MDEV-25829 Change default Unicode collation to uca1400_ai_ci

Step#1 - Changing collation derivation for string user variables
from IMPLICIT to COERCIBLE.

Retionale:

Without this preparatory change, switching the default collation for
Unicode character sets from xxx_general_ci to uca1400_ai_ci would cause
"Illegal mix of collations" errors in scenarios comparing a column with
a non-default collation to a string user variable

This is especially important for queries to INFORMATION_SCHEMA tables,
whose columns use utf8mb3_general_ci.

See the description of MDEV-25829 for more details and SQL script examples.

mysql-test/main/func_regexp_pcre.result

@@ -859,11 +859,22 @@ REGEXP_REPLACE('1 foo and bar', '(\\d+) foo and (\\d+ )?bar', '\\1 this and \\2t
 # Testing a warning
 SET NAMES latin1;
 SET @regCheck= '\\xE0\\x01';
-SELECT 0xE001 REGEXP @regCheck;
-0xE001 REGEXP @regCheck
+SELECT 0xE001 REGEXP @regCheck COLLATE latin1_bin;
+0xE001 REGEXP @regCheck COLLATE latin1_bin
+0
+Warnings:
+Warning	1139	Regex error 'UTF-8 error: 1 byte missing at end'
+SELECT 0xE001 REGEXP CAST(@regCheck AS CHAR);
+0xE001 REGEXP CAST(@regCheck AS CHAR)
 0
 Warnings:
 Warning	1139	Regex error 'UTF-8 error: 1 byte missing at end'
+# Since 11.4 user variables have DERIVATION_COERCIBLE
+# so a user variable and a literal in the pattern give equal results
+SET @regCheck= '\\xE0\\x01';
+SELECT 0xE001 REGEXP '\\xE0\\x01' AS c1, 0xE001 REGEXP @regCheck AS c2;
+c1	c2
+1	1
 # Testing workaround N1: This makes the pattern to be a binary string:
 SET NAMES latin1;
 SET @regCheck= X'E001';

mysql-test/main/func_regexp_pcre.test

@@ -428,7 +428,13 @@ SELECT REGEXP_REPLACE('1 foo and bar', '(\\d+) foo and (\\d+ )?bar', '\\1 this a
 --echo # Testing a warning
 SET NAMES latin1;
 SET @regCheck= '\\xE0\\x01';
-SELECT 0xE001 REGEXP @regCheck;
+SELECT 0xE001 REGEXP @regCheck COLLATE latin1_bin;
+SELECT 0xE001 REGEXP CAST(@regCheck AS CHAR);
+
+--echo # Since 11.4 user variables have DERIVATION_COERCIBLE
+--echo # so a user variable and a literal in the pattern give equal results
+SET @regCheck= '\\xE0\\x01';
+SELECT 0xE001 REGEXP '\\xE0\\x01' AS c1, 0xE001 REGEXP @regCheck AS c2;
 
 --echo # Testing workaround N1: This makes the pattern to be a binary string:
 SET NAMES latin1;

mysql-test/main/get_diagnostics.result

@@ -774,10 +774,10 @@ Warning	1916	Got overflow when converting '-19999999999999999999' to INT. Value
 GET DIAGNOSTICS CONDITION 1 @var1 = MESSAGE_TEXT, @var2 = CLASS_ORIGIN;
 SELECT CHARSET(@var1), COLLATION(@var1), COERCIBILITY(@var1);
 CHARSET(@var1)	COLLATION(@var1)	COERCIBILITY(@var1)
-utf8mb3	utf8mb3_general_ci	2
+utf8mb3	utf8mb3_general_ci	4
 SELECT CHARSET(@var2), COLLATION(@var2), COERCIBILITY(@var2);
 CHARSET(@var2)	COLLATION(@var2)	COERCIBILITY(@var2)
-utf8mb3	utf8mb3_general_ci	2
+utf8mb3	utf8mb3_general_ci	4
 #
 # Command statistics
 #

mysql-test/main/user_var.result

@@ -137,7 +137,7 @@ drop table t1;
 set @a=_latin2'test';
 select charset(@a),collation(@a),coercibility(@a);
 charset(@a)	collation(@a)	coercibility(@a)
-latin2	latin2_general_ci	2
+latin2	latin2_general_ci	4
 select @a=_latin2'TEST';
 @a=_latin2'TEST'
 1
@@ -147,7 +147,7 @@ select @a=_latin2'TEST' collate latin2_bin;
 set @a=_latin2'test' collate latin2_general_ci;
 select charset(@a),collation(@a),coercibility(@a);
 charset(@a)	collation(@a)	coercibility(@a)
-latin2	latin2_general_ci	2
+latin2	latin2_general_ci	4
 select @a=_latin2'TEST';
 @a=_latin2'TEST'
 1
@@ -162,19 +162,19 @@ collation(@a:=_latin2'test')
 latin2_general_ci
 select coercibility(@a:=_latin2'test');
 coercibility(@a:=_latin2'test')
-2
+4
 select collation(@a:=_latin2'test' collate latin2_bin);
 collation(@a:=_latin2'test' collate latin2_bin)
 latin2_bin
 select coercibility(@a:=_latin2'test' collate latin2_bin);
 coercibility(@a:=_latin2'test' collate latin2_bin)
-2
+4
 select (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST';
 (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST'
 0
 select charset(@a),collation(@a),coercibility(@a);
 charset(@a)	collation(@a)	coercibility(@a)
-latin2	latin2_bin	2
+latin2	latin2_bin	4
 select (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST' collate latin2_general_ci;
 (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST' collate latin2_general_ci
 1
@@ -184,11 +184,11 @@ my_column
 0
 select @v, coercibility(@v);
 @v	coercibility(@v)
-NULL	2
+NULL	4
 set @v1=null, @v2=1, @v3=1.1, @v4=now();
 select coercibility(@v1),coercibility(@v2),coercibility(@v3),coercibility(@v4);
 coercibility(@v1)	coercibility(@v2)	coercibility(@v3)	coercibility(@v4)
-2	5	5	2
+4	5	5	4
 set session @honk=99;
 ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '@honk=99' at line 1
 select @@local.max_allowed_packet;

sql/item_func.cc

@@ -4734,7 +4734,7 @@ bool Item_func_set_user_var::fix_fields(THD *thd, Item **ref)
                              &my_charset_numeric : args[0]->collation.collation);
   collation.set(m_var_entry->charset(),
                 args[0]->collation.derivation == DERIVATION_NUMERIC ?
-                DERIVATION_NUMERIC : DERIVATION_IMPLICIT);
+                DERIVATION_NUMERIC : DERIVATION_COERCIBLE);
   switch (args[0]->result_type()) {
   case STRING_RESULT:
   case TIME_RESULT:
@@ -4799,7 +4799,7 @@ Item_func_set_user_var::fix_length_and_dec(THD *thd)
   }
   else
   {
-    collation.set(DERIVATION_IMPLICIT);
+    collation.set(DERIVATION_COERCIBLE);
     fix_length_and_charset(args[0]->max_char_length(),
                            args[0]->collation.collation);
   }
@@ -5676,7 +5676,7 @@ bool Item_func_get_user_var::fix_length_and_dec(THD *thd)
         set_handler(&type_handler_slonglong);
       break;
     case STRING_RESULT:
-      collation.set(m_var_entry->charset(), DERIVATION_IMPLICIT);
+      collation.set(m_var_entry->charset(), DERIVATION_COERCIBLE);
       max_length= MAX_BLOB_WIDTH - 1;
       set_handler(&type_handler_long_blob);
       if (m_var_entry->type_handler()->field_type() == MYSQL_TYPE_GEOMETRY)
@@ -5696,7 +5696,7 @@ bool Item_func_get_user_var::fix_length_and_dec(THD *thd)
   }
   else
   {
-    collation.set(&my_charset_bin, DERIVATION_IMPLICIT);
+    collation.set(&my_charset_bin, DERIVATION_COERCIBLE);
     null_value= 1;
     set_handler(&type_handler_long_blob);
     max_length= MAX_BLOB_WIDTH;