tpl/collections: Fix some index cases where the indices given is a slice and be more lenient with nil inputs

See adjusted tests for detail.

Fixes #10489

tpl/collections/index.go

@@ -35,12 +35,9 @@ import (
 func (ns *Namespace) Index(item any, args ...any) (any, error) {
 	v := reflect.ValueOf(item)
 	if !v.IsValid() {
-		return nil, errors.New("index of untyped nil")
-	}
-
-	lowerm, ok := item.(maps.Params)
-	if ok {
-		return lowerm.Get(cast.ToStringSlice(args)...), nil
+		// See issue 10489
+		// This used to be an error.
+		return nil, nil
 	}
 
 	var indices []any
@@ -51,18 +48,25 @@ func (ns *Namespace) Index(item any, args ...any) (any, error) {
 			for i := 0; i < v.Len(); i++ {
 				indices = append(indices, v.Index(i).Interface())
 			}
+		} else {
+			indices = append(indices, args[0])
 		}
+	} else {
+		indices = args
 	}
 
-	if indices == nil {
-		indices = args
+	lowerm, ok := item.(maps.Params)
+	if ok {
+		return lowerm.Get(cast.ToStringSlice(indices)...), nil
 	}
 
 	for _, i := range indices {
 		index := reflect.ValueOf(i)
 		var isNil bool
 		if v, isNil = indirect(v); isNil {
-			return nil, errors.New("index of nil pointer")
+			// See issue 10489
+			// This used to be an error.
+			return nil, nil
 		}
 		switch v.Kind() {
 		case reflect.Array, reflect.Slice, reflect.String:

tpl/collections/index_test.go

@@ -30,6 +30,11 @@ func TestIndex(t *testing.T) {
 	c := qt.New(t)
 	ns := New(&deps.Deps{Language: langs.NewDefaultLanguage(config.New())})
 
+	var (
+		emptyInterface any
+		nilPointer     *int
+	)
+
 	for i, test := range []struct {
 		item    any
 		indices []any
@@ -45,16 +50,23 @@ func TestIndex(t *testing.T) {
 		{map[string]map[string]string{"a": {"b": "c"}}, []any{"a", "b"}, "c", false},
 		{[]map[string]map[string]string{{"a": {"b": "c"}}}, []any{0, "a", "b"}, "c", false},
 		{map[string]map[string]any{"a": {"b": []string{"c", "d"}}}, []any{"a", "b", 1}, "d", false},
-		{map[string]map[string]string{"a": {"b": "c"}}, []any{[]string{"a", "b"}}, "c", false},
 		{maps.Params{"a": "av"}, []any{"A"}, "av", false},
 		{maps.Params{"a": map[string]any{"b": "bv"}}, []any{"A", "B"}, "bv", false},
+
+		// These used to be errors.
+		// See issue 10489.
+		{nil, nil, nil, false},
+		{nil, []any{0}, nil, false},
+		{emptyInterface, []any{0}, nil, false},
+		{nilPointer, []any{0}, nil, false},
+
 		// errors
-		{nil, nil, nil, true},
 		{[]int{0, 1}, []any{"1"}, nil, true},
 		{[]int{0, 1}, []any{nil}, nil, true},
 		{tstNoStringer{}, []any{0}, nil, true},
 	} {
-		c.Run(fmt.Sprint(i), func(c *qt.C) {
+
+		c.Run(fmt.Sprintf("vararg %d", i), func(c *qt.C) {
 			errMsg := qt.Commentf("[%d] %v", i, test)
 
 			result, err := ns.Index(test.item, test.indices...)
@@ -66,5 +78,18 @@ func TestIndex(t *testing.T) {
 			c.Assert(err, qt.IsNil, errMsg)
 			c.Assert(result, qt.DeepEquals, test.expect, errMsg)
 		})
+
+		c.Run(fmt.Sprintf("slice %d", i), func(c *qt.C) {
+			errMsg := qt.Commentf("[%d] %v", i, test)
+
+			result, err := ns.Index(test.item, test.indices)
+
+			if test.isErr {
+				c.Assert(err, qt.Not(qt.IsNil), errMsg)
+				return
+			}
+			c.Assert(err, qt.IsNil, errMsg)
+			c.Assert(result, qt.DeepEquals, test.expect, errMsg)
+		})
 	}
 }