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/append_test.go

@@ -0,0 +1,70 @@
+// 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 (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAppend(t *testing.T) {
+	t.Parallel()
+
+	for i, test := range []struct {
+		start    interface{}
+		addend   []interface{}
+		expected interface{}
+	}{
+		{[]string{"a", "b"}, []interface{}{"c"}, []string{"a", "b", "c"}},
+		{[]string{"a", "b"}, []interface{}{"c", "d", "e"}, []string{"a", "b", "c", "d", "e"}},
+		{[]string{"a", "b"}, []interface{}{[]string{"c", "d", "e"}}, []string{"a", "b", "c", "d", "e"}},
+		{nil, []interface{}{"a", "b"}, []string{"a", "b"}},
+		{nil, []interface{}{nil}, []interface{}{nil}},
+		{[]interface{}{}, []interface{}{[]string{"c", "d", "e"}}, []string{"c", "d", "e"}},
+		{tstSlicers{&tstSlicer{"a"}, &tstSlicer{"b"}},
+			[]interface{}{&tstSlicer{"c"}},
+			tstSlicers{&tstSlicer{"a"}, &tstSlicer{"b"}, &tstSlicer{"c"}}},
+		{&tstSlicers{&tstSlicer{"a"}, &tstSlicer{"b"}},
+			[]interface{}{&tstSlicer{"c"}},
+			tstSlicers{&tstSlicer{"a"},
+				&tstSlicer{"b"},
+				&tstSlicer{"c"}}},
+		// Errors
+		{"", []interface{}{[]string{"a", "b"}}, false},
+		// No string concatenation.
+		{"ab",
+			[]interface{}{"c"},
+			false},
+	} {
+
+		errMsg := fmt.Sprintf("[%d]", i)
+
+		result, err := Append(test.start, test.addend...)
+
+		if b, ok := test.expected.(bool); ok && !b {
+			require.Error(t, err, errMsg)
+			continue
+		}
+
+		require.NoError(t, err, errMsg)
+
+		if !reflect.DeepEqual(test.expected, result) {
+			t.Fatalf("%s got\n%T: %v\nexpected\n%T: %v", errMsg, result, result, test.expected, test.expected)
+		}
+	}
+
+}