Fix for BUG#7455.

The fix checks if the trim string argument is NULL. If so, the standard
mandates that the function result must be also NULL.

mysql-test/r/func_str.result

@@ -697,3 +697,9 @@ quote(ltrim(concat('    ', 'a')))
 select quote(trim(concat('    ', 'a')));
 quote(trim(concat('    ', 'a')))
 'a'
+select trim(null from 'kate') as "must_be_null";
+must_be_null
+NULL
+select trim('xyz' from null) as "must_be_null";
+must_be_null
+NULL

mysql-test/t/func_str.test

@@ -435,3 +435,11 @@ drop table t1;
 
 select quote(ltrim(concat('    ', 'a')));
 select quote(trim(concat('    ', 'a')));
+
+#
+# Bug#7455 unexpected result: TRIM(<NULL> FROM <whatever>) gives NOT NULL
+# According to ANSI if one of the TRIM arguments is NULL, then the result
+# must be NULL too.
+#
+select trim(null from 'kate') as "must_be_null";
+select trim('xyz' from null) as "must_be_null";

sql/item_strfunc.cc

@@ -1302,9 +1302,18 @@ String *Item_func_trim::val_str(String *str)
     return 0;					/* purecov: inspected */
   char buff[MAX_FIELD_WIDTH];
   String tmp(buff,sizeof(buff),res->charset());
-  String *remove_str= (arg_count==2) ? args[1]->val_str(&tmp) : &remove;
   uint remove_length;
   LINT_INIT(remove_length);
+  String *remove_str; /* The string to remove from res. */
+
+  if (arg_count == 2)
+  {
+    remove_str= args[1]->val_str(&tmp);
+    if ((null_value= args[1]->null_value))
+      return 0;
+  }
+  else
+    remove_str= &remove; /* Default value. */
 
   if (!remove_str || (remove_length=remove_str->length()) == 0 ||
       remove_length > res->length())