xslt 3.0, and use of 'as' attribute with xsl:variable

Hi all,
   I'm trying to understand a simple concept with XSLT 3.0, when we use
'as' attribute with xsl:variable instruction. I'm using Saxon HE 12.2.

I'd like to understand these concepts with a non schema aware XSLT
transformation.

My XSLT stylesheet is as follows (this one doesn't use 'as' attribute on
xsl:variable instruction),

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                         version="3.0">

    <xsl:output method="xml" indent="yes"/>

    <xsl:variable name="var1" select="'4'"/>

    <xsl:template match="/">
         <result>
             <xsl:value-of select="$var1 + 3"/>
         </result>
    </xsl:template>

</xsl:stylesheet>

Running the above XSLT transformation, produces following error,

Error in xsl:value-of/@select on line 13 column 44 of test1.xsl:
  XPTY0004  Arithmetic operator is not defined for arguments of types
(xs:string, xs:integer)

I'm fine with above produced XSLT transformation error.

Now I change the above cited XSLT stylesheet to following (I'm introducing
an 'as' attribute to xsl:variable instruction),

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                         xmlns:xs="http://www.w3.org/2001/XMLSchema"
                         exclude-result-prefixes="xs"
                         version="3.0">

    <xsl:output method="xml" indent="yes"/>

    <xsl:variable name="var1" select="'4'" as="xs:integer"/>

    <xsl:template match="/">
        <result>
            <xsl:value-of select="$var1 + 3"/>
        </result>
    </xsl:template>

</xsl:stylesheet>

When I run this new XSLT transformation, I get following error,

Error at xsl:variable on line 9 column 61 of test1.xsl:
  XTTE0570  The required item type of the value of variable $var1 is
xs:integer. The
  supplied value is of type xs:string

This error confuses me. I think that, the new XSLT transformation that I've
tried should have succeeded.

The XSLT 3.0 spec says following, about xsl:variable instruction,

[1]
<quote>
An optional "as" attribute specifies the required type of the variable. The
value of the "as" attribute is a 'SequenceType'.

If the "as" attribute is specified, then the supplied value of the variable
is converted to the required type. [ERR XTTE0570] It is a type error if the
supplied value of a variable cannot be converted to the required type. If
the "as" attribute is omitted, the supplied value of the variable is used
directly, and no conversion takes place.
</quote>

I think the string value '4' of variable var1, could be easily converted to
an xs:integer value (since, the string value '4', contains all numeric
characters).

By reading the above quoted text [1] from XSLT 3.0 spec, I think that, the
second XSLT transformation that I've cited should have been able to do an
addition $var1 + 3 without issues.

Any thoughts please.


-- 
Regards,
Mukul Gandhi