Fix WeightedPages in union etc.

We introduced a callback func() to get the owner Page in 0.55.0.

Sadly, funcs is  not comparable type in Go.

This commit replaces the func with a struct pointer that wraps the Page.

Fixes #5850

hugolib/collections_test.go

@@ -87,6 +87,37 @@ tags_weight: %d
 		`weightedPages:2::page.WeightedPages:[WeightedPage(10,"Page") WeightedPage(20,"Page")]`)
 }
 
+func TestUnionFunc(t *testing.T) {
+	assert := require.New(t)
+
+	pageContent := `
+---
+title: "Page"
+tags: ["blue", "green"]
+tags_weight: %d
+---
+
+`
+	b := newTestSitesBuilder(t)
+	b.WithSimpleConfigFile().
+		WithContent("page1.md", fmt.Sprintf(pageContent, 10), "page2.md", fmt.Sprintf(pageContent, 20),
+			"page3.md", fmt.Sprintf(pageContent, 30)).
+		WithTemplatesAdded("index.html", `
+{{ $unionPages := first 2 .Site.RegularPages | union .Site.RegularPages  }}
+{{ $unionWeightedPages := .Site.Taxonomies.tags.blue | union .Site.Taxonomies.tags.green }}
+{{ printf "unionPages: %T %d" $unionPages (len $unionPages) }} 
+{{ printf "unionWeightedPages: %T %d" $unionWeightedPages (len $unionWeightedPages) }}
+`)
+	b.CreateSites().Build(BuildCfg{})
+
+	assert.Equal(1, len(b.H.Sites))
+	require.Len(t, b.H.Sites[0].RegularPages(), 3)
+
+	b.AssertFileContent("public/index.html",
+		"unionPages: page.Pages 3",
+		"unionWeightedPages: page.WeightedPages 6")
+}
+
 func TestAppendFunc(t *testing.T) {
 	assert := require.New(t)