IFNULL didn't work as expected.

[AndrewDi]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

tidb>create table task_progress(sum_record bigint(20) not null);
Query OK, 0 rows affected (0.00 sec)

tidb>select ifnull(max(sum_record),0),max(sum_record) from task_progress;
+---------------------------+-----------------+
| ifnull(max(sum_record),0) | max(sum_record) |
+---------------------------+-----------------+
|                      NULL |            NULL |
+---------------------------+-----------------+
1 row in set (0.00 sec)

2. What did you expect to see?
   0

3. What did you see instead?
   NULL

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   current master branch build

[XuHuaiyu]

func (a *baseFuncDesc) typeInfer4MaxMin(ctx sessionctx.Context) {
	_, argIsScalaFunc := a.Args[0].(*expression.ScalarFunction)
	if argIsScalaFunc && a.Args[0].GetType().Tp == mysql.TypeFloat {
		// For scalar function, the result of "float32" is set to the "float64"
		// field in the "Datum". If we do not wrap a cast-as-double function on a.Args[0],
		// error would happen when extracting the evaluation of a.Args[0] to a ProjectionExec.
		tp := types.NewFieldType(mysql.TypeDouble)
		tp.Flen, tp.Decimal = mysql.MaxRealWidth, types.UnspecifiedLength
		types.SetBinChsClnFlag(tp)
		a.Args[0] = expression.BuildCastFunction(ctx, a.Args[0], tp)
	}
	a.RetTp = a.Args[0].GetType() // We should call a.Args[0].GetType().Clone() and reset the `NotNullFlag` for max/min in this line.
	if a.RetTp.Tp == mysql.TypeEnum || a.RetTp.Tp == mysql.TypeSet {
		a.RetTp = &types.FieldType{Tp: mysql.TypeString, Flen: mysql.MaxFieldCharLength}
	}
}

[AndrewDi]

fixed