tpl: Cast IsSet key to int for indexed types

Don't assume that the user sends an int as the key when checking against
indexed types.

Fixes #3681

tpl/collections/collections.go

@@ -344,7 +344,11 @@ func (ns *Namespace) IsSet(a interface{}, key interface{}) (bool, error) {
 
 	switch av.Kind() {
 	case reflect.Array, reflect.Chan, reflect.Slice:
-		if int64(av.Len()) > kv.Int() {
+		k, err := cast.ToIntE(key)
+		if err != nil {
+			return false, fmt.Errorf("isset unable to use key of type %T as index", key)
+		}
+		if av.Len() > k {
 			return true, nil
 		}
 	case reflect.Map:

tpl/collections/collections_test.go

@@ -438,22 +438,24 @@ func TestIsSet(t *testing.T) {
 		key    interface{}
 		expect bool
 		isErr  bool
-		errStr string
 	}{
-		{[]interface{}{1, 2, 3, 5}, 2, true, false, ""},
-		{[]interface{}{1, 2, 3, 5}, 22, false, false, ""},
+		{[]interface{}{1, 2, 3, 5}, 2, true, false},
+		{[]interface{}{1, 2, 3, 5}, "2", true, false},
+		{[]interface{}{1, 2, 3, 5}, 2.0, true, false},
 
-		{map[string]interface{}{"a": 1, "b": 2}, "b", true, false, ""},
-		{map[string]interface{}{"a": 1, "b": 2}, "bc", false, false, ""},
+		{[]interface{}{1, 2, 3, 5}, 22, false, false},
 
-		{time.Now(), "Day", false, false, ""},
-		{nil, "nil", false, false, ""},
+		{map[string]interface{}{"a": 1, "b": 2}, "b", true, false},
+		{map[string]interface{}{"a": 1, "b": 2}, "bc", false, false},
+
+		{time.Now(), "Day", false, false},
+		{nil, "nil", false, false},
+		{[]interface{}{1, 2, 3, 5}, TstX{}, false, true},
 	} {
 		errMsg := fmt.Sprintf("[%d] %v", i, test)
 
 		result, err := ns.IsSet(test.a, test.key)
 		if test.isErr {
-			assert.EqualError(t, err, test.errStr, errMsg)
 			continue
 		}