xsl:use-attribute-sets in libxslt not conforming to the XSLT spec

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Hi,

I noticed since version 1.1.30 of libxslt, the attributes on the literal 
element are not merged correctly with the attributes specified on the 
element itself. The language in the spec [1] ("7.1.4 Named Attribute 
Sets") is as follows:

 > Thus, for a literal result element, attributes from attribute sets 
named in an xsl:use-attribute-sets attribute will be added first, in the 
order listed in the attribute; next, attributes specified on the literal 
result element will be added; finally, any attributes specified by 
xsl:attribute elements will be added. Since adding an attribute to an 
element replaces any existing attribute of that element with the same 
name, this means that attributes specified in attribute sets can be 
overridden by attributes specified on the literal result element itself.

Below is a small test case that has two attribute sets ("as1", "as2"; 
used in that order) + attributes specified on the element itself 
("element") and attributes specified via <xsl:attribute/> in the content 
of the literal element. Per spec, the output of the test case should be:

<bar a1="attr" a2="element" a3="as2" a4="as1"/>

With 1.1.30 and newer, the output is:

<bar a1="attr" a2="as2" a3="as2" a4="as1"/>

Before 1.1.30, the output was:

<bar a1="attr" a2="element" a3="as1" a4="as1"/>

The commit [2] that changed this behavior apparently fixed the order of 
attributes merged from multiple attribute sets (see a3/a4 attributes 
above) but in doing so, broke the handling of the attributes from the 
literal element (see a2 attribute above).

This, for example, breaks the docbook stylesheet customizations for the 
titlepage (which auto-generate an XSLT stylesheet using both an 
attribute sets and attributes on a literal element).

The test case:

---- a.xsl ----
<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
version="1.0">
   <xsl:template match="*">
     <xsl:apply-templates/>
   </xsl:template>

   <xsl:attribute-set name="as1">
     <xsl:attribute name="a1">as1</xsl:attribute>
     <xsl:attribute name="a2">as1</xsl:attribute>
     <xsl:attribute name="a3">as1</xsl:attribute>
     <xsl:attribute name="a4">as1</xsl:attribute>
   </xsl:attribute-set>

   <xsl:attribute-set name="as2">
     <xsl:attribute name="a1">as2</xsl:attribute>
     <xsl:attribute name="a2">as2</xsl:attribute>
     <xsl:attribute name="a3">as2</xsl:attribute>
   </xsl:attribute-set>

   <xsl:template match="foo">
     <bar xsl:use-attribute-sets="as1 as2" a1="element" a2="element">
       <xsl:attribute name="a1">attr</xsl:attribute>
     </bar>
   </xsl:template>
</xsl:stylesheet>

---- a.xml ----
<?xml version="1.0"?>
<foo/>

[1] https://www.w3.org/TR/1999/REC-xslt-19991116
[2] 
https://gitlab.gnome.org/GNOME/libxslt/commit/05f70130433478c1075ce8b6fdc4c4dadf51d33e

Regards,
Alexey.


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<?xml version="1.0"?>
<foo/>

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