Type declarations: Circular dependencies

[ChristianGruen]

(: x.xq :)
import module namespace a = 'A' at 'a.xqm';
()

(: a.xqm :)
module namespace a = 'A';
import module namespace b = 'B' at 'b.xqm';
declare type a:TYPE as item();

(: b.xqm :)
module namespace b = 'B';
import module namespace a = 'A' at 'a.xqm';
declare type b:TYPE as a:TYPE;

…raises: [XPST0051] Unknown type: Q{A}TYPE.

[GuntherRademacher]

The XQuery spec until April 8, 2025 contained this restriction:

The declaration of an item type (whether locally declared in a module or imported from a public declaration in an imported module) must precede any use of the item type name: that is, the name only becomes available in the static context of constructs that lexically follow the relevant item type declaration or module import. A consequence of this rule is that cyclic and self-referential definitions are not allowed.

In response to qtspecs issue #1520, this was lifted, allowing forward references of type declarations. But BaseX currently still has this restriction.