Bug#11926811 / Bug#60625 Illegal mix of collations

Problem: comparison of a DATETIME sp variable and NOW()
led to Illegal mix of collations error when 
character_set_connection=utf8.
Introduced by "WL#2649 Number-to-string conversions".

Error happened in Arg_comparator::set_compare_func(),
because the first argument was errouneously converted to utf8,
while the second argument was not.

Fix: separate agg_arg_charsets_for_comparison() into two functions:

- agg_arg_charsets_for_comparison() - for pure comparison,
  when we don't need to return any string result and therefore
  don't need to convert arguments to @@character_set_connection:
    SELECT a = b;

- agg_arg_charsets_for_string_results_with_comparison() - when
  we need to return a string result, but we also need to do
  comparison internally: SELECT REPLACE(a,b,c)
  If all arguments are numbers:
    SELECT REPLACE(123,2,3) -> 133
  we convert arguments to @@character_set_connection.


  @ mysql-test/include/ctype_numconv.inc
  @ mysql-test/r/ctype_binary.result
  @ mysql-test/r/ctype_cp1251.result
  @ mysql-test/r/ctype_latin1.result
  @ mysql-test/r/ctype_ucs.result
  @ mysql-test/r/ctype_utf8.result
  Adding tests

  @ sql/item.cc
  @ sql/item.h
  @ sql/item_func.cc
  @ sql/item_func.h
  @ sql/item_strfunc.cc

  Introducing and using new function
   agg_item_charsets_for_string_result_with_comparison() and
  its Item_func wrapper agg_arg_charsets_for_string_result_with_comparison().

1 files changed, 2 insertions, 1 deletions

diff --git a/sql/item.cc b/sql/item.cc
index a1482a6d634..af3917c09c1 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -1746,7 +1746,8 @@ bool agg_item_collations(DTCollation &c, const char *fname,
   }
   
   /* If all arguments where numbers, reset to @@collation_connection */
-  if (c.derivation == DERIVATION_NUMERIC)
+  if (flags & MY_COLL_ALLOW_NUMERIC_CONV &&
+      c.derivation == DERIVATION_NUMERIC)
     c.set(Item::default_charset(), DERIVATION_COERCIBLE, MY_REPERTOIRE_NUMERIC);
 
   return FALSE;