MDEV-20292 REPEAT(x,-1) returns a wrong data type

mysql-test/main/func_str.result

@@ -5068,5 +5068,21 @@ NULL
 
 DROP TABLE t1;
 #
+# MDEV-20292 REPEAT(x,-1) returns a wrong data type
+#
+CREATE OR REPLACE TABLE t1 (i BIGINT);
+INSERT INTO t1 VALUES (42);
+CREATE OR REPLACE TABLE t2 AS SELECT
+REPEAT(i,0) AS c0,
+REPEAT(i,-1) AS c1
+FROM t1;
+SHOW CREATE TABLE t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `c0` char(0) CHARACTER SET utf8 DEFAULT NULL,
+  `c1` char(0) CHARACTER SET utf8 DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1, t2;
+#
 # End of 10.3 tests
 #

mysql-test/main/func_str.test

@@ -2028,6 +2028,19 @@ SELECT LPAD( c, 0, '?' ) FROM t1;
 SELECT RPAD( c, 0, '?' ) FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-20292 REPEAT(x,-1) returns a wrong data type
+--echo #
+
+CREATE OR REPLACE TABLE t1 (i BIGINT);
+INSERT INTO t1 VALUES (42);
+CREATE OR REPLACE TABLE t2 AS SELECT
+  REPEAT(i,0) AS c0,
+  REPEAT(i,-1) AS c1
+FROM t1;
+SHOW CREATE TABLE t2;
+DROP TABLE t1, t2;
+
 
 --echo #
 --echo # End of 10.3 tests

sql/item_strfunc.cc

@@ -2967,21 +2967,21 @@ bool Item_func_repeat::fix_length_and_dec()
   if (agg_arg_charsets_for_string_result(collation, args, 1))
     return TRUE;
   DBUG_ASSERT(collation.collation != NULL);
-  if (args[1]->const_item())
+  if (args[1]->const_item() && !args[1]->is_expensive())
+  {
+    Longlong_hybrid nr= args[1]->to_longlong_hybrid();
+    if (args[1]->null_value || nr.neg())
+      fix_char_length(0);
+    else
     {
-    /* must be longlong to avoid truncation */
-    longlong count= args[1]->val_int();
-
       /* Assumes that the maximum length of a String is < INT_MAX32. */
-    /* Set here so that rest of code sees out-of-bound value as such. */
-    if (args[1]->null_value)
-      count= 0;
-    else if (count > INT_MAX32)
+      longlong count= nr.value();
+      if (count > INT_MAX32)
         count= INT_MAX32;
-
       ulonglong char_length= (ulonglong) args[0]->max_char_length() * count;
       fix_char_length_ulonglong(char_length);
     }
+  }
   else
   {
     max_length= MAX_BLOB_WIDTH;