tpl: Extend merge to accept multiple parameters

Fixes #7595

tpl/collections/merge.go

@@ -17,17 +17,35 @@ import (
 	"reflect"
 	"strings"
 
-	"github.com/gohugoio/hugo/common/maps"
-
 	"github.com/gohugoio/hugo/common/hreflect"
+	"github.com/gohugoio/hugo/common/maps"
 
 	"github.com/pkg/errors"
 )
 
-// Merge creates a copy of dst and merges src into it.
-// Currently only maps supported. Key handling is case insensitive.
-func (ns *Namespace) Merge(src, dst interface{}) (interface{}, error) {
+// Merge creates a copy of the final parameter and merges the preceeding
+// parameters into it in reverse order.
+// Currently only maps are supported. Key handling is case insensitive.
+func (ns *Namespace) Merge(params ...interface{}) (interface{}, error) {
+	if len(params) < 2 {
+		return nil, errors.New("merge requires at least two parameters")
+	}
 
+	var err error
+	result := params[len(params)-1]
+
+	for i := len(params) - 2; i >= 0; i-- {
+		result, err = ns.merge(params[i], result)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return result, nil
+}
+
+// merge creates a copy of dst and merges src into it.
+func (ns *Namespace) merge(src, dst interface{}) (interface{}, error) {
 	vdst, vsrc := reflect.ValueOf(dst), reflect.ValueOf(src)
 
 	if vdst.Kind() != reflect.Map {
@@ -60,14 +78,12 @@ func caseInsensitiveLookup(m, k reflect.Value) (reflect.Value, bool) {
 		if strings.EqualFold(k.String(), key.String()) {
 			return m.MapIndex(key), true
 		}
-
 	}
 
 	return reflect.Value{}, false
 }
 
 func mergeMap(dst, src reflect.Value) reflect.Value {
-
 	out := reflect.MakeMap(dst.Type())
 
 	// If the destination is Params, we must lower case all keys.