compare, hugolib, tpl: Add Eqer interface

And use it in `eq` and `ne` so `Page` values can be compared directly in the templates without thinking about it being a `Page` or a `PageOutput` wrapper.

Fixes #3807

2 files changed, 33 insertions, 0 deletions

diff --git a/tpl/compare/compare.go b/tpl/compare/compare.go
index 1482c0afe..2df03b7db 100644
--- a/tpl/compare/compare.go
+++ b/tpl/compare/compare.go
@@ -18,6 +18,8 @@ import (
 	"reflect"
 	"strconv"
 	"time"
+
+	"github.com/gohugoio/hugo/compare"
 )
 
 // New returns a new instance of the compare-namespaced template functions.
@@ -85,6 +87,14 @@ func (*Namespace) Default(dflt interface{}, given ...interface{}) (interface{},
 
 // Eq returns the boolean truth of arg1 == arg2.
 func (*Namespace) Eq(x, y interface{}) bool {
+
+	// hugolib.Page implements compare.Eqer to make Page and PageOutput comparable.
+	if e1, ok := x.(compare.Eqer); ok {
+		if e2, ok := y.(compare.Eqer); ok {
+			return e1.Eq(e2)
+		}
+	}
+
 	normalize := func(v interface{}) interface{} {
 		vv := reflect.ValueOf(v)
 		switch vv.Kind() {
diff --git a/tpl/compare/compare_test.go b/tpl/compare/compare_test.go
index 57f061f4d..9adbcf574 100644
--- a/tpl/compare/compare_test.go
+++ b/tpl/compare/compare_test.go
@@ -26,6 +26,25 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+type tstEqerType1 string
+type tstEqerType2 string
+
+func (t tstEqerType2) Eq(other interface{}) bool {
+	return cast.ToString(t) == cast.ToString(other)
+}
+
+func (t tstEqerType2) String() string {
+	return string(t)
+}
+
+func (t tstEqerType1) Eq(other interface{}) bool {
+	return cast.ToString(t) == cast.ToString(other)
+}
+
+func (t tstEqerType1) String() string {
+	return string(t)
+}
+
 type tstCompareType int
 
 const (
@@ -148,6 +167,10 @@ func doTestCompare(t *testing.T, tp tstCompareType, funcUnderTest func(a, b inte
 		{"a", "a", 0},
 		{"a", "b", -1},
 		{"b", "a", 1},
+		{tstEqerType1("a"), tstEqerType1("a"), 0},
+		{tstEqerType1("a"), tstEqerType2("a"), 0},
+		{tstEqerType2("a"), tstEqerType1("a"), 0},
+		{tstEqerType2("a"), tstEqerType1("b"), -1},
 	} {
 		result := funcUnderTest(test.left, test.right)
 		success := false