[BUG] xs:QName castable as returns incorrect results

[joewiz]

Describe the bug

"0" castable as xs:QName should return false(), and it does so in BaseX and Saxon, but in eXist it returns true().

(Oddly, eXist recognizes that "0" castable as xs:NCName is false(). I say "oddly," because QName consists of either a single NCName or 2 NCNames joined by a colon.)

Expected behavior

"0" castable as xs:QName should return false()

For more context, see https://en.wikipedia.org/wiki/QName and https://www.w3.org/TR/xml-names/#ns-qualnames.

To Reproduce

xquery version "3.1";

module namespace t="http://exist-db.org/xquery/test";

declare namespace test="http://exist-db.org/xquery/xqsuite";

declare
    %test:assertFalse
function t:test() {
    "0" castable as xs:QName
};

Screenshots

n/a

Context (please always complete the following information):

• OS: macOS 10.13.6
• eXist-db version: 5.2.0 and 5.3.0-SNAPSHOT with Fix issues with types and comparisons #3363
• Java Version: OpenJDK 1.8.0_242 (Zulu 8.44.0.11-CA-macosx) (build 1.8.0_242-b20)

Additional context

• How is eXist-db installed? 5.2.0 release via DMG and 5.3.0-SNAPSHOT via maven build
• Any custom changes in e.g. conf.xml? none

[joewiz]

The current develop branch (eXist 5.3.0-SNAPSHOT e92ea8f 20200915110349) is still affected.