expression: fix incorrect copy of flag in truncate function (#55284)

close #53546

Author: tuziemon

pkg/expression/builtin_math.go

@@ -2047,7 +2047,10 @@ func (c *truncateFunctionClass) getFunction(ctx BuildContext, args []Expression)
 		bf.tp.SetDecimalUnderLimit(calculateDecimal4RoundAndTruncate(ctx, args, argTp))
 		bf.tp.SetFlenUnderLimit(args[0].GetType(ctx.GetEvalCtx()).GetFlen() - args[0].GetType(ctx.GetEvalCtx()).GetDecimal() + bf.tp.GetDecimal())
 	}
-	bf.tp.AddFlag(args[0].GetType(ctx.GetEvalCtx()).GetFlag())
+	argFieldTp := args[0].GetType(ctx.GetEvalCtx())
+	if mysql.HasUnsignedFlag(argFieldTp.GetFlag()) {
+		bf.tp.AddFlag(mysql.UnsignedFlag)
+	}
 
 	var sig builtinFunc
 	switch argTp {

tests/integrationtest/r/expression/constant_fold.result

@@ -24,3 +24,9 @@ utf8mb4_general_ci
 select collation(ifnull(concat(NULL),ifnull(concat(NULL),id))) from t1;
 collation(ifnull(concat(NULL),ifnull(concat(NULL),id)))
 utf8mb4_general_ci
+drop table if exists t1;
+create table t1 (c1 int);
+insert into t1 values (null);
+select truncate(1,c1), truncate(1,c1) is not NULL from t1;
+truncate(1,c1)	truncate(1,c1) is not NULL
+NULL	0

tests/integrationtest/t/expression/constant_fold.test

@@ -11,3 +11,9 @@ select collation(ifnull(concat(NULL),ifnull(concat(id),'~'))) from t;
 select collation(ifnull(concat(id),ifnull(concat(id),'~'))) from t;
 select collation(ifnull(concat(NULL),id)) from t1;
 select collation(ifnull(concat(NULL),ifnull(concat(NULL),id))) from t1;
+
+# https://github.com/pingcap/tidb/issues/53546
+drop table if exists t1;
+create table t1 (c1 int);
+insert into t1 values (null);
+select truncate(1,c1), truncate(1,c1) is not NULL from t1;