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

tpl/collections/index_test.go

@@ -14,6 +14,7 @@
 package collections
 
 import (
+	"fmt"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
@@ -40,22 +41,26 @@ func TestIndex(t *testing.T) {
 		{map[string]map[string]string{"a": {"b": "c"}}, []interface{}{"a", "b"}, "c", false},
 		{[]map[string]map[string]string{{"a": {"b": "c"}}}, []interface{}{0, "a", "b"}, "c", false},
 		{map[string]map[string]interface{}{"a": {"b": []string{"c", "d"}}}, []interface{}{"a", "b", 1}, "d", false},
+		{map[string]map[string]string{"a": {"b": "c"}}, []interface{}{[]string{"a", "b"}}, "c", false},
+
 		// errors
 		{nil, nil, nil, true},
 		{[]int{0, 1}, []interface{}{"1"}, nil, true},
 		{[]int{0, 1}, []interface{}{nil}, nil, true},
 		{tstNoStringer{}, []interface{}{0}, nil, true},
 	} {
-		errMsg := qt.Commentf("[%d] %v", i, test)
 
-		result, err := ns.Index(test.item, test.indices...)
+		c.Run(fmt.Sprint(i), func(c *qt.C) {
+			errMsg := qt.Commentf("[%d] %v", i, test)
 
-		if test.isErr {
+			result, err := ns.Index(test.item, test.indices...)
-			c.Assert(err, qt.Not(qt.IsNil), errMsg)
-			continue
-		}
 
-		c.Assert(err, qt.IsNil, errMsg)
+			if test.isErr {
-		c.Assert(result, qt.DeepEquals, test.expect, errMsg)
+				c.Assert(err, qt.Not(qt.IsNil), errMsg)
+				return
+			}
+			c.Assert(err, qt.IsNil, errMsg)
+			c.Assert(result, qt.DeepEquals, test.expect, errMsg)
+		})
 	}
 }