A date can be represented as an int (like 20060101) and as a string (like
"2006.01.01"). When a DATE/TIME field is compared in one SELECT against both
representations the constant propagation mechanism leads to comparison
of DATE as a string and DATE as an int. In this example it compares 2006 and
20060101 integers. Obviously it fails comparison although they represents the
same date.


Now the Item_bool_func2::fix_length_and_dec() function sets the comparison
context for items being compared. I.e. if items compared as strings the
comparison context is STRING.
The constant propagation mechanism now doesn't mix items used in different
comparison contexts. The context check is done in the
Item_field::equal_fields_propagator() and in the change_cond_ref_to_const() 
functions.

Also the better fix for bug 21159 is introduced.


mysql-test/t/type_datetime.test:
  Added a test case for bug#21475: Wrongly applied constant propagation leads to a false comparison.
mysql-test/r/type_datetime.result:
  Added a test case for bug#21475: Wrongly applied constant propagation leads to a false comparison.
sql/sql_select.cc:
  Fixed bug#21475: Wrongly applied constant propagation leads to a false comparison.
  The constant propagation mechanism now doesn't mix items used in different
  comparison contexts. The check is done in the change_cond_ref_to_const() function.
sql/item_cmpfunc.cc:
  Fixed bug#21475: Wrongly applied constant propagation leads to a false comparison.
  Now the Item_bool_func2::fix_length_and_dec() function sets the comparison
  context for items being compared.
sql/item.h:
  Fixed bug#21475: Wrongly applied constant propagation leads to a false comparison.
  To the Item class a new field called cmp_context is added.
  It represents the comparison context of an item.
sql/item.cc:
  Fixed bug#21475: Wrongly applied constant propagation leads to a false comparison.
  The constant propagation mechanism now doesn't mix items used in different
  comparison contexts. The context check is done in the
  Item_field::equal_fields_propagator() function.