Fixed bug #21698: erroneously a field could be replaced by an

equal constant under any circumstances.
In fact this substitution can be allowed if the field is
not of a type string or if the field reference serves as 
an argument of a comparison predicate.  


mysql-test/r/func_str.result:
  Added test cases for bug #21698.
mysql-test/r/heap_hash.result:
  Adjusted results after the fix for bug #21198.
mysql-test/t/func_str.test:
  Added test cases for bug #21698.
sql/item.cc:
  Fixed bug #21198.
  Added a method to check whether a field reference can be 
  substituted for a constant equal to the field.
  This substitution is allowed if the field is not of a type string
  or if the field reference serves as an argument of a comparison
  predicate.
sql/item.h:
  Fixed bug #21698.
  Added a new virtual transformation method for a item 'compile'
  with two callback function parameters.
  Added a new virtual method 'subst_argument_checker' to be used
  as an amnalyzer method.
  This method is supposed to set its in/out argument to NULL for
  the nodes where substitution of a string field for a constant
  is not valid.
sql/item_cmpfunc.cc:
  Fixed bug #21698.
  Added an implementation of the compile method for class Item_cond.
  First it processes the Item_cond node with a callback function and if
  the latter returns TRUE it proceeds with a transformation performed by
  another callback function.
sql/item_cmpfunc.h:
  Fixed bug #21698.
  Added the implementations of 'subst_argument_checker'
  for the Item_func and Item_cond classes.
  This method is supposed to set its in/out argument to NULL for
  the nodes where substitution of a string field for a constant
  is not valid.
  Added the declaration of an implementation of the compile method for 
  class Item_cond.
  First it processes the Item_cond node with a callback function and if
  the latter returns TRUE it proceeds with a transformation performed by
  another callback function.
sql/item_func.cc:
  Fixed bug #21698.
  Added an implementation of the compile method for class Item_func.
  First it processes the Item_func node with a callback function and if
  the latter returns TRUE it proceeds with a transformation performed by
  another callback function.
sql/item_func.h:
  Fixed bug #21698.
  Added the declaration of the implementation of the compile method for
  class Item_func.
  First it processes the Item_func node with a callback function and if
  the latter returns TRUE it proceeds with a transformation performed by
  another callback function.
sql/sql_select.cc:
  Fixed bug #21698.
  Limited the conditions at which a field can be substituted 
  a for an equal constant in a formula. 
  This substitution is allowed if the field is not of a type string
  or if the field reference serves as an argument of a comparison
  predicate.

sql/item.cc

@@ -3745,14 +3745,49 @@ Item_equal *Item_field::find_item_equal(COND_EQUAL *cond_equal)
 }
 
 
+/*
+  Check whether a field can be substituted by an equal item
+
+  SYNOPSIS
+    equal_fields_propagator()
+      arg - *arg != NULL <-> the field is in the context where
+            substitution for an equal item is valid
+   
+  DESCRIPTION
+    The function checks whether a substitution of the field
+    occurrence for an equal item is valid.
+
+  NOTES
+    The following statement is not always true:
+    x=y => F(x)=F(x/y).
+    This means substitution of an item for an equal item not always
+    yields an equavalent condition.
+    Here's an example:
+      'a'='a '
+      (LENGTH('a')=1) != (LENGTH('a ')=2)
+    Such a substitution is surely valid if either the substituted
+    field is not of a STRING type or if it is an argument of
+    a comparison  predicate.  
+
+  RETURN
+    TRUE   substitution is valid
+    FALSE  otherwise
+*/
+
+bool Item_field::subst_argument_checker(byte **arg)
+{
+  return (result_type() != STRING_RESULT) || (*arg);
+}
+
+
 /*
   Set a pointer to the multiple equality the field reference belongs to
   (if any)
    
   SYNOPSIS
     equal_fields_propagator()
-    arg - reference to list of multiple equalities where
-          the field (this object) is to be looked for
+      arg - reference to list of multiple equalities where
+            the field (this object) is to be looked for
   
   DESCRIPTION
     The function looks for a multiple equality containing the field item
@@ -3764,7 +3799,7 @@ Item_equal *Item_field::find_item_equal(COND_EQUAL *cond_equal)
 
   NOTES
     This function is supposed to be called as a callback parameter in calls
-    of the transform method.  
+    of the compile method.  
 
   RETURN VALUES
     pointer to the replacing constant item, if the field item was substituted