I'm afraid I don't quite understand what you are after.  Could you 
please provide a sample stylesheet + document which demonstrates 
what is lacking, or where an error occurs?
Thanks,
Bill

Created attachment 24773 [details]
Example stylesheet and input document

I added an example stylesheet and input document. When you initially
run it, it will fail, complaining about being unable to evaluate the
expression $param3.
If you edit the input document and remove the reference to param3, the
processing will complete.

It appears to me that your request is actually for an additional 
function built into libxslt in order to test whether or not an XPath 
expression is valid.  If this is correct, I'm afraid this will not be 
possible, since it would make any stylesheet using it incompatible 
with other conforming processors.

However, I really don't understand why you want to use the 
exslt "dynamic" function dyn:evaluate, rather than a simpler approach 
such as (modifying your 'gallery.xsl'):

<?xml version='1.0'?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
version='1.1'>
<xsl:output method="html" />
<xsl:param name="param1" select="'hello world'"/>
<xsl:param name="param2" select="'goodbye world'"/>

<xsl:template name="decodeParams">
    <xsl:choose>
        <xsl:when test="@name='param1'">
            <xsl:value-of select="$param1"/>
        </xsl:when>
        <xsl:when test="@name='param2'">
            <xsl:value-of select="$param2"/>
        </xsl:when>
        <xsl:otherwise>
            <xsl:value-of select="concat('Undefined parameter ', 
@name)"/>
        </xsl:otherwise>
    </xsl:choose>
</xsl:template>

<xsl:template match="@*|node()">
    <xsl:choose>
    <xsl:when test="name() = 'param'">
        <xsl:call-template name="decodeParams"/>
    </xsl:when>
    <xsl:otherwise>
        <xsl:copy>
        <xsl:apply-templates select="@*|node()" />
        </xsl:copy>
    </xsl:otherwise>
    </xsl:choose>
</xsl:template>
</xsl:stylesheet>

which certainly works with all conforming xslt processors.

Yes, I understand it would be an extension. 
 
I can think of two reasons for such functionality being beneficial:

1. The solution you propose is obvious but, in my opinion, it violates
the coding maxim of avoiding redundancy. Every time you add or remove
a parameter you need to do so in two places. Yes, it is a small
inconvenience but with many documents over long periods of time and
many different coders, it reduces maintainability dramatically.

2. uniformity of functionality. You can find out if in an input
document a particular node or particular element exists in a
particular place. Why not be able to find out if a particular input
parameter exists? 

It is not a huge feature, just a convenience. Thanks for considering it!
 

I'm still considering your request, and although I am still not 
agreeable to implementing some "special function" for this purpose 
within libxslt, I am willing to seriously consider an argument that, 
according to the exslt.org site, the dyn:evaluate function should 
have the characteristic that: 
"... If the expression string passed as the second argument is an 
invalid XPath expression (including an empty string), this function 
returns an empty node set."

I guess we could argue whether or not a non-existent parameter is 
an "invalid XPath expression", but if you were to claim that it is, 
you would be on pretty strong ground :-).

I'm not sure how hard it would be to enhance the coding of 
dyn:evaluate to allow (i.e. not error-out on) this condition, but 
I'm wondering whether this would be an acceptable alternative for 
you?

Bill

I'm not familiar with the machinations of the exslt specification but
as an alternative to creatively interpreting the definition of
dyn:evaluate() would it not be possible to define another exslt
function to perform the test?

Otherwise, the your suggestion would work for me and sounds very
defensible from a technical standpoint :) 

GNOME is going to shut down bugzilla.gnome.org in favor of gitlab.gnome.org.
As part of that, we are mass-closing older open tickets in bugzilla.gnome.org
which have not seen updates for a longer time (resources are unfortunately
quite limited so not every ticket can get handled).

If you can still reproduce the situation described in this ticket in a recent
and supported software version, then please follow
  https://wiki.gnome.org/GettingInTouch/BugReportingGuidelines
and create a new ticket at
  https://gitlab.gnome.org/GNOME/libxslt/-/issues/

Thank you for your understanding and your help.