If you have:

module A {
namespace ...;
prefix A;
// without revision element
...
}

module B {
namespace ...;
prefix B;
import A

// also without revision-date
...
}

then the getImports() method on module B does not provide module A. It seems that this case is skipped over in AbstractEffectiveModule.resolveModuleImports().

I checked the RFC, and the situation seems legal, although it says:

• on revision: "A module SHOULD have at least one initial "revision" statement."
• on import: "If no "revision-date" substatement is present, it is undefined from which revision of the module they are taken."

But if there is only 1 revision of a module available, it seems logical to take that one.

Workaround is to add a revision to the imported module, then it works (and it's also the logical thing to do, to add the revision). So the severity of this bug is low.