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.go

@@ -0,0 +1,83 @@
+// 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"
+)
+
+// Append appends from to a slice to and returns the resulting slice.
+// If lenght of from is one and the only element is a slice of same type as to,
+// it will be appended.
+func Append(to interface{}, from ...interface{}) (interface{}, error) {
+	tov, toIsNil := indirect(reflect.ValueOf(to))
+
+	toIsNil = toIsNil || to == nil
+	var tot reflect.Type
+
+	if !toIsNil {
+		if tov.Kind() != reflect.Slice {
+			return nil, fmt.Errorf("expected a slice, got %T", to)
+		}
+
+		tot = tov.Type().Elem()
+		toIsNil = tov.Len() == 0
+
+		if len(from) == 1 {
+			fromv := reflect.ValueOf(from[0])
+			if fromv.Kind() == reflect.Slice {
+				if toIsNil {
+					// If we get nil []string, we just return the []string
+					return from[0], nil
+				}
+
+				fromt := reflect.TypeOf(from[0]).Elem()
+
+				// If we get []string []string, we append the from slice to to
+				if tot == fromt {
+					return reflect.AppendSlice(tov, fromv).Interface(), nil
+				}
+			}
+		}
+	}
+
+	if toIsNil {
+		return Slice(from...), nil
+	}
+
+	for _, f := range from {
+		fv := reflect.ValueOf(f)
+		if tot != fv.Type() {
+			return nil, fmt.Errorf("append element type mismatch: expected %v, got %v", tot, fv.Type())
+		}
+		tov = reflect.Append(tov, fv)
+	}
+
+	return tov.Interface(), nil
+}
+
+// indirect is borrowed from the Go stdlib: 'text/template/exec.go'
+// TODO(bep) consolidate
+func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
+	for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
+		if v.IsNil() {
+			return v, true
+		}
+		if v.Kind() == reflect.Interface && v.NumMethod() > 0 {
+			break
+		}
+	}
+	return v, false
+}