tpl/collections: Unwrap any interface value in sort and where

Hugo `0.55.0` introduced some new interface types for `Page` etc.

This worked great in general, but there were cases where this would fail in `where` and `sort`.

One such example would be sorting by `MenuItem.Page.Date` where `Page` on `MenuItem` was a small subset of the bigger `page.Page` interface.

This commit fixes that by unwrapping such interface values.

Fixes #5989

4 files changed, 153 insertions, 22 deletions

diff --git a/hugolib/menu_test.go b/hugolib/menu_test.go
index f1db3cb3a..4a2b17603 100644
--- a/hugolib/menu_test.go
+++ b/hugolib/menu_test.go
@@ -222,3 +222,54 @@ menu: "main"
 	b.AssertFileContent("public/index.html", "AMP and HTML|/blog/html-amp/|AMP only|/amp/blog/amp/|HTML only|/blog/html/|Home Sweet Home|/|")
 	b.AssertFileContent("public/amp/index.html", "AMP and HTML|/amp/blog/html-amp/|AMP only|/amp/blog/amp/|HTML only|/blog/html/|Home Sweet Home|/amp/|")
 }
+
+// https://github.com/gohugoio/hugo/issues/5989
+func TestMenuPageSortByDate(t *testing.T) {
+
+	b := newTestSitesBuilder(t).WithSimpleConfigFile()
+
+	b.WithContent("blog/a.md", `
+---
+Title: A
+date: 2019-01-01
+menu:
+  main:
+    identifier: "a"
+    weight: 1
+---
+
+`)
+
+	b.WithContent("blog/b.md", `
+---
+Title: B
+date: 2018-01-02
+menu:
+  main:
+    parent: "a"
+    weight: 100
+---
+
+`)
+
+	b.WithContent("blog/c.md", `
+---
+Title: C
+date: 2019-01-03
+menu:
+  main:
+    parent: "a"
+    weight: 10
+---
+
+`)
+
+	b.WithTemplatesAdded("index.html", `{{ range .Site.Menus.main }}{{ .Title }}|Children: 
+{{- $children := sort .Children ".Page.Date" "desc" }}{{ range $children }}{{ .Title }}|{{ end }}{{ end }}
+	
+`)
+
+	b.Build(BuildCfg{})
+
+	b.AssertFileContent("public/index.html", "A|Children:C|B|")
+}
diff --git a/tpl/collections/collections_test.go b/tpl/collections/collections_test.go
index 137c6fa3a..662536a24 100644
--- a/tpl/collections/collections_test.go
+++ b/tpl/collections/collections_test.go
@@ -819,6 +819,10 @@ func (x TstX) TstRv() string {
 	return "r" + x.B
 }
 
+func (x TstX) TstRv2() string {
+	return "r" + x.B
+}
+
 func (x TstX) unexportedMethod() string {
 	return x.unexported
 }
@@ -850,6 +854,33 @@ type TstX struct {
 	unexported string
 }
 
+type TstXIHolder struct {
+	XI TstXI
+}
+
+// Partially implemented by the TstX struct.
+type TstXI interface {
+	TstRv2() string
+}
+
+func ToTstXIs(slice interface{}) []TstXI {
+	s := reflect.ValueOf(slice)
+	if s.Kind() != reflect.Slice {
+		return nil
+	}
+	tis := make([]TstXI, s.Len())
+
+	for i := 0; i < s.Len(); i++ {
+		tsti, ok := s.Index(i).Interface().(TstXI)
+		if !ok {
+			return nil
+		}
+		tis[i] = tsti
+	}
+
+	return tis
+}
+
 func newDeps(cfg config.Provider) *deps.Deps {
 	l := langs.NewLanguage("en", cfg)
 	l.Set("i18nDir", "i18n")
diff --git a/tpl/collections/where.go b/tpl/collections/where.go
index 17d6552e6..42f0d370f 100644
--- a/tpl/collections/where.go
+++ b/tpl/collections/where.go
@@ -280,8 +280,16 @@ func evaluateSubElem(obj reflect.Value, elemName string) (reflect.Value, error)
 	typ := obj.Type()
 	obj, isNil := indirect(obj)
 
+	if obj.Kind() == reflect.Interface {
+		// If obj is an interface, we need to inspect the value it contains
+		// to see the full set of methods and fields.
+		// Indirect returns the value that it points to, which is what's needed
+		// below to be able to reflect on its fields.
+		obj = reflect.Indirect(obj.Elem())
+	}
+
 	// first, check whether obj has a method. In this case, obj is
-	// an interface, a struct or its pointer. If obj is a struct,
+	// a struct or its pointer. If obj is a struct,
 	// to check all T and *T method, use obj pointer type Value
 	objPtr := obj
 	if objPtr.Kind() != reflect.Interface && objPtr.CanAddr() {
diff --git a/tpl/collections/where_test.go b/tpl/collections/where_test.go
index 295b89051..cdef7aefb 100644
--- a/tpl/collections/where_test.go
+++ b/tpl/collections/where_test.go
@@ -38,13 +38,31 @@ func TestWhere(t *testing.T) {
 	d5 := d4.Add(1 * time.Hour)
 	d6 := d5.Add(1 * time.Hour)
 
-	for i, test := range []struct {
+	type testt struct {
 		seq    interface{}
 		key    interface{}
 		op     string
 		match  interface{}
 		expect interface{}
-	}{
+	}
+
+	createTestVariants := func(test testt) []testt {
+		testVariants := []testt{test}
+		if islice := ToTstXIs(test.seq); islice != nil {
+			variant := test
+			variant.seq = islice
+			expect := ToTstXIs(test.expect)
+			if expect != nil {
+				variant.expect = expect
+			}
+			testVariants = append(testVariants, variant)
+		}
+
+		return testVariants
+
+	}
+
+	for i, test := range []testt{
 		{
 			seq: []map[int]string{
 				{1: "a", 2: "m"}, {1: "c", 2: "d"}, {1: "e", 3: "m"},
@@ -217,6 +235,24 @@ func TestWhere(t *testing.T) {
 			},
 		},
 		{
+			seq: []TstXIHolder{
+				{&TstX{A: "a", B: "b"}}, {&TstX{A: "c", B: "d"}}, {&TstX{A: "e", B: "f"}},
+			},
+			key: "XI.TstRp", match: "rc",
+			expect: []TstXIHolder{
+				{&TstX{A: "c", B: "d"}},
+			},
+		},
+		{
+			seq: []TstXIHolder{
+				{&TstX{A: "a", B: "b"}}, {&TstX{A: "c", B: "d"}}, {&TstX{A: "e", B: "f"}},
+			},
+			key: "XI.A", match: "e",
+			expect: []TstXIHolder{
+				{&TstX{A: "e", B: "f"}},
+			},
+		},
+		{
 			seq: []map[string]Mid{
 				{"foo": Mid{Tst: TstX{A: "a", B: "b"}}}, {"foo": Mid{Tst: TstX{A: "c", B: "d"}}}, {"foo": Mid{Tst: TstX{A: "e", B: "f"}}},
 			},
@@ -522,28 +558,33 @@ func TestWhere(t *testing.T) {
 			},
 		},
 	} {
-		t.Run(fmt.Sprintf("test case %d for key %s", i, test.key), func(t *testing.T) {
-			var results interface{}
-			var err error
 
-			if len(test.op) > 0 {
-				results, err = ns.Where(test.seq, test.key, test.op, test.match)
-			} else {
-				results, err = ns.Where(test.seq, test.key, test.match)
-			}
-			if b, ok := test.expect.(bool); ok && !b {
-				if err == nil {
-					t.Errorf("[%d] Where didn't return an expected error", i)
-				}
-			} else {
-				if err != nil {
-					t.Errorf("[%d] failed: %s", i, err)
+		testVariants := createTestVariants(test)
+		for j, test := range testVariants {
+			name := fmt.Sprintf("[%d/%d] %T %s %s", i, j, test.seq, test.op, test.key)
+			t.Run(name, func(t *testing.T) {
+				var results interface{}
+				var err error
+
+				if len(test.op) > 0 {
+					results, err = ns.Where(test.seq, test.key, test.op, test.match)
+				} else {
+					results, err = ns.Where(test.seq, test.key, test.match)
 				}
-				if !reflect.DeepEqual(results, test.expect) {
-					t.Errorf("[%d] Where clause matching %v with %v, got %v but expected %v", i, test.key, test.match, results, test.expect)
+				if b, ok := test.expect.(bool); ok && !b {
+					if err == nil {
+						t.Fatalf("[%d] Where didn't return an expected error", i)
+					}
+				} else {
+					if err != nil {
+						t.Fatalf("[%d] failed: %s", i, err)
+					}
+					if !reflect.DeepEqual(results, test.expect) {
+						t.Fatalf("Where clause matching %v with %v in seq %v (%T),\ngot\n%v (%T) but expected\n%v (%T)", test.key, test.match, test.seq, test.seq, results, results, test.expect, test.expect)
+					}
 				}
-			}
-		})
+			})
+		}
 	}
 
 	var err error