common/maps: Improve append in Scratch

This commit consolidates the reflective collections handling in `.Scratch` vs the `tpl` package so they use the same code paths.

This commit also adds support for a corner case where a typed slice is appended to a nil or empty `[]interface{}`.

Fixes #5275

common/collections/slice.go

@@ -0,0 +1,66 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package collections
+
+import (
+	"reflect"
+)
+
+// Slicer definse a very generic way to create a typed slice. This is used
+// in collections.Slice template func to get types such as Pages, PageGroups etc.
+// instead of the less useful []interface{}.
+type Slicer interface {
+	Slice(items interface{}) (interface{}, error)
+}
+
+// Slice returns a slice of all passed arguments.
+func Slice(args ...interface{}) interface{} {
+	if len(args) == 0 {
+		return args
+	}
+
+	first := args[0]
+	firstType := reflect.TypeOf(first)
+
+	if firstType == nil {
+		return args
+	}
+
+	if g, ok := first.(Slicer); ok {
+		v, err := g.Slice(args)
+		if err == nil {
+			return v
+		}
+
+		// If Slice fails, the items are not of the same type and
+		// []interface{} is the best we can do.
+		return args
+	}
+
+	if len(args) > 1 {
+		// This can be a mix of types.
+		for i := 1; i < len(args); i++ {
+			if firstType != reflect.TypeOf(args[i]) {
+				// []interface{} is the best we can do
+				return args
+			}
+		}
+	}
+
+	slice := reflect.MakeSlice(reflect.SliceOf(firstType), len(args), len(args))
+	for i, arg := range args {
+		slice.Index(i).Set(reflect.ValueOf(arg))
+	}
+	return slice.Interface()
+}