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, 4 insertions, 3 deletions

diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index ad18d5dabbe..e5c47c110f4 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -1168,7 +1168,7 @@ void Item_func_replace::fix_length_and_dec()
     char_length+= max_substrs * (uint) diff;
   }
 
-  if (agg_arg_charsets_for_comparison(collation, args, 3))
+  if (agg_arg_charsets_for_string_result_with_comparison(collation, args, 3))
     return;
   fix_char_length_ulonglong(char_length);
 }
@@ -1458,7 +1458,7 @@ void Item_func_substr::fix_length_and_dec()
 
 void Item_func_substr_index::fix_length_and_dec()
 { 
-  if (agg_arg_charsets_for_comparison(collation, args, 2))
+  if (agg_arg_charsets_for_string_result_with_comparison(collation, args, 2))
     return;
   fix_char_length(args[0]->max_char_length());
 }
@@ -1797,7 +1797,8 @@ void Item_func_trim::fix_length_and_dec()
   {
     // Handle character set for args[1] and args[0].
     // Note that we pass args[1] as the first item, and args[0] as the second.
-    if (agg_arg_charsets_for_comparison(collation, &args[1], 2, -1))
+    if (agg_arg_charsets_for_string_result_with_comparison(collation,
+                                                           &args[1], 2, -1))
       return;
   }
   fix_char_length(args[0]->max_char_length());