func_str.result, func_str.test:

  Adding test case.
item_strfunc.cc:
  bug#11728 string function LEFT, strange undocumented behaviour
  Fixing LEFT and RIGHT return NULL if the second
  argument is NULL.

mysql-test/r/func_str.result

@@ -33,6 +33,9 @@ instr('hello','HE')	instr('hello',binary 'HE')	instr(binary 'hello','HE')
 select position(binary 'll' in 'hello'),position('a' in binary 'hello');
 position(binary 'll' in 'hello')	position('a' in binary 'hello')
 3	0
+select left('hello',null), right('hello',null);
+left('hello',null)	right('hello',null)
+NULL	NULL
 select left('hello',2),right('hello',2),substring('hello',2,2),mid('hello',1,5) ;
 left('hello',2)	right('hello',2)	substring('hello',2,2)	mid('hello',1,5)
 he	lo	el	hello

mysql-test/t/func_str.test

@@ -19,6 +19,11 @@ select hex(char(256));
 select locate('he','hello'),locate('he','hello',2),locate('lo','hello',2) ;
 select instr('hello','HE'), instr('hello',binary 'HE'), instr(binary 'hello','HE'); 
 select position(binary 'll' in 'hello'),position('a' in binary 'hello');
+#
+# Bug#11728 string function LEFT,
+# strange undocumented behaviour, strict mode
+#
+select left('hello',null), right('hello',null);
 select left('hello',2),right('hello',2),substring('hello',2,2),mid('hello',1,5) ;
 select concat('',left(right(concat('what ',concat('is ','happening')),9),4),'',substring('monty',5,1)) ;
 select substring_index('www.tcx.se','.',-2),substring_index('www.tcx.se','.',1);

sql/item_strfunc.cc

@@ -1038,7 +1038,7 @@ String *Item_func_left::val_str(String *str)
   long length  =(long) args[1]->val_int();
   uint char_pos;
 
-  if ((null_value=args[0]->null_value))
+  if ((null_value=(args[0]->null_value || args[1]->null_value)))
     return 0;
   if (length <= 0)
     return &my_empty_string;
@@ -1078,7 +1078,7 @@ String *Item_func_right::val_str(String *str)
   String *res  =args[0]->val_str(str);
   long length  =(long) args[1]->val_int();
 
-  if ((null_value=args[0]->null_value))
+  if ((null_value=(args[0]->null_value || args[1]->null_value)))
     return 0; /* purecov: inspected */
   if (length <= 0)
     return &my_empty_string; /* purecov: inspected */