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_func.cc b/sql/item_func.cc
index 427910c3625..82310365d27 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -2537,7 +2537,8 @@ void Item_func_min_max::fix_length_and_dec()
   }
   if (cmp_type == STRING_RESULT)
   {
-    agg_arg_charsets_for_comparison(collation, args, arg_count);
+    agg_arg_charsets_for_string_result_with_comparison(collation,
+                                                       args, arg_count);
     if (datetime_found)
     {
       thd= current_thd;