Thank you, Christian, now I understand!

Incidentally, nevertheless I am not sure the following behaviour is correct:

basex "<_ xmlns='bar'>{element foo {namespace {''} {'bar'}}}</_>/*"

[XQDY0102] Duplicate namespace declaration: ''.

As the dynamic constructor is located in a static context with a default namespace, the foo should be interpreted as a name in that namespace, thus "if the name of an element in an element constructor is in no namespace" does not apply.

Cordial greetings,

Hans-Jürgen

PS: By contrast, this works:

basex "<_ xmlns='bar'>{<foo/>}</_>/*"

PPS: And this also works:

basex "<_ xmlns:f='bar'>{element f:foo {namespace {'f'} {'bar'}}}</_>/*"

<f:foo xmlns:f="bar"/>

Am Sonntag, 30. Januar 2022, 17:54:24 MEZ hat Christian Grün <christian.gruen@gmail.com> Folgendes geschrieben:

Hi Hans-Jürgen,

> basex "<foo>{namespace {''}{'bar'}}</foo>"
> =>
> [XQDY0102] Duplicate namespace declaration: ''.

This is expected. The specification states that “[…] if the name of an
element in an element constructor is in no namespace, creating a
default namespace for that element using a computed namespace
constructor is an error [err:XQDY0102]”

You’ll need to include a (static) namespace URI as part of the direct
element constructor:

<foo xmlns='bar'/>

Best,
Christian

[1]

https://www.w3.org/TR/xquery-31/#id-ns-nodes-on-elements