MDEV-23499 Assertion c.same_type(*o) failed

dict_col_t::same_encoding(), dict_col_t::same_format(): Allow
an instantaneous change of a column to a compatible encoding,
just like ha_innobase::can_convert_string() and similar functions do.

mysql-test/suite/innodb/r/instant_alter_bugs.result

@@ -407,4 +407,10 @@ CREATE TABLE t (i INT PRIMARY KEY) ENGINE=InnoDB;
 INSERT INTO t SET i=1;
 ALTER TABLE t ADD e CHAR(255) CHARACTER SET UTF32 FIRST, ALGORITHM=INSTANT;
 DROP TABLE t;
+#
+# MDEV-23499 Assertion c.same_type(*o) failed
+#
+CREATE TABLE t (pk SERIAL, b TEXT CHARACTER SET utf8) ENGINE=InnoDB;
+ALTER TABLE t MODIFY b TEXT CHARACTER SET utf8mb4 FIRST;
+DROP TABLE t;
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

mysql-test/suite/innodb/t/instant_alter_bugs.test

@@ -429,4 +429,11 @@ INSERT INTO t SET i=1;
 ALTER TABLE t ADD e CHAR(255) CHARACTER SET UTF32 FIRST, ALGORITHM=INSTANT;
 DROP TABLE t;
 
+--echo #
+--echo # MDEV-23499 Assertion c.same_type(*o) failed
+--echo #
+CREATE TABLE t (pk SERIAL, b TEXT CHARACTER SET utf8) ENGINE=InnoDB;
+ALTER TABLE t MODIFY b TEXT CHARACTER SET utf8mb4 FIRST;
+DROP TABLE t;
+
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

storage/innobase/dict/dict0mem.cc

@@ -120,7 +120,7 @@ bool dict_col_t::same_encoding(uint16_t a, uint16_t b)
 {
   if (const CHARSET_INFO *acs= get_charset(a, MYF(MY_WME)))
     if (const CHARSET_INFO *bcs= get_charset(b, MYF(MY_WME)))
-      return Charset(acs).same_encoding(bcs);
+      return Charset(bcs).encoding_allows_reinterpret_as(acs);
   return false;
 }
 

storage/innobase/include/dict0mem.h

@@ -728,7 +728,7 @@ struct dict_col_t{
   bool same_format(const dict_col_t &other) const
   {
     return same_type(other) && len >= other.len &&
-      mbminlen == other.mbminlen && mbmaxlen == other.mbmaxlen &&
+      mbminlen == other.mbminlen && mbmaxlen >= other.mbmaxlen &&
       !((prtype ^ other.prtype) & ~(DATA_NOT_NULL | DATA_VERSIONED |
                                     CHAR_COLL_MASK << 16 |
                                     DATA_LONG_TRUE_VARCHAR));