bug#26533: 26.0.50; xml-parse-region's symbol-qname argument is ignored

[David Engster <deng@randomsample.de>]

Christopher Wellons writes:
> A bug was introduced in aea67018 that causes the special "symbol-qnames"
> value for PARSE-NS to be ignored, as if it were nil. This information is
> discarded by the change to xml-parse-attlist, so functions further down
> the line see the argument as if it was set to nil.
>
> Here's an example of the bug:
>
>     (with-temp-buffer
>       (insert "<root a:b='c'></root>")
>       (let ((xml-default-ns ()))
>         (xml-parse-region nil nil nil nil 'symbol-qnames)))
>
> Prior to this commit (Emacs 25.1 and earlier) the result is:
>
>     ((root ((b . "c"))))
>
> After this commit:
>
>     ((root ((a:b . "c"))))
>
> This is the same as PARSE-NS being set to nil.

Thanks for the report.

You are right that the fix for bug #23440 was not correct. I now pushed
a hopefully better version to master.

Note however that your test above has two problems: First, it's invalid
XML since you're using an undeclared prefix (so the parser should rather
throw an error, but I'm not eager to make the xml parser more strict, as
there's a lot of invalid XML in the wild). Second, I don't understand
why you let-bind `xml-default-ns' to nil. This will break namespace
expansion, and it will actually do this for the whole Emacs session if
xml.el gets autoloaded during the above.

-David