<decimal expr>,<int column>)

was erroneously converted to double, while the result of
ROUND(<decimal expr>, <int literal>) was preserved as decimal.
As a result of such a conversion the value of ROUND(D,A) could
differ from the value of ROUND(D,val(A)) if D was a decimal expression.

Now the result of the ROUND function is never converted to 
double if the first argument is decimal.  

mysql-test/r/type_decimal.result

@@ -790,3 +790,12 @@ Warning	1292	Truncated incorrect datetime value: '0000-00-00'
 Warning	1292	Truncated incorrect datetime value: '0000-00-00'
 Warning	1292	Truncated incorrect datetime value: '0000-00-00'
 drop table t1;
+CREATE TABLE t1 (
+qty decimal(16,6) default NULL, 
+dps tinyint(3) unsigned default NULL 
+);
+INSERT INTO t1 VALUES (1.1325,3);
+SELECT ROUND(qty,3), dps, ROUND(qty,dps) FROM t1;
+ROUND(qty,3)	dps	ROUND(qty,dps)
+1.133	3	1.133
+DROP TABLE t1;

mysql-test/t/type_decimal.test

@@ -394,3 +394,17 @@ create table t1 as
  from (select 1 as s,'t' as t union select null, null ) as sub1;
 select group_concat(t) from t1 group by week(date)/10;
 drop table t1;
+
+#
+# Bug#28980: ROUND(<dec expr>, <int col>) returned double values
+#
+
+CREATE TABLE t1 (
+  qty decimal(16,6) default NULL, 
+  dps tinyint(3) unsigned default NULL 
+);
+INSERT INTO t1 VALUES (1.1325,3);
+
+SELECT ROUND(qty,3), dps, ROUND(qty,dps) FROM t1;
+
+DROP TABLE t1;

sql/item_func.cc

@@ -1957,7 +1957,13 @@ void Item_func_round::fix_length_and_dec()
   {
     max_length= args[0]->max_length;
     decimals= args[0]->decimals;
-    hybrid_type= REAL_RESULT;
+    if (args[0]->result_type() == DECIMAL_RESULT)
+    {
+      max_length++;
+      hybrid_type= DECIMAL_RESULT;
+    }
+    else
+      hybrid_type= REAL_RESULT;
     return;
   }