scala.xml.parsing.XhtmlParser silently ignores invalid XML

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=== What steps will reproduce the problem (please be specific and use wikiformatting)? ===

scala.xml.parsing.XhtmlParser(scala.io.Source.fromString("<p/><b/>"))

=== What is the expected behavior? ===

Expected it to throw an exception saying "document must contain exactly one element"

=== What do you see instead? ===

Repl output:

:1:8: document must contain exactly one element ^
List(

, )
res1: scala.xml.NodeSeq = Document()

(Its printing an error to the console???)

=== Additional information ===
(for instance, a link to a relevant mailing list discussion)

=== What versions of the following are you using? ===

• Scala:
• Java:
• Operating system:

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4296?orig=1
Reporter: Alex Black (alexblack)

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@dcsobral said:
Speaking only of Xhtml, we have the following rules:

• It must be an XML Document
• It must conform to one of certain pre-defined DTDs.
• The root element of the document must be html.
• The root element of the document must contain an xmlns declaration.
• There must be a DOCTYPE declaration in the document prior to the root element.
• The DTD subset must not be used to override any parameter entities in the DTD

From the pre-requisite of it needing to be an XML Document, we have this requirement, indicating that it must follow the production rule prolog element Misc*, where prolog and Misc contain only the xml declaration, comments, processing instructions, spaces and newlines.

So, taken all together, XhtmlParser should complain about the following:

• There's more than one root.
• It does not conform to any of the valid DTDs.
• The root is not called html.
• The root does not contain a xmlns declaration.
• There's no DOCTYPE declaration prior to the root.

I expect such strictness to be detrimental instead of a feature for most applications. However, failing and still returning something -- that does not represent the input given -- is definitely very bad.

For whatever it is worth, this reminds me a lot of #4520. Though the symptom is rather different, I suspect they both arise from the lack of an exception.

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@SethTisue said:
The scala-xml library is now community-maintained. Issues with it are now tracked at https://github.com/scala/scala-xml/issues instead of here in the Scala JIRA.

Interested community members: if you consider this issue significant, feel free to open a new issue for it on GitHub, with links in both directions.