parser/pageparser: Fix handling of commented out front matter

When the page parser was rewritten in 0.51, this was interpreted literally, but commented out front matter is used in the wild to "hide it from GitHub", e.g:

```
<!--
+++
title = "hello"
+++
-->
```

Fixes #5478

6 files changed, 65 insertions, 13 deletions

diff --git a/go.sum b/go.sum
index 9f5b5bea3..951e4c6f9 100644
--- a/go.sum
+++ b/go.sum
@@ -68,6 +68,7 @@ github.com/magefile/mage v1.4.0 h1:RI7B1CgnPAuu2O9lWszwya61RLmfL0KCdo+QyyI/Bhk=
 github.com/magefile/mage v1.4.0/go.mod h1:IUDi13rsHje59lecXokTfGX0QIzO45uVPlXnJYsXepA=
 github.com/magiconair/properties v1.8.0 h1:LLgXmsheXeRoUOBOjtwPQCWIYqM/LU1ayDtDePerRcY=
 github.com/magiconair/properties v1.8.0/go.mod h1:PppfXfuXeibc/6YijjN8zIbojt8czPbwD3XqdrwzmxQ=
+github.com/markbates/inflect v0.0.0-20171215194931-a12c3aec81a6 h1:LZhVjIISSbj8qLf2qDPP0D8z0uvOWAW5C85ly5mJW6c=
 github.com/markbates/inflect v0.0.0-20171215194931-a12c3aec81a6/go.mod h1:oTeZL2KHA7CUX6X+fovmK9OvIOFuqu0TwdQrZjLTh88=
 github.com/matryer/try v0.0.0-20161228173917-9ac251b645a2/go.mod h1:0KeJpeMD6o+O4hW7qJOT7vyQPKrWmj26uf5wMc/IiIs=
 github.com/mattn/go-isatty v0.0.4 h1:bnP0vzxcAdeI1zdubAl5PjU6zsERjGZb7raWodagDYs=
diff --git a/hugolib/page_content.go b/hugolib/page_content.go
index 75124a1df..a363249d5 100644
--- a/hugolib/page_content.go
+++ b/hugolib/page_content.go
@@ -83,8 +83,6 @@ Loop:
 
 		switch {
 		case it.Type == pageparser.TypeIgnore:
-		case it.Type == pageparser.TypeHTMLComment:
-			// Ignore. This is only a leading Front matter comment.
 		case it.Type == pageparser.TypeHTMLStart:
 			// This is HTML without front matter. It can still have shortcodes.
 			p.renderable = false
diff --git a/hugolib/page_test.go b/hugolib/page_test.go
index f9c26754b..cd8bd8ffc 100644
--- a/hugolib/page_test.go
+++ b/hugolib/page_test.go
@@ -1586,6 +1586,33 @@ CONTENT:{{ .Content }}
 	)
 }
 
+// https://github.com/gohugoio/hugo/issues/5478
+func TestPageWithCommentedOutFrontMatter(t *testing.T) {
+	b := newTestSitesBuilder(t)
+	b.WithSimpleConfigFile()
+
+	b.WithContent("page.md", `<!--
++++
+title = "hello"
++++
+-->
+This is the content.
+`)
+
+	b.WithTemplatesAdded("layouts/_default/single.html", `
+Title: {{ .Title }}
+Content:{{ .Content }}
+`)
+
+	b.CreateSites().Build(BuildCfg{})
+
+	b.AssertFileContent("public/page/index.html",
+		"Title: hello",
+		"Content:<p>This is the content.</p>",
+	)
+
+}
+
 // TODO(bep) this may be useful for other tests.
 func compareObjects(a interface{}, b interface{}) bool {
 	aStr := strings.Split(fmt.Sprintf("%v", a), "")
diff --git a/parser/pageparser/item.go b/parser/pageparser/item.go
index 644c20e87..b9d47b16e 100644
--- a/parser/pageparser/item.go
+++ b/parser/pageparser/item.go
@@ -108,7 +108,6 @@ const (
 
 	// page items
 	TypeHTMLStart          // document starting with < as first non-whitespace
-	TypeHTMLComment        // We ignore leading comments
 	TypeLeadSummaryDivider // <!--more-->,  # more
 	TypeFrontMatterYAML
 	TypeFrontMatterTOML
diff --git a/parser/pageparser/pagelexer.go b/parser/pageparser/pagelexer.go
index 94c1ff26b..5802c318b 100644
--- a/parser/pageparser/pagelexer.go
+++ b/parser/pageparser/pagelexer.go
@@ -53,6 +53,8 @@ type pageLexer struct {
 	summaryDivider []byte
 	// Set when we have parsed any summary divider
 	summaryDividerChecked bool
+	// Whether we're in a HTML comment.
+	isInHTMLComment bool
 
 	lexerShortcodeState
 
@@ -120,7 +122,7 @@ var (
 	delimYAML         = []byte("---")
 	delimOrg          = []byte("#+")
 	htmlCommentStart  = []byte("<!--")
-	htmlCOmmentEnd    = []byte("-->")
+	htmlCommentEnd    = []byte("-->")
 )
 
 func (l *pageLexer) next() rune {
@@ -195,6 +197,15 @@ func (l *pageLexer) consumeCRLF() bool {
 	return consumed
 }
 
+func (l *pageLexer) consumeToNextLine() {
+	for {
+		r := l.next()
+		if r == eof || isEndOfLine(r) {
+			return
+		}
+	}
+}
+
 func (l *pageLexer) consumeSpace() {
 	for {
 		r := l.next()
@@ -206,6 +217,10 @@ func (l *pageLexer) consumeSpace() {
 }
 
 func lexMainSection(l *pageLexer) stateFunc {
+	if l.isInHTMLComment {
+		return lexEndFromtMatterHTMLComment
+	}
+
 	// Fast forward as far as possible.
 	var l1, l2 int
 
@@ -312,16 +327,15 @@ LOOP:
 		case r == byteOrderMark:
 			l.emit(TypeIgnore)
 		case !isSpace(r) && !isEndOfLine(r):
-			// No front matter.
 			if r == '<' {
 				l.backup()
 				if l.hasPrefix(htmlCommentStart) {
-					right := l.index(htmlCOmmentEnd)
-					if right == -1 {
-						return l.errorf("starting HTML comment with no end")
-					}
-					l.pos += right + len(htmlCOmmentEnd)
-					l.emit(TypeHTMLComment)
+					// This may be commented out front mattter, which should
+					// still be read.
+					l.consumeToNextLine()
+					l.isInHTMLComment = true
+					l.emit(TypeIgnore)
+					continue LOOP
 				} else {
 					if l.pos > l.start {
 						l.emit(tText)
@@ -341,6 +355,19 @@ LOOP:
 	return lexMainSection
 }
 
+func lexEndFromtMatterHTMLComment(l *pageLexer) stateFunc {
+	l.isInHTMLComment = false
+	right := l.index(htmlCommentEnd)
+	if right == -1 {
+		return l.errorf("starting HTML comment with no end")
+	}
+	l.pos += right + len(htmlCommentEnd)
+	l.emit(TypeIgnore)
+
+	// Now move on to the shortcodes.
+	return lexMainSection
+}
+
 func lexDone(l *pageLexer) stateFunc {
 
 	// Done!
diff --git a/parser/pageparser/pageparser_intro_test.go b/parser/pageparser/pageparser_intro_test.go
index ba48a3ee3..901b216c8 100644
--- a/parser/pageparser/pageparser_intro_test.go
+++ b/parser/pageparser/pageparser_intro_test.go
@@ -60,7 +60,8 @@ var frontMatterTests = []lexerTest{
 	{"No front matter", "\nSome text.\n", []Item{tstSomeText, tstEOF}},
 	{"YAML front matter", "---\nfoo: \"bar\"\n---\n\nSome text.\n", []Item{tstFrontMatterYAML, tstSomeText, tstEOF}},
 	{"YAML empty front matter", "---\n---\n\nSome text.\n", []Item{nti(TypeFrontMatterYAML, ""), tstSomeText, tstEOF}},
-	{"YAML commented out front matter", "<!--\n---\nfoo: \"bar\"\n---\n-->\nSome text.\n", []Item{nti(TypeHTMLComment, "<!--\n---\nfoo: \"bar\"\n---\n-->"), tstSomeText, tstEOF}},
+	{"YAML commented out front matter", "<!--\n---\nfoo: \"bar\"\n---\n-->\nSome text.\n", []Item{nti(TypeIgnore, "<!--\n"), tstFrontMatterYAML, nti(TypeIgnore, "-->"), tstSomeText, tstEOF}},
+	{"YAML commented out front matter, no end", "<!--\n---\nfoo: \"bar\"\n---\nSome text.\n", []Item{nti(TypeIgnore, "<!--\n"), tstFrontMatterYAML, nti(tError, "starting HTML comment with no end")}},
 	// Note that we keep all bytes as they are, but we need to handle CRLF
 	{"YAML front matter CRLF", "---\r\nfoo: \"bar\"\r\n---\n\nSome text.\n", []Item{tstFrontMatterYAMLCRLF, tstSomeText, tstEOF}},
 	{"TOML front matter", "+++\nfoo = \"bar\"\n+++\n\nSome text.\n", []Item{tstFrontMatterTOML, tstSomeText, tstEOF}},
@@ -78,7 +79,6 @@ var frontMatterTests = []lexerTest{
 func TestFrontMatter(t *testing.T) {
 	t.Parallel()
 	for i, test := range frontMatterTests {
-
 		items := collect([]byte(test.input), false, lexIntroSection)
 		if !equal(items, test.items) {
 			got := crLfReplacer.Replace(fmt.Sprint(items))