tpl/collections: Fix union when the first slice is empty

Fixes #3686

Author: bep

tpl/collections/collections.go

@@ -494,6 +494,7 @@ type intersector struct {
 }
 
 func (i *intersector) appendIfNotSeen(v reflect.Value) {
+
 	vi := v.Interface()
 	if !i.seen[vi] {
 		i.r = reflect.Append(i.r, v)
@@ -565,6 +566,14 @@ func (ns *Namespace) Union(l1, l2 interface{}) (interface{}, error) {
 				}
 			}
 
+			if !l1vv.IsValid() {
+				// The first slice may be empty. Pick the first value of the second
+				// to use as a prototype.
+				if l2v.Len() > 0 {
+					l1vv = l2v.Index(0)
+				}
+			}
+
 			for j := 0; j < l2v.Len(); j++ {
 				l2vv := l2v.Index(j)
 

tpl/collections/collections_test.go

@@ -666,6 +666,13 @@ func TestUnion(t *testing.T) {
 		{pagesVals{p1v}, pagesVals{p3v, p3v}, pagesVals{p1v, p3v}, false},
 		{[]interface{}{p1, p4}, []interface{}{p4, p2, p2}, []interface{}{p1, p4, p2}, false},
 		{[]interface{}{p1v}, []interface{}{p3v, p3v}, []interface{}{p1v, p3v}, false},
+		// #3686
+		{[]interface{}{p1v}, []interface{}{}, []interface{}{p1v}, false},
+		{[]interface{}{}, []interface{}{p1v}, []interface{}{p1v}, false},
+		{pagesPtr{p1}, pagesPtr{}, pagesPtr{p1}, false},
+		{pagesVals{p1v}, pagesVals{}, pagesVals{p1v}, false},
+		{pagesPtr{}, pagesPtr{p1}, pagesPtr{p1}, false},
+		{pagesVals{}, pagesVals{p1v}, pagesVals{p1v}, false},
 
 		// errors
 		{"not array or slice", []string{"a"}, false, true},