tpl/collections: Make index work with slice as the last arg

Fixes #6496

tpl/collections/index.go

@@ -28,11 +28,27 @@ import (
 // We deviate from the stdlib due to https://github.com/golang/go/issues/14751.
 //
 // TODO(moorereason): merge upstream changes.
-func (ns *Namespace) Index(item interface{}, indices ...interface{}) (interface{}, error) {
+func (ns *Namespace) Index(item interface{}, args ...interface{}) (interface{}, error) {
 	v := reflect.ValueOf(item)
 	if !v.IsValid() {
 		return nil, errors.New("index of untyped nil")
 	}
+
+	var indices []interface{}
+
+	if len(args) == 1 {
+		v := reflect.ValueOf(args[0])
+		if v.Kind() == reflect.Slice {
+			for i := 0; i < v.Len(); i++ {
+				indices = append(indices, v.Index(i).Interface())
+			}
+		}
+	}
+
+	if indices == nil {
+		indices = args
+	}
+
 	for _, i := range indices {
 		index := reflect.ValueOf(i)
 		var isNil bool