[yxshee]

When a document has no XML declaration, a comment or DOCTYPE that comes before the root element is dropped from the parsed tree:

doc, _ := xmlquery.Parse(strings.NewReader(`<!-- license --><!DOCTYPE r><r/>`))
fmt.Println(doc.OutputXML(true))
// <?xml version="1.0"?><r></r>
fmt.Println(len(xmlquery.Find(doc, "//comment()")))
// 0

With a declaration first (<?xml version="1.0"?><!-- license --><!DOCTYPE r><r/>) both nodes are kept, and a processing instruction in the same position (<?pi?><r/>) is kept as well.

In parser.parse, the xml.Comment and xml.Directive cases attach the node relative to p.prev. Before anything else has been seen, p.prev is the document node and p.level == p.prev.level == 0, so the node is added with AddSibling(p.prev, node): it becomes a sibling of the document node and is unreachable from it. The xml.ProcInst case already handles this position by making the node a child of the document at level 1.

I noticed it while comparing xmlquery's OutputXML against libxml2's canonical form on generated documents; documents that started with a comment or a DOCTYPE and had no declaration lost those nodes.

The comment and directive cases now do what the processing-instruction case does when p.prev is the document node: add the node as a child of the document at level 1 and make it p.prev. The following root element is then attached as its sibling, as it is after a leading processing instruction today, so no synthetic <?xml version="1.0"?> is inserted in front of it (the same as for <?pi?><r/> on master).

Documents with a declaration, or with nothing before the root element, are unaffected: TestMissDeclaration still gets the synthetic declaration, TestDirective and TestMultipleProcInst are unchanged.

TestLeadingCommentAndDoctypeWithoutDeclaration checks the document children, OutputXML, //comment() and that the root's own comment child is untouched; TestLeadingCommentWithWhitespace covers a CRLF between the comment and the root. Both fail on master and pass here; go test ./... and go vet ./... are clean.