[xsl] Cannot mix nodes and atomic values - how comes?

["Michael Müller-Hillebrand mmh@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Hi experts,

We ran into an error that made me think about the idea of mixed data types in
sequences. Take this instance:

<?xml version="1.0" encoding="UTF-8"?>
<bars>
  <bar foo="x"/>
  <bar/>
</bars>

We need to process all <bar> and want to have a default value if @foo is not
present. When trying this XPath:

for $i in //bar/(@foo, 'no-foo')[1] return $i

we get the error "XPath failed due to: Cannot mix nodes and atomic values in
the result of a path expression"

When building the XPath the following way it runs fine and creates a sequence
of an attribute and a string:

for $b in //bar return for $i in $b/(@foo, 'no-foo')[1] return $i

What is the difference? Did I miss something in the specification?

Thanks,

- Michael

(Tested with Saxon 9.2/Kernow, 9.6/OxygenXML)