Bug#23411: ... MOD-ing zero returns strange result

The Item_func_mod objects never had maybe_null set, so users had no reason 
to expect that they can be NULL, and may therefore deduce wrong results.
Now, set maybe_null.

BitKeeper/etc/collapsed

@@ -1 +1,3 @@
 452a92d0-31-8wSzSfZi165fcGcXPA
+454a7ef8gdvE_ddMlJyghvOAkKPNOQ
+454f8960jsVT_kMKJtZ9OCgXoba0xQ

mysql-test/r/func_test.result

@@ -183,3 +183,78 @@ select 5.1 mod 3, 5.1 mod -3, -5.1 mod 3, -5.1 mod -3;
 select 5 mod 3, 5 mod -3, -5 mod 3, -5 mod -3;
 5 mod 3	5 mod -3	-5 mod 3	-5 mod -3
 2	2	-2	-2
+select (12%0) <=> null      as '1';
+1
+1
+select (12%0) is null       as '1';
+1
+1
+select 12%0                 as 'NULL';
+NULL
+NULL
+select 12%2                 as '0';
+0
+0
+select 12%NULL              as 'NULL';
+NULL
+NULL
+select 12 % null            as 'NULL';
+NULL
+NULL
+select null % 12            as 'NULL';
+NULL
+NULL
+select null % 0             as 'NULL';
+NULL
+NULL
+select 0 % null             as 'NULL';
+NULL
+NULL
+select null % null          as 'NULL';
+NULL
+NULL
+select (12 mod 0) <=> null  as '1';
+1
+1
+select (12 mod 0) is null   as '1';
+1
+1
+select 12 mod 0             as 'NULL';
+NULL
+NULL
+select 12 mod 2             as '0';
+0
+0
+select 12 mod null          as 'NULL';
+NULL
+NULL
+select null mod 12          as 'NULL';
+NULL
+NULL
+select null mod 0           as 'NULL';
+NULL
+NULL
+select 0 mod null           as 'NULL';
+NULL
+NULL
+select null mod null        as 'NULL';
+NULL
+NULL
+select mod(12.0, 0)         as 'NULL';
+NULL
+NULL
+select mod(12, 0.0)         as 'NULL';
+NULL
+NULL
+select mod(12, NULL)        as 'NULL';
+NULL
+NULL
+select mod(12.0, NULL)      as 'NULL';
+NULL
+NULL
+select mod(NULL, 2)         as 'NULL';
+NULL
+NULL
+select mod(NULL, 2.0)       as 'NULL';
+NULL
+NULL

mysql-test/t/func_test.test

@@ -108,4 +108,36 @@ select 5.1 mod 3, 5.1 mod -3, -5.1 mod 3, -5.1 mod -3;
 
 select 5 mod 3, 5 mod -3, -5 mod 3, -5 mod -3;
 
+#
+# Bug#23411: The "%" (MOD) operator is not documented; MOD-ing zero returns strange result
+# Manual: "Division by zero produces a NULL result"
+#
+select (12%0) <=> null      as '1';
+select (12%0) is null       as '1';
+select 12%0                 as 'NULL';
+select 12%2                 as '0';
+select 12%NULL              as 'NULL';
+select 12 % null            as 'NULL';
+select null % 12            as 'NULL';
+select null % 0             as 'NULL';
+select 0 % null             as 'NULL';
+select null % null          as 'NULL';
+
+select (12 mod 0) <=> null  as '1';
+select (12 mod 0) is null   as '1';
+select 12 mod 0             as 'NULL';
+select 12 mod 2             as '0';
+select 12 mod null          as 'NULL';
+select null mod 12          as 'NULL';
+select null mod 0           as 'NULL';
+select 0 mod null           as 'NULL';
+select null mod null        as 'NULL';
+
+select mod(12.0, 0)         as 'NULL';
+select mod(12, 0.0)         as 'NULL';
+select mod(12, NULL)        as 'NULL';
+select mod(12.0, NULL)      as 'NULL';
+select mod(NULL, 2)         as 'NULL';
+select mod(NULL, 2.0)       as 'NULL';
+
 # End of 4.1 tests

sql/item_func.cc

@@ -96,7 +96,7 @@ Item_func::Item_func(THD *thd, Item_func *item)
 
 
 /*
-  Resolve references to table column for a function and it's argument
+  Resolve references to table column for a function and its argument
 
   SYNOPSIS:
   fix_fields()
@@ -707,6 +707,7 @@ longlong Item_func_mod::val_int()
 void Item_func_mod::fix_length_and_dec()
 {
   Item_num_op::fix_length_and_dec();
+  maybe_null= 1;
 }