ErrIllegalTagType prints tag type

tag.go

@@ -27,10 +27,16 @@ func (errTooManyTags) Error() string {
 
 var ErrTooManyTags = errTooManyTags{}
 
-type errIllegalTagType struct{}
+type errIllegalTagType struct {
+	tag any
+}
+
+func (e errIllegalTagType) Error() string {
+	return fmt.Sprintf("illegal tag type %T", e.tag)
+}
 
-func (errIllegalTagType) Error() string {
-	return "illegal tag type"
+func (errIllegalTagType) Is(other error) bool {
+	return other == ErrIllegalTagType
 }
 
 var ErrIllegalTagType = errIllegalTagType{}
@@ -82,7 +88,7 @@ func tagExpand(l int, rq *Request, tag interface{}, result []interface{}) ([]int
 	default:
 		return append(result, data), nil
 	}
-	return result, ErrIllegalTagType
+	return result, errIllegalTagType{tag: tag}
 }
 
 func TagExpand(rq *Request, tag interface{}) ([]interface{}, error) {

tag_test.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"html/template"
 	"reflect"
+	"strings"
 	"sync/atomic"
 	"testing"
 )
@@ -80,7 +81,10 @@ func TestTagExpand_IllegalTypesPanic(t *testing.T) {
 				if !ok {
 					t.Fail()
 				}
-				if e.Error() != ErrIllegalTagType.Error() {
+				if !errors.Is(e, ErrIllegalTagType) {
+					t.Fail()
+				}
+				if !strings.Contains(e.Error(), fmt.Sprintf("%T", tag)) {
 					t.Fail()
 				}
 			}()