hugolib: Fix Related when called from shortcode

Fixes #5071

hugolib/shortcode_test.go

@@ -150,6 +150,17 @@ func TestPositionalParamIndexOutOfBounds(t *testing.T) {
 	CheckShortCodeMatch(t, "{{< video 47238zzb >}}", "Playing Video Missing", wt)
 }
 
+// #5071
+func TestShortcodeRelated(t *testing.T) {
+	t.Parallel()
+	wt := func(tem tpl.TemplateHandler) error {
+		tem.AddTemplate("_internal/shortcodes/a.html", `{{ len (.Site.RegularPages.Related .Page) }}`)
+		return nil
+	}
+
+	CheckShortCodeMatch(t, "{{< a >}}", "0", wt)
+}
+
 // some repro issues for panics in Go Fuzz testing
 
 func TestNamedParamSC(t *testing.T) {

hugolib/site_sections.go

@@ -145,15 +145,16 @@ func (p *Page) Eq(other interface{}) bool {
 }
 
 func unwrapPage(in interface{}) (*Page, error) {
-	if po, ok := in.(*PageOutput); ok {
-		in = po.Page
-	}
-
-	pp, ok := in.(*Page)
-	if !ok {
+	switch v := in.(type) {
+	case *Page:
+		return v, nil
+	case *PageOutput:
+		return v.Page, nil
+	case *PageWithoutContent:
+		return v.Page, nil
+	default:
 		return nil, fmt.Errorf("%T not supported", in)
 	}
-	return pp, nil
 }
 
 // Sections returns this section's subsections, if any.