tpl/tplimpl: Fix template truth logic

Before this commit, due to a bug in Go's `text/template` package, this would print different output for typed nil interface values:

```
{{ if .AuthenticatedUser }}User is authenticated!{{ else }}{{ end }}
{{ if not .AuthenticatedUser }}{{ else }}}User is authenticated!{{ end }}
```

This commit works around this by wrapping every `if` and `with` with a custom `getif` template func with truth logic that matches `not`, `and` and `or`.

Those 3 template funcs from Go's stdlib are now pulled into Hugo's source tree and adjusted to support custom zero values, e.g. types that implement `IsZero`.

This means that you can now do:

```
{{ with .Date }}{{ . }}{{ end }}
```

And it would work as expected.

Fixes #5738

tpl/compare/init.go

@@ -71,6 +71,27 @@ func init() {
 			[][2]string{},
 		)
 
+		ns.AddMethodMapping(ctx.And,
+			[]string{"and"},
+			[][2]string{},
+		)
+
+		ns.AddMethodMapping(ctx.Or,
+			[]string{"or"},
+			[][2]string{},
+		)
+
+		// getif is used internally by Hugo. Do not document.
+		ns.AddMethodMapping(ctx.getIf,
+			[]string{"getif"},
+			[][2]string{},
+		)
+
+		ns.AddMethodMapping(ctx.Not,
+			[]string{"not"},
+			[][2]string{},
+		)
+
 		ns.AddMethodMapping(ctx.Conditional,
 			[]string{"cond"},
 			[][2]string{

tpl/compare/truth.go

@@ -0,0 +1,73 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+// The functions in this file is based on the Go source code, copyright
+// The Go Authors and  governed by a BSD-style license.
+//
+// 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 compare provides template functions for comparing values.
+package compare
+
+import (
+	"reflect"
+
+	"github.com/gohugoio/hugo/common/hreflect"
+)
+
+// Boolean logic, based on:
+// https://github.com/golang/go/blob/178a2c42254166cffed1b25fb1d3c7a5727cada6/src/text/template/funcs.go#L302
+
+func truth(arg reflect.Value) bool {
+	return hreflect.IsTruthfulValue(arg)
+}
+
+// getIf will return the given arg if it is considered truthful, else an empty string.
+func (*Namespace) getIf(arg reflect.Value) reflect.Value {
+	if truth(arg) {
+		return arg
+	}
+	return reflect.ValueOf("")
+}
+
+// And computes the Boolean AND of its arguments, returning
+// the first false argument it encounters, or the last argument.
+func (*Namespace) And(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
+	if !truth(arg0) {
+		return arg0
+	}
+	for i := range args {
+		arg0 = args[i]
+		if !truth(arg0) {
+			break
+		}
+	}
+	return arg0
+}
+
+// Or computes the Boolean OR of its arguments, returning
+// the first true argument it encounters, or the last argument.
+func (*Namespace) Or(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
+	if truth(arg0) {
+		return arg0
+	}
+	for i := range args {
+		arg0 = args[i]
+		if truth(arg0) {
+			break
+		}
+	}
+	return arg0
+}
+
+// Not returns the Boolean negation of its argument.
+func (*Namespace) Not(arg reflect.Value) bool {
+	return !truth(arg)
+}

tpl/compare/truth_test.go

@@ -0,0 +1,60 @@
+// Copyright 2019 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 compare
+
+import (
+	"reflect"
+	"testing"
+	"time"
+
+	"github.com/gohugoio/hugo/common/hreflect"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTruth(t *testing.T) {
+	n := New()
+
+	truthv, falsev := reflect.ValueOf(time.Now()), reflect.ValueOf(false)
+
+	assertTruth := func(t *testing.T, v reflect.Value, expected bool) {
+		if hreflect.IsTruthfulValue(v) != expected {
+			t.Fatal("truth mismatch")
+		}
+	}
+
+	t.Run("And", func(t *testing.T) {
+		assertTruth(t, n.And(truthv, truthv), true)
+		assertTruth(t, n.And(truthv, falsev), false)
+
+	})
+
+	t.Run("Or", func(t *testing.T) {
+		assertTruth(t, n.Or(truthv, truthv), true)
+		assertTruth(t, n.Or(falsev, truthv, falsev), true)
+		assertTruth(t, n.Or(falsev, falsev), false)
+	})
+
+	t.Run("Not", func(t *testing.T) {
+		assert := require.New(t)
+		assert.True(n.Not(falsev))
+		assert.False(n.Not(truthv))
+	})
+
+	t.Run("getIf", func(t *testing.T) {
+		assert := require.New(t)
+		assertTruth(t, n.getIf(reflect.ValueOf(nil)), false)
+		s := reflect.ValueOf("Hugo")
+		assert.Equal(s, n.getIf(s))
+	})
+}