MDEV-39530 Collation mix issue after update

mysql-test/main/user_var.result

@@ -162,13 +162,13 @@ collation(@a:=_latin2'test')
 latin2_general_ci
 select coercibility(@a:=_latin2'test');
 coercibility(@a:=_latin2'test')
-6
+5
 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)
-6
+5
 select (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST';
 (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST'
 0

mysql-test/main/user_var_collation.result

@@ -0,0 +1,24 @@
+#
+# MDEV-39530 Collation mix issue after update
+#
+# Reproduce the environment from the report: character_set_collations
+# maps utf8mb3 to utf8mb3_uca1400_ai_ci and the connection uses that
+# collation.
+SET NAMES utf8mb3 COLLATE utf8mb3_uca1400_ai_ci;
+SELECT @@character_set_collations, @@collation_connection;
+@@character_set_collations	@@collation_connection
+utf8mb3=utf8mb3_uca1400_ai_ci,ucs2=ucs2_uca1400_ai_ci,utf8mb4=utf8mb4_uca1400_ai_ci,utf16=utf16_uca1400_ai_ci,utf32=utf32_uca1400_ai_ci	utf8mb3_uca1400_ai_ci
+CREATE TABLE file (
+`ID` int(10) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
+`Path` varchar(800) NOT NULL DEFAULT ''
+) ENGINE=MyISAM DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_unicode_ci;
+INSERT INTO file VALUES (1,"text");
+# This query used to work in 11.4 but raises
+# ER_CANT_AGGREGATE_2COLLATIONS in 12.1+
+SELECT IF((@p:=(SELECT f.Path FROM file f WHERE f.ID=1 AND f.Path REGEXP "^xx"))!="",1,2);
+IF((@p:=(SELECT f.Path FROM file f WHERE f.ID=1 AND f.Path REGEXP "^xx"))!="",1,2)
+2
+DROP TABLE file;
+#
+# End of test
+#

mysql-test/main/user_var_collation.test

@@ -0,0 +1,26 @@
+--echo #
+--echo # MDEV-39530 Collation mix issue after update
+--echo #
+
+--echo # Reproduce the environment from the report: character_set_collations
+--echo # maps utf8mb3 to utf8mb3_uca1400_ai_ci and the connection uses that
+--echo # collation.
+SET NAMES utf8mb3 COLLATE utf8mb3_uca1400_ai_ci;
+SELECT @@character_set_collations, @@collation_connection;
+
+CREATE TABLE file (
+  `ID` int(10) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
+  `Path` varchar(800) NOT NULL DEFAULT ''
+) ENGINE=MyISAM DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_unicode_ci;
+
+INSERT INTO file VALUES (1,"text");
+
+--echo # This query used to work in 11.4 but raises
+--echo # ER_CANT_AGGREGATE_2COLLATIONS in 12.1+
+SELECT IF((@p:=(SELECT f.Path FROM file f WHERE f.ID=1 AND f.Path REGEXP "^xx"))!="",1,2);
+
+DROP TABLE file;
+
+--echo #
+--echo # End of test
+--echo #

sql/item_func.cc

@@ -4840,9 +4840,17 @@ bool Item_func_set_user_var::fix_fields(THD *thd, Item **ref)
   if (!m_var_entry->charset() || !null_item)
     m_var_entry->set_charset(args[0]->collation.derivation == DERIVATION_NUMERIC ?
                              &my_charset_numeric : args[0]->collation.collation);
+  /*
+    A user variable assignment (e.g. @p:=expr) must expose the same collation
+    derivation as reading the variable back with Item_func_get_user_var, which
+    uses DERIVATION_USERVAR (see MDEV-35041). Using DERIVATION_COERCIBLE here
+    made the assignment as strong as a string literal, causing "Illegal mix of
+    collations" conflicts against literals that carry the (possibly different)
+    connection collation (MDEV-39530).
+  */
   collation.set(m_var_entry->charset(),
                 args[0]->collation.derivation == DERIVATION_NUMERIC ?
-                DERIVATION_NUMERIC : DERIVATION_COERCIBLE);
+                DERIVATION_NUMERIC : DERIVATION_USERVAR);
   switch (args[0]->result_type()) {
   case STRING_RESULT:
   case TIME_RESULT:
@@ -4907,7 +4915,7 @@ Item_func_set_user_var::fix_length_and_dec(THD *thd)
   }
   else
   {
-    collation.set(DERIVATION_COERCIBLE);
+    collation.set(DERIVATION_USERVAR);
     fix_length_and_charset(args[0]->max_char_length(),
                            args[0]->collation.collation);
   }