select.result:

  Added test case for bug#18759 Incorrect string to numeric conversion.  
select.test:
  Added test case for bug#18759 Incorrect string to numeric conversion.
item_cmpfunc.cc:
  Cleanup after fix for bug#18360 removal

mysql-test/r/select.result

@@ -2738,3 +2738,9 @@ ERROR HY000: Key 'a' doesn't exist in table 't1'
 EXPLAIN SELECT * FROM t1 FORCE INDEX (a);
 ERROR HY000: Key 'a' doesn't exist in table 't1'
 DROP TABLE t1;
+CREATE TABLE t1 (i BIGINT UNSIGNED NOT NULL);
+INSERT INTO t1 VALUES (10);
+SELECT i='1e+01',i=1e+01, i in (1e+01), i in ('1e+01') FROM t1;
+i='1e+01'	i=1e+01	i in (1e+01)	i in ('1e+01')
+0	1	1	1
+DROP TABLE t1;

mysql-test/t/select.test

@@ -2278,4 +2278,23 @@ EXPLAIN SELECT * FROM t1 FORCE INDEX (a);
 
 DROP TABLE t1;
 
+#
+# Bug #18759 "Incorrect string to numeric conversion"
+#
+# This test is here so that the behavior will not be changed to 4.1
+# and not to 5.0 either. In 4.1 and 5.0 sending an integer as a string
+# will be converted internally to real (double) value and it is not
+# as accurate as bigint (longlong) for integers. Thus the results may
+# vary. In 5.1 internally it is decimal, which is a string type and
+# will be more accurate. Due to rather big changes needed to fix this
+# in 4.1 or 5.0 it is not desired to do it in the stable versions.
+#
+# This test is here only to make sure that behavior is not changed in
+# 4.1 and 5.0
+#
+CREATE TABLE t1 (i BIGINT UNSIGNED NOT NULL);
+INSERT INTO t1 VALUES (10);
+SELECT i='1e+01',i=1e+01, i in (1e+01), i in ('1e+01') FROM t1;
+DROP TABLE t1;
+
 # End of 4.1 tests

sql/item_cmpfunc.cc

@@ -136,7 +136,8 @@ static void agg_cmp_type(THD *thd, Item_result *type, Item **items, uint nitems)
       }
       continue;
     }
-    if ((res= items[i]->real_item()->type()) == Item::FIELD_ITEM)
+    if ((res= items[i]->real_item()->type()) == Item::FIELD_ITEM &&
+        items[i]->result_type() != INT_RESULT)
     {
       field= ((Item_field *)items[i]->real_item())->field;
       break;