MDEV-23388 Assertion `args[0]->decimals == 0' failed in Item_func_round::fix_arg_int

Type_handler_temporal_result::Item_func_min_max_fix_attributes()
in an expression GREATEST(string,date), e.g:

  SELECT GREATEST('1', CAST('2020-12-12' AS DATE));

incorrectly evaluated decimals as 6 (like for DATETIME).

Adding a separate virtual implementation:
  Type_handler_date_common::Item_func_min_max_fix_attributes()
This makes the code simpler.

mysql-test/main/type_date.result

@@ -1103,5 +1103,35 @@ t2	CREATE TABLE `t2` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t2,t1;
 #
+# MDEV-23388 Assertion `args[0]->decimals == 0' failed in Item_func_round::fix_arg_int
+#
+SELECT ROUND(GREATEST('1', CAST('2020-12-12' AS DATE)));
+ROUND(GREATEST('1', CAST('2020-12-12' AS DATE)))
+20201212
+Warnings:
+Warning	1292	Truncated incorrect datetime value: '1'
+SELECT GREATEST('1', CAST('2020-12-12' AS DATE));
+Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
+def					GREATEST('1', CAST('2020-12-12' AS DATE))	10	10	10	Y	128	0	63
+GREATEST('1', CAST('2020-12-12' AS DATE))
+2020-12-12
+Warnings:
+Warning	1292	Truncated incorrect datetime value: '1'
+CREATE TABLE t1 (c_date DATE NOT NULL, c_int INT NOT NULL);
+CREATE TABLE t2 AS SELECT
+GREATEST(c_date,c_date),
+GREATEST(c_date,c_int),
+GREATEST(c_int,c_date)
+FROM t1;
+SHOW CREATE TABLE t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `GREATEST(c_date,c_date)` date NOT NULL,
+  `GREATEST(c_date,c_int)` date DEFAULT NULL,
+  `GREATEST(c_int,c_date)` date DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t2;
+DROP TABLE t1;
+#
 # End of 10.4 tests
 #

mysql-test/main/type_date.test

@@ -749,6 +749,28 @@ CREATE TABLE t2 AS SELECT FLOOR(a), CEIL(a),ROUND(a),TRUNCATE(a,0) FROM t1;
 SHOW CREATE TABLE t2;
 DROP TABLE t2,t1;
 
+--echo #
+--echo # MDEV-23388 Assertion `args[0]->decimals == 0' failed in Item_func_round::fix_arg_int
+--echo #
+
+SELECT ROUND(GREATEST('1', CAST('2020-12-12' AS DATE)));
+
+--disable_ps_protocol
+--enable_metadata
+SELECT GREATEST('1', CAST('2020-12-12' AS DATE));
+--disable_metadata
+--enable_ps_protocol
+
+CREATE TABLE t1 (c_date DATE NOT NULL, c_int INT NOT NULL);
+CREATE TABLE t2 AS SELECT
+  GREATEST(c_date,c_date),
+  GREATEST(c_date,c_int),
+  GREATEST(c_int,c_date)
+FROM t1;
+SHOW CREATE TABLE t2;
+DROP TABLE t2;
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

sql/sql_type.cc

@@ -4120,6 +4120,7 @@ bool Type_handler_temporal_result::
        Item_func_min_max_fix_attributes(THD *thd, Item_func_min_max *func,
                                         Item **items, uint nitems) const
 {
+  DBUG_ASSERT(func->field_type() != MYSQL_TYPE_DATE);
   bool rc= Type_handler::Item_func_min_max_fix_attributes(thd, func,
                                                           items, nitems);
   bool is_time= func->field_type() == MYSQL_TYPE_TIME;
@@ -4174,7 +4175,6 @@ bool Type_handler_temporal_result::
         DATETIME             DATETIME       no conversion
         DATETIME             TIMESTAMP      safe conversion
         DATETIME             DATE           safe conversion
-        DATE                 DATE           no conversion
         TIME                 TIME           no conversion
 
         Note, a function cannot return TIMESTAMP if it has non-TIMESTAMP
@@ -4191,9 +4191,6 @@ bool Type_handler_temporal_result::
       -------------------- ------------- -------
       TIMESTAMP            TIME          Not possible
       DATETIME             TIME          depends on OLD_MODE_ZERO_DATE_TIME_CAST
-      DATE                 TIMESTAMP     Not possible
-      DATE                 DATETIME      Not possible
-      DATE                 TIME          Not possible
       TIME                 TIMESTAMP     Not possible
       TIME                 DATETIME      Not possible
       TIME                 DATE          Not possible
@@ -4216,6 +4213,30 @@ bool Type_handler_temporal_result::
 }
 
 
+bool Type_handler_date_common::
+       Item_func_min_max_fix_attributes(THD *thd, Item_func_min_max *func,
+                                        Item **items, uint nitems) const
+{
+  func->fix_attributes_date();
+  if (func->maybe_null)
+    return false;
+  /*
+    We cannot trust the generic maybe_null value calculated during fix_fields().
+    If a conversion from non-temoral types to DATE happens,
+    then the result can be NULL (even if all arguments are not NULL).
+  */
+  for (uint i= 0; i < nitems; i++)
+  {
+    if (items[i]->type_handler()->cmp_type() != TIME_RESULT)
+    {
+      func->maybe_null= true;
+      break;
+    }
+  }
+  return false;
+}
+
+
 bool Type_handler_real_result::
        Item_func_min_max_fix_attributes(THD *thd, Item_func_min_max *func,
                                         Item **items, uint nitems) const

sql/sql_type.h

@@ -5508,6 +5508,8 @@ class Type_handler_date_common: public Type_handler_temporal_with_date
                                        Type_handler_hybrid_field_type *,
                                        Type_all_attributes *atrr,
                                        Item **items, uint nitems) const;
+  bool Item_func_min_max_fix_attributes(THD *thd, Item_func_min_max *func,
+                                        Item **items, uint nitems) const;
   void Item_param_set_param_func(Item_param *param,
                                  uchar **pos, ulong len) const;
 };